@article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/105117, title ="Measuring Basal Force Fluctuations of Debris Flows Using Seismic Recordings and Empirical Green's Functions", author = "Allstadt, Kate E. and Farin, Maxime", journal = "Journal of Geophysical Research. Earth Surface", volume = "125", number = "9", pages = "Art. No. e2020JF005590", month = "September", year = "2020", doi = "10.1029/2020jf005590", issn = "2169-9003", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200826-103628260", note = "© 2020 The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. \n\nIssue Online: 07 September 2020; Version of Record online: 07 September 2020; Accepted manuscript online: 25 August 2020; Manuscript accepted: 19 August 2020; Manuscript revised: 17 July 2020; Manuscript received: 21 February 2020. \n\nWe would like to acknowledge all the people who were instrumental in making the 2017 flume experiments happen, including Joel Smith, Chris Lockett, David George, Kelly Swinford, Corina Cerovski‐Darriau, John Perkins, Erin Bessette‐Kirton, Ben Mirus, and Francis Rengers. Nodal seismic instruments were provided by IRIS PASSCAL. V. C. T. acknowledges support from NSF EAR‐1939227. We also acknowledge Robert Anthony and three anonymous reviewers for their insightful reviews that helped substantially improve the manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. \n\nData Availability Statement: Seismic data from the 2017 flume experiments are available from the IRIS Data Management Center PH5 repository under network code YV (Allstadt, 2017, https://doi.org/10.7914/SN/YV_2017). Other data used in this study are available from USGS ScienceBase (Obryk & Iverson, 2019, https://doi.org/10.5066/P9NAFXW7). Video recordings of these two experiments and many other similar experiments are available for online viewing at https://pubs.usgs.gov/of/2007/1315 (Logan et al., 2018).", revision_no = "26", abstract = "We present a novel method for measuring the fluctuating basal normal and shear stresses of debris flows by using along‐channel seismic recordings. Our method couples a simple parameterization of a debris flow as a seismic source with direct measurements of seismic path effects using empirical Green's functions generated with a force hammer. We test this method using two large‐scale (8 and 10 m³) experimental flows at the U.S. Geological Survey debris‐flow flume that were recorded by dozens of three‐component seismic sensors. The seismically derived basal stress fluctuations compare well in amplitude and timing to independent force plate measurements within the valid frequency range (15–50 Hz). We show that although the high‐frequency seismic signals provide band‐limited forcing information, there are systematic relations between the fluctuating stresses and independently measured flow properties, especially mean basal shear stress and flow thickness. However, none of the relationships are simple, and since the flow properties also correlate with one another, we cannot isolate a single factor that relates in a simple way to the fluctuating forces. Nevertheless, our observations, most notably the gradually declining ratio of fluctuating to mean basal stresses during flow passage and the distinctive behavior of the coarse, unsaturated flow front, imply that flow style may be a primary control on the conversion of translational to vibrational kinetic energy. This conversion ultimately controls the radiation of high‐frequency seismic waves. Thus, flow style may provide the key to revealing the nature of the relationship between fluctuating forces and other flow properties.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/104947, title ="Time-Dependent Stresses From Fluid Extraction and Diffusion With Applications to Induced Seismicity", author = "Lambert, Valère and Tsai, Victor C.", journal = "Journal of Applied Mechanics", volume = "87", number = "8", pages = "Art. No. 081002", month = "August", year = "2020", doi = "10.1115/1.4047034", issn = "0021-8936", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200813-074456541", note = "© 2020 by ASME. \n\nPaper No: JAM-20-1033. \n\nContributed by the Applied Mechanics Division of ASME for publication in the Journal of Applied Mechanics. Manuscript received January 20, 2020; final manuscript received April 20, 2020; published online May 14, 2020.\n\n\n", revision_no = "11", abstract = "Over recent decades, it has become clear that the extraction of fluids from underground reservoirs can be linked to seismicity and aseismic deformation around producing fields. Using a simple model with uniform fluid extraction from a reservoir, Segall (1989, “Earthquakes Triggered by Fluid Extraction,” Geology, 17(10), pp. 942–946) illustrated how poroelastic stresses resulting from fluid withdrawal may be consistent with earthquake focal mechanisms surrounding some producing fields. Since these stress fields depend on the spatial gradient of the change in pore fluid content within the reservoir, both quantitative and qualitative predictions of the stress changes surrounding a reservoir may be considerably affected by assumptions in the geometry and hydraulic properties of the producing zone. Here, we expand upon the work of Segall (1989, “Earthquakes Triggered by Fluid Extraction,” Geology, 17, pp. 942–946 and 1985, “Stress and Subsidence Resulting From Subsurface Fluid Withdrawal in the Epicentral Region of the 1983 Coalinga Earthquake,” J. Geophys. Res. Solid Earth, 90, pp. 6801–6816) to provide a quantitative analysis of the surrounding stresses resulting from fluid extraction and diffusion in a horizontal reservoir. In particular, when considering the diffusion of fluids, the spatial pattern and magnitude of imposed stresses is controlled by the ratio between the volumetric rate of fluid extraction and the reservoir diffusivity. Moreover, the effective reservoir length expands over time along with the diffusion front, predicting a time-dependent rotation of the induced principal stresses from relative tension to compression along the ends of the producing zone. This reversal in perturbed principal stress directions may manifest as a rotation in earthquake focal mechanisms or varied sensitivity to poroelastic triggering, depending upon the criticality of the pre-existing stress state and fault orientations, which may explain inferred rotations in principal stress directions associated with some induced seismicity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/103037, title ="Extension of the Basin Rayleigh-Wave Amplification Theory to Include Basin-Edge Effects", author = "Brissaud, Quentin and Bowden, Daniel C.", journal = "Bulletin of the Seismological Society of America", volume = "110", number = "3", pages = "1305-1322", month = "June", year = "2020", doi = "10.1785/0120190161", issn = "0037-1106", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200506-125019256", note = "© 2020 Seismological Society of America. \n\nManuscript received 21 June 2019; Published online 7 April 2020. \n\nThe authors thank Arjun Datta for discussion about his work and for providing his code to compute surface‐wave transmission and reflection coefficients. \n\nData and Resources: The velocity and density profiles used in this study were extracted from the Southern California Earthquake Data Center (SCEDC), model CVMS4.26 https://scec.usc.edu/scecpedia/UCVM/ (last accessed March 2020). Scripts to compute surface‐wave transmission and reflection coefficients (SWRT) are available at https://github.com/arjundatta23/SWRT/ (last accessed March 2020). The SPECtral Finite EleMents (SPECFEM) package is available at https://geodynamics.org/cig/software/specfem2d/ (last accessed March 2020). The supplementary material contains additional information about the Green’s functions analytical expressions and extra figures describing near‐field effects, the variations of dominant frequency with velocity ratios, the horizontal maximum amplification sensitivity against basin‐shape ratio and location within the basin, and comparing amplification spectra from the different theories at another location in the Los Angeles basin. The supplementary material also describes the wavefield composition in semi‐infinite basins and the spatial and frequency dependence of vertical and horizontal amplification spectra for semi‐infinite and closed basins.", revision_no = "16", abstract = "The presence of sediments near the Earth’s surface can significantly amplify the strength of shaking during earthquakes. Such basin or site amplification effects have been well documented in numerous regions, yet the complex and often situational dependence of competing reasons for this amplification makes it hard to quantify in a general sense or to determine the most significant contributions. Simple 1D seismic profiles can be used to estimate the amplitude differences between a basin site and a hard‐rock reference site, but this ignores any reflections or conversions at the basin edge or a resonance effect depending on the basin’s geometry. In this article, we explore an analytic model based on coupling coefficients for surface Rayleigh waves to account for the lateral discontinuities at a basin’s edge (Datta 2018). We use this simple tool to explore the relationship between the basin’s Rayleigh‐wave amplification spectrum and various parameters such as basin depth, edge slope angle, and impedance contrast. The step‐by‐step construction of the model allows us to quantify the contributions from various wave propagation effects with the goal of identifying situations under which various basin‐edge effects must be considered in addition to purely 1D estimates. For the most velocity contrasts (less than a factor of 5), the error made by the 1D theory in predicting maximum Rayleigh‐wave basin amplification is under 35% for both the horizontal and the vertical components. For simple basins, the vertical amplification dominates at larger high frequencies and the horizontal at lower frequencies. Finally, we demonstrate from comparisons with spectral‐element wavefield simulations that realistic velocity structures can be reduced to a simpler “box” shape for the semi‐analytic formulation used here with reasonable results. For the purposes of estimating site‐amplification or microzonation, an improved model that accounts for basin‐edge effects can be implemented without high‐computational cost.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/103041, title ="Did Oldham Discover the Core After All? Handling Imprecise Historical Data with Hierarchical Bayesian Model Selection Methods", author = "Muir, Jack B. and Tsai, Victor C.", journal = "Seismological Research Letters", volume = "91", number = "3", pages = "1377-1383", month = "May", year = "2020", doi = "10.1785/0220190266", issn = "0895-0695", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200506-132625904", note = "© 2020 Seismological Society of America. \n\nManuscript received 20 September 2019; Published online 15 January 2020. \n\nThe authors would like to thank two anonymous reviewers for their feedback and the SRL Editor‐in‐Chief Allison Bent for managing the review process. The authors would also like to thank Luis Rivera for providing an internal review of this article. J. B. M. would like to thank the General Sir John Monash Foundation and the Origin Energy Foundation for financial support during his graduate studies. \n\nData and Resources: Historical data were taken from Oldham (1906), either from the reported tables of averaged events or by digitizing the presented travel‐time curves. All calculations were performed using the PyStan wrapper of the Stan statistical software package (Carpenter et al., 2017). The supplemental material contains inversion results for the five models not presented in the article (Figs. S1–S5). Additional discussion regarding hierarchical Markov chain Monte Carlo (MCMC) sampling and leave‐one‐out cross validation (LOO‐CV) versus k‐fold CV are also present in the supplement.", revision_no = "14", abstract = "Historical seismic data are essential to fill in the gaps in geophysical knowledge caused by the low rate of significant seismic events. Handling historical data in the context of geophysical inverse problems requires special care, due to the large errors in the data collection process. Using Oldham’s data for the discovery of Earth’s core as a case study, we illustrate how a hierarchical Bayesian model selection methodology using leave‐one‐out cross validation can robustly and efficiently answer quantitative questions using even poor‐quality geophysical data. We find that there is statistically significant evidence for the existence of the core using only the P‐wave data that Oldham effectively discarded in his discussion.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/101009, title ="Geometric and Level Set Tomography using Ensemble Kalman Inversion", author = "Muir, Jack B. and Tsai, Victor C.", journal = "Geophysical Journal International", volume = "220", number = "2", pages = "967-980", month = "February", year = "2020", doi = "10.1093/gji/ggz472", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200130-142308238", note = "© 2019 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). \n\nAccepted 2019 October 17. Received 2019 October 13; in original form 2019 June 11. Published: 21 October 2019. \n\nThe authors would like to thank Nicholas Rawlinson and an anonymous reviewer for providing useful commentary that has significantly improved the quality of the manuscript. We would also like to thank Editor Michael Ritzwoller and the anonymous assistant editor for managing the review process. JBM would like to thank Andrew Stuart (Caltech Computational and Mathematical Sciences) and the 2018 Gene Golub SIAM summer school for useful discussions regarding this study. Data from Carrizo Plains was collected during the 2017 Caltech Applied Geophysics Field Course, for which JBM was a Teaching Assistant. JBM would like to thank the instructors Rob Clayton and Mark Simons, and co-TA Voon Hui Lai, as well as the students, for the course. JBM would also like to thank the General Sir John Monash Foundation and the Origin Energy Foundation for financial support. This study was supported by NSF grant EAR-1453263. \n\nAll calculations were computed using the Julia language (Bezanson et al.2017). Code for our model specification language can be found at https://github.com/jbmuir/EarthModels.jl. Code for the EKI optimizer can be found at https://github.com/jbmuir/EnsembleKalmanInversion.jl. Code for a Julia 1.0+ compliant factored Eikonal fast marching method forward solver can be found at https://github.com/jbmuir/FEFMM.jl.", revision_no = "7", abstract = "Tomography is one of the cornerstones of geophysics, enabling detailed spatial descriptions of otherwise invisible processes. However, due to the fundamental ill-posedness of tomography problems, the choice of parametrizations and regularizations for inversion significantly affect the result. Parametrizations for geophysical tomography typically reflect the mathematical structure of the inverse problem. We propose, instead, to parametrize the tomographic inverse problem using a geologically motivated approach. We build a model from explicit geological units that reflect the a priori knowledge of the problem. To solve the resulting large-scale nonlinear inverse problem, we employ the efficient Ensemble Kalman Inversion scheme, a highly parallelizable, iteratively regularizing optimizer that uses the ensemble Kalman filter to perform a derivative-free approximation of the general iteratively regularized Levenberg–Marquardt method. The combination of a model specification framework that explicitly encodes geological structure and a robust, derivative-free optimizer enables the solution of complex inverse problems involving non-differentiable forward solvers and significant a priori knowledge. We illustrate the model specification framework using synthetic and real data examples of near-surface seismic tomography using the factored eikonal fast marching method as a forward solver for first arrival traveltimes. The geometrical and level set framework allows us to describe geophysical hypotheses in concrete terms, and then optimize and test these hypotheses, helping us to answer targeted geophysical questions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/100477, title ="Frequency-Dependent P Wave Polarization and Its Subwavelength Near-Surface Depth Sensitivity", author = "Park, Sunyoung and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "46", number = "24", pages = "14377-14384", month = "December", year = "2019", doi = "10.1029/2019gl084892", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200103-074516932", note = "© 2019 American Geophysical Union. \n\nReceived 9 AUG 2019; Accepted 8 NOV 2019; Accepted article online 15 NOV 2019. \n\nThe authors thank the editor Jeroen Ritsema and two anonymous reviewers for helpful comments. S.P. thanks Hiroo Kanamori for valuable discussion about the Median Tectonic Line. The early phase of this work was supported by NSF grant EAR‐1735960. S. P. was also supported by the Caltech Texaco Postdoctoral Fellowship. The waveform simulation code was obtained from QUEST (http://www.quest‐itn.org/library/software/reflectivity‐method.html).", revision_no = "21", abstract = "Near‐surface structure is crucial to assessing seismic hazards and understanding earthquakes and surface processes yet is a major challenge to robustly image. Recently, an approach based on body‐wave polarization was introduced for constraining shallow seismic structure, but the depth sensitivity of the polarization measurement has remained unclear. Using waveform simulations based on a layer over a half space, we find that the depth sensitivity of P wave polarization peaks at the surface and decreases abruptly over a depth range shorter than its wavelength. A strong frequency dependence provides constraints on local 1‐D structure, with frequencies between 0.1 and 10 Hz illuminating structure at depths of 10 m to several kilometers. Applying these results to teleseismic recordings in Japan provides constraints on structure at about 120 to 750 m, including a distinctive weak zone along the Median Tectonic Line in the Kii peninsula and Awaji Island.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/99581, title ="Validation of a fast semi-analytic method for surface-wave propagation in layered media", author = "Brissaud, Quentin and Tsai, Victor C.", journal = "Geophysical Journal International", volume = "219", number = "2", pages = "1405-1420", month = "November", year = "2019", doi = "10.1093/gji/ggz351", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20191031-104134697", note = "© The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). \n\nAccepted 2019 September 2. Received 2019 May 17; in original form 2019 January 30. Published: 05 September 2019. \n\nThe authors would like to thank the two anonymous reviewers that greatly helped improving the paper.", revision_no = "9", abstract = "Green’s functions provide an efficient way to model surface-wave propagation and estimate physical quantities for near-surface processes. Several surface-wave Green’s function approximations (far-field, no mode conversions and no higher mode surface waves) have been employed for numerous applications such as estimating sediment flux in rivers, determining the properties of landslides, identifying the seismic signature of debris flows or to study seismic noise through cross-correlations. Based on those approximations, simple empirical scalings exist to derive phase velocities and amplitudes for pure power-law velocity structures providing an exact relationship between the velocity model and the Green’s functions. However, no quantitative estimates of the accuracy of these simple scalings have been reported for impulsive sources in complex velocity structures. In this paper, we address this gap by comparing the theoretical predictions to high-order numerical solutions for the vertical component of the wavefield. The Green’s functions computation shows that attenuation-induced dispersion of phase and group velocity plays an important role and should be carefully taken into account to correctly describe how surface-wave amplitudes decay with distance. The comparisons confirm the general reliability of the semi-analytic model for power-law and realistic shear velocity structures to describe fundamental-mode Rayleigh waves in terms of characteristic frequencies, amplitudes and envelopes. At short distances from the source, and for large near-surface velocity gradients or high Q values, the low-frequency energy can be dominated by higher mode surface waves that can be captured by introducing additional higher mode Rayleigh-wave power-law scalings. We also find that the energy spectral density for realistic shear-velocity models close to piecewise power-law models can be accurately modelled using the same non-dimensional scalings. The frequency range of validity of each power-law scaling can be derived from the corresponding phase velocities. Finally, highly discontinuous near-surface velocity profiles can also be approximated by a combination of power-law scalings. Analytical Green’s functions derived from the non-dimensionalization provide a good estimate of the amplitude and variations of the energy distribution, although the predictions are quite poor around the frequency bounds of each power-law scaling.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/96125, title ="A physical model of the high-frequency seismic signal generated by debris flows", author = "Farin, Maxime and Tsai, Victor C.", journal = "Earth Surface Processes and Landforms", volume = "44", number = "13", pages = "2529-2543", month = "October", year = "2019", doi = "10.1002/esp.4677", issn = "0197-9337", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190604-151340833", note = "© 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. \n\nReceived 20 November 2018; Revised 11 April 2019; Accepted 20 May 2019. \n\nWe thank Dick Iverson for helpful comments on an early draft of the manuscript and we thank two anonymous reviewers for their comments. This work was supported by NSF grants EAR 1558479, EAR 1349115 and the Terrestrial Hazards Observation and Reporting Center at Caltech.", revision_no = "24", abstract = "We propose a physical model for the high‐frequency (>1 Hz) spectral distribution of seismic power generated by debris flows. The modeled debris flow is assumed to have four regions where the impact rate and impulses are controlled by different mechanisms: the flow body, a coarser‐grained snout, a snout lip where particles fall from the snout on the bed, and a dilute front composed of saltating particles. We calculate the seismic power produced by this impact model in two end‐member scenarios, a thin‐flow and thick‐flow limit, which assume that the ratio of grain sizes to flow thicknesses are either near unity or much less than unity. The thin‐flow limit is more appropriate for boulder‐rich flows that are most likely to generate large seismic signals. As a flow passes a seismic station, the rise phase of the seismic amplitude is generated primarily by the snout while the decay phase is generated first by the snout and then the main flow body. The lip and saltating front generate a negligible seismic signal. When ground properties are known, seismic power depends most strongly on both particle diameter and average flow speed cubed, and also depends on length and width of the flow. The effective particle diameter for producing seismic power is substantially higher than the median grain size and close to the 73rd percentile for a realistic grain size distribution. We discuss how the model can be used to estimate effective particle diameter and average flow speed from an integrated measure of seismic power. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/96682, title ="Direct Observations of Surface‐Wave Eigenfunctions at the Homestake 3D Array", author = "Meyers, Patrick and Bowden, Daniel C.", journal = "Bulletin of the Seismological Society of America", volume = "109", number = "4", pages = "1194-1202", month = "August", year = "2019", doi = "10.1785/0120190026", issn = "0037-1106", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190625-083425940", note = "© 2019 Seismological Society of America. \n\nManuscript received 7 February 2019; Published Online 25 June 2019. \n\nData and Resources: Data presented here were collected at the Homestake 3D seismometer array. The data are available at the Incorporated Research Institutions for Seismology (IRIS) Data Management Center available at www.iris.edu (last accessed July 2018), designation X6. Finding a model to fit dispersion data was done using Computer Programs in Seismology, v.3.30, at http://www.eas.slu.edu/eqc/eqccps.html (last accessed March 2016). \n\nThe authors are grateful to staff at the Sanford Underground Research Facility and Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL) for their assistance. Specifically, they thank Jaret Heise, Tom Regan, Bryce Pietzyk, and Jamey Tollefson. Vital technical contributions related to the operation and maintenance of the Homestake 3D seismometer array were made by Terry Stigall. The authors also thank two anonymous reviewers for their helpful comments and suggestions. This work was supported by National Science Foundation (NSF) INSPIRE (Integrated NSF Support Promoting Interdisciplinary Research and Education) Grant PHY1344265. Parts of this research were conducted by the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), through Project Number CE170100004. The authors are grateful for computational resources provided by the Laser Interferometer Gravitational‐Wave Observatory (LIGO) Laboratory and supported by National Science Foundation Grants PHY‐0757058 and PHY‐0823459.", revision_no = "17", abstract = "Despite the theory for both Rayleigh and Love waves being well accepted and the theoretical predictions accurately matching observations, the direct observation of their quantifiable decay with depth has never been measured in the Earth’s crust. In this work, we present observations of the quantifiable decay with depth of surface‐wave eigenfunctions. This is done by making direct observations of both Rayleigh‐wave and Love‐wave eigenfunction amplitudes over a range of depths using data collected at the 3D Homestake array for a suite of nearby mine blasts. Observations of amplitudes over a range of frequencies from 0.4 to 1.2 Hz are consistent with theoretical eigenfunction predictions. They show a clear exponential decay of amplitudes with increasing depth and a reversal in sign of the radial‐component Rayleigh‐wave eigenfunction at large depths, as predicted for fundamental‐mode Rayleigh waves. Minor discrepancies between the observed eigenfunctions and those predicted using estimates of the local velocity structure suggest that the observed eigenfunctions could be used to improve the velocity model. Our results confirm that both Rayleigh and Love waves have the depth dependence that they have long been assumed to have. This is an important direct validation of a classic theoretical result in geophysics and provides new observational evidence that classical seismological surface‐wave theory can be used to accurately infer properties of Earth structure and earthquake sources.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/93731, title ="Coherence-based approaches for estimating the composition of the seismic wavefield", author = "Coughlin, M. and Harms, J.", journal = "Journal of Geophysical Research. Solid Earth", volume = "124", number = "3", pages = "2941-2956", month = "March", year = "2019", doi = "10.1029/2018jb016608", issn = "2169-9313", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190312-101923328", note = "© 2019 American Geophysical Union. \n\nReceived 23 AUG 2018; Accepted 5MAR 2019; Accepted article online 12MAR 2019; Published online 25MAR 2019. \n\nData used in this project are available from the Incorporated Research Institutions for Seismology (IRIS; Mandic et al., 2014). The seismic instruments used for this array were provided by IRIS through the PASSCAL Instrument Center at New Mexico Tech. We thank Nicholas Harmon and an anonymous reviewer for suggestions and improvements to the text. M. C. was supported by the David and Ellen Lee Postdoctoral Fellowship at the California Institute of Technology. We thank the staff at the Sanford Underground Research Facility and PASSCAL for assistance, particularly the help of Tom Regan, Jaret Heise, Jamey Tollefson, and Bryce Pietzyk. This work was supported by National Science Foundation INSPIRE grant PHY1344265. This paper has been assigned LIGO document number LIGO‐P1700422.", revision_no = "22", abstract = "As new techniques exploiting the Earth's ambient seismic noise field are developed and applied, such as for the observation of temporal changes in seismic velocity structure, it is crucial to quantify the precision with which wave‐type measurements can be made. This work uses array data at the Homestake mine in Lead, South Dakota, and an array at Sweetwater, Texas, to consider two aspects that control this precision: the types of seismic wave contributing to the ambient noise field at microseism frequencies and the effect of array geometry. Both are quantified using measurements of wavefield coherence between stations in combination with Wiener filters. We find a strong seasonal change between body‐wave and surface‐wave content. Regarding the inclusion of underground stations, we quantify the lower limit to which the ambient noise field can be characterized and reproduced; the applications of the Wiener filters are about 4 times more successful in reproducing ambient noise waveforms when underground stations are included in the array, resulting in predictions of seismic time series with less than a 1% residual, and are ultimately limited by the geometry and aperture of the array, as well as by temporal variations in the seismic field. We discuss the implications of these results for the geophysics community performing ambient seismic noise studies, as well as for the cancellation of seismic Newtonian gravity noise in ground‐based, sub‐Hertz, gravitational‐wave detectors.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/91204, title ="Particle transport mechanics and induced seismic noise in steep flume experiments with accelerometer-embedded tracers", author = "Gimbert, Florent and Fuller, Brian M.", journal = "Earth Surface Processes and Landforms", volume = "44", number = "1", pages = "219-241", month = "January", year = "2019", doi = "10.1002/esp.4495", issn = "0197-9337", url = "https://resolver.caltech.edu/CaltechAUTHORS:20181127-075323708", note = "© 2018 John Wiley & Sons, Ltd. \n\nReceived 29 November 2017; Revised 14 August 2018; Accepted 20 August 2018; Issue Online: 17 January 2019. \n\nThis work was supported by NSF grant EAR‐1558479, the Terrestrial Hazard Observation and Reporting Center at the California Institute of Technology and the ANR grant ANR‐17‐CE01‐0008‐01. We thank Anne‐Sophie Drouet for help in processing the smart rock data, Fanny Brun and Jeff Prancevic for help in conducting preliminary flume and smart rock experiments, and Bismark Wong for help in inferring sediment flux at high rates from video analysis.", revision_no = "17", abstract = "Recent advances in fluvial seismology have provided solid observational and theoretical evidence that near‐river seismic ground motion may be used to monitor and quantify coarse sediment transport. However, inversions of sediment transport rates from seismic observations have not been fully tested against independent measurements, and thus have unknown but potentially large uncertainties. In the present study, we provide the first robust test of existing theory by conducting dedicated sediment transport experiments in a flume laboratory under fully turbulent and rough flow conditions. We monitor grain‐scale physics with the use of ‘smart rocks’ that consist of accelerometers embedded into manufactured rocks, and we quantitatively link bedload mechanics and seismic observations under various prescribed flow and sediment transport conditions. From our grain‐scale observations, we find that bedload grain hop times are widely distributed, with impacts being on average much more frequent than predicted by existing saltation models. Impact velocities are observed to be a linear function of average downstream cobble velocities, and both velocities show a bed‐slope dependency that is not represented in existing saltation models. Incorporating these effects in an improved bedload‐induced seismic noise model allows sediment flux to be inverted from seismic noise within a factor of two uncertainty. This result holds over nearly two orders of magnitude of prescribed sediment fluxes with different sediment sizes and channel‐bed slopes, and particle–particle collisions observed at the highest investigated rates are found to have negligible effect on the generated seismic power. These results support the applicability of the seismic‐inversion framework to mountain rivers, although further experiments remain to be conducted at sediment transport near transport capacity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/90319, title ="A Simple Model for Deglacial Meltwater Pulses", author = "Robel, Alexander A. and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "45", number = "21", pages = "11742-11750", month = "November", year = "2018", doi = "10.1029/2018GL080884", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20181022-091459376", note = "© 2018 American Geophysical Union. \n\nReceived 11 OCT 2018; Accepted 14 OCT 2018; Accepted article online 18 OCT 2018; Published online 4 NOV 2018. \n\nA simple numerical implementation of the mathematical\nmodel described in this study is available as a public repository on GitHub: https://github.com/aarobel/. This work was primarily supported by the NSF Arctic Natural Sciences Program (Grant OPP 1735715). AAR was also supported by the NOAA Climate and Global Change Postdoctoral Fellowship and the Caltech Stanback Postdoctoral Fellowship during an early phase of this project. Thanks to Lev Tarasov for providing the output from his observationally-constrained Laurentide Ice Sheet model.", revision_no = "28", abstract = "Evidence from radiocarbon dating and complex ice sheet modeling suggests that the fastest rate of sea level rise in Earth's recent history coincided with collapse of the ice saddle between the Laurentide and Cordilleran ice sheets during the last deglaciation. In this study, we derive a simple, two‐equation model of two ice sheets intersecting in an ice saddle. We show that two conditions are necessary for producing the acceleration in ice sheet melt associated with meltwater pulses: the positive height‐mass balance feedback and an ice saddle geometry. The amplitude and timing of meltwater pulses is sensitively dependent on the rate of climate warming during deglaciation and the relative size of ice sheets undergoing deglaciation. We discuss how simulations of meltwater pulses can be improved and the prospect for meltwater pulses under continued climate warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/89064, title ="A 3D Broadband Seismometer Array Experiment at the Homestake Mine", author = "Mandic, Vuk and Tsai, Victor C.", journal = "Seismological Research Letters", volume = "89", number = "6", pages = "2420-2429", month = "November", year = "2018", doi = "10.1785/0220170228", issn = "0895-0695", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180822-151337473", note = "© 2018 Seismological Society of America. \n\nPublished Online 22 August 2018. \n\nThe authors thank the staff at the Sanford Underground Research Facility and Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL) for assistance, particularly the help of Tom Regan, Jaret Heise, Jamey Tollefson, and Bryce Pietzyk. Terry Stigall made important technical contributions to operate and maintain the array. The authors also thank C. Langston and an anonymous reviewer for comments. The seismic instruments used for this array were provided by the Incorporated Research Institutions for Seismology (IRIS) through the PASSCAL Instrument Center at New Mexico Tech. Data collected are available through the IRIS Data Management Center. The facilities of the IRIS Consortium are supported by the National Science Foundation under Cooperative Agreement EAR‐1261681 and the Department of Energy (DOE) National Nuclear Security Administration. This work was supported by National Science Foundation INSPIRE Grant PHY1344265. \n\nData and Resources: Data collected by the Homestake array and presented here are available at the Incorporated Research Institutions for Seismology (IRIS) Data Management Center available at www.iris.edu (last accessed July 2018) in 2018, designation X6. Also used are data for the array network facility of USArray website available at http://anf.ucsd.edu/events/ (last accessed April 2017).", revision_no = "19", abstract = "Seismometer deployments are often confined to near the Earth’s surface for practical reasons, despite the clear advantages of deeper seismometer installations related to lower noise levels and more homogeneous conditions. Here, we describe a 3D broadband seismometer array deployed at the inactive Homestake Mine in South Dakota, which takes advantage of infrastructure originally setup for mining and is now used for a range of scientific experiments. The array consists of 24 stations, of which 15 were underground, with depths ranging from 300 ft (91 m) to 4850 ft (1478 m), and with a 3D aperture of ∼1.5\u2009km in each direction, thus spanning a 3D volume of about 3.4\u2009km^3. We describe unique research opportunities and challenges related to the 3D geometry, including the generally low ambient noise levels, the strong coherency between observed event waveforms across the array, and the technical challenges of running the network. This article summarizes preliminary results obtained using data acquired by the Homestake array, illustrating the range of possible studies supported by the data.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/89583, title ="A simple physics-based improvement to the positive degree day model", author = "Tsai, Victor C. and Ruan, Xiaozhou", journal = "Journal of Glaciology", volume = "64", number = "246", pages = "661-668", month = "August", year = "2018", doi = "10.1017/jog.2018.55", issn = "0022-1430", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180912-135435614", note = "© The Author(s) 2018. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. \n\nMS received 20 November 2017 and accepted in revised form 11 June 2018; first published online 6 July 2018. \n\nWe thank three anonymous reviewers for their constructive comments. This work was partly supported by NSF grant OPP-1735715.", revision_no = "10", abstract = "Meltwater is important to understanding glacier health and dynamics. Since melt measurements are uncommon, ice ablation estimates are often based on models including the positive degree day (PDD) model. The PDD estimate is popular since it only requires air temperature as input, but suffers from the lack of physical motivation of an energy-balance model. We present a physics-based alternative to the PDD model that still only takes air/surface temperature as input. The model resembles the PDD model except accounting for time lags in ablation when cold ice needs to be warmed. The model is expressed as a differential equation with a single extra parameter related to the efficiency of heating a near-surface layer of ice. With zero thickness, the model reduces to the PDD model, providing a physical basis for the PDD model. Applying the model to data from Greenland, it improves modestly upon the PDD model, with the main improvement being better prediction of early season melting. This new model is a useful compromise, with some of the physics of more realistic models and the simplicity of a PDD model. The model should improve estimates of meltwater production and help constrain PDD parameters when empirical calibration is challenging.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/86939, title ="Observations and modeling of long-period ground-motion amplification across northeast China", author = "Chen, Haichao and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "45", number = "12", pages = "5968-5976", month = "June", year = "2018", doi = "10.1029/2018GL078212", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180608-144450318", note = "© 2018 American Geophysical Union. \n\nReceived 5 APR 2018; Accepted 2 JUN 2018; Accepted article online 8 JUN 2018; Published online 22 JUN 2018. \n\nWe thank all the people involved in the NECESSArray project for providing the waveform data and Guoliang Li for providing the velocity model used in this study. We also thank Morgan Moschetti and one anonymous reviewer for their constructive comments that improved the quality of this manuscript. Waveform data of the permanent stations were provided by Data Management Centre of the China National Seismic Network at the Institute of Geophysics (SEISDMC, https://doi.org/10.11998/SeisDmc/SN). The NECESSArray data are available from the IRIS data center. This work was jointly supported by the National Key Research and Development Program of China (no. 2017YFC1500300), the National Basic Research Program of China (no. 2015CB250903), the National Natural Science Foundation of China (nos. 41604043 and 41630209), and NSF grants EAR‐1453263, SCEC‐17061, and EAR‐1547228. All the plots were made using the Generic Mapping Tools version 5.4.2 (Wessel et al., 2013).", revision_no = "31", abstract = "Basin resonances can significantly amplify and prolong ground shaking, and accurate site‐amplification estimates are crucial for mitigating potential seismic hazards within metropolitan basins. In this work, we estimate the site amplification of long‐period (2–10 s) ground motions across northeast China for both surface waves and vertically incident shear waves. The spatial distribution of relatively large site amplifications correlates strongly with known sedimentary basins for both wave types. However, the site response of surface waves is typically twice as high as that of shear waves at most basin sites. We further show that these site‐amplification features can be well explained by predictions based on the local one‐dimensional structure at each site. Our results highlight the importance of accounting for surface‐wave contributions and demonstrate the usefulness of semi‐analytical theory for surface‐wave amplification, which may be broadly applicable in future seismic hazard analysis.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/86701, title ="The Seismic Signature of Debris Flows: Flow Mechanics and Early Warning at Montecito, California", author = "Lai, Voon Hui and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "45", number = "11", pages = "5528-5535", month = "June", year = "2018", doi = "10.1029/2018GL077683", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180530-083814651", note = "© 2018 American Geophysical Union. \n\nReceived 25 FEB 2018; Accepted 19 MAY 2018; Accepted article online 30 MAY 2018; Published online 12 JUN 2018. \n\nThe authors thank three anonymous reviewers for comments. Funding was provided by the U.S. National Science Foundation EAR‐1558479 to V.C.T. and M.P.L. and EAR‐1346115 to M.P.L. A.R.B. acknowledges support from the Swiss National Science Foundation. V.L. processed the seismic data, V.C.T. designed the model, M.P.L. analyzed the geomorphic data, T.P.U. measured the geographical data, and A.R.B. measured boulder data. V.C.T. and M.P.L. wrote the paper, with editorial contributions from all authors. All authors contributed to the interpretation of results. All waveform data were accessed through the Southern California Earthquake Data Center (SCEDC) at Caltech, https://doi.org/10.7909/C3WD3xH1.", revision_no = "24", abstract = "Debris flows are concentrated slurries of water and sediment that shape the landscape and pose a major hazard to human life and infrastructure. Seismic ground motion‐based observations promise to provide new, remote constraints on debris flow physics, but the lack of data and a theoretical basis for interpreting them hinders progress. Here we present a new mechanistic physical model for the seismic ground motion of debris flows and apply this to the devastating debris flows in Montecito, California on 9 January 2018. The amplitude and frequency characteristics of the seismic data can distinguish debris flows from other seismic sources and enable the estimation of debris‐flow speed, width, boulder sizes, and location. Results suggest that present instrumentation could have provided 5 min of early warning over limited areas, whereas a seismic array designed for debris flows would have provided 10 min of warning for most of the city.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/83622, title ="Expected seismicity and the seismic noise environment of Europa", author = "Panning, Mark P. and Stähler, Simon C.", journal = "Journal of Geophysical Research. Planets", volume = "123", number = "1", pages = "163-179", month = "January", year = "2018", doi = "10.1002/2017JE005332", issn = "2169-9097", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171201-112529873", note = "© 2017 American Geophysical Union. \n\nReceived 4 MAY 2017; Accepted 14 AUG 2017; Accepted article online 30 NOV 2017; Published online 25 JAN 2018. \n\nThe authors acknowledge computational support in the project pr63qo “3-D wave propagation and rupture: forward and inverse problem” at Leibniz-Rechenzentrum Garching. S. C. S. was supported by grant SI1538/4-1 of Deutsche Forschungsgemeinschaft DFG. This work was partially supported by strategic research and technology funds from the Jet Propulsion Laboratory, Caltech and by the Icy Worlds node of NASA's Astrobiology Institute (13-13NAI7_2-0024). Noise waveform records and seismic catalogs are available via GitHub at http://github.com/mpanning/EuropaNoise. Axisem waveform databases are maintained by S. C. S. and can be accessed for use in Instaseis scripts via http://instaseis.ethz.ch/icy_ocean_worlds/. Sound files in the supporting information were created with Matlab scripts from Zhigang Peng (http://geophysics.eas.gatech.edu/people/zpeng/EQ_Music/). Work by M. P. was started at the University of Florida and was completed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement by the United States Government or the Jet Propulsion Laboratory, California Institute of Technology.", revision_no = "28", abstract = "Seismic data will be a vital geophysical constraint on internal structure of Europa if we land instruments on the surface. Quantifying expected seismic activity on Europa both in terms of large, recognizable signals and ambient background noise is important for understanding dynamics of the moon, as well as interpretation of potential future data. Seismic energy sources will likely include cracking in the ice shell and turbulent motion in the oceans. We define a range of models of seismic activity in Europa's ice shell by assuming each model follows a Gutenberg-Richter relationship with varying parameters. A range of cumulative seismic moment release between 10^(16) and 10^(18) Nm/yr is defined by scaling tidal dissipation energy to tectonic events on the Earth's moon. Random catalogs are generated and used to create synthetic continuous noise records through numerical wave propagation in thermodynamically self-consistent models of the interior structure of Europa. Spectral characteristics of the noise are calculated by determining probabilistic power spectral densities of the synthetic records. While the range of seismicity models predicts noise levels that vary by 80 dB, we show that most noise estimates are below the self-noise floor of high-frequency geophones but may be recorded by more sensitive instruments. The largest expected signals exceed background noise by ∼50 dB. Noise records may allow for constraints on interior structure through autocorrelation. Models of seismic noise generated by pressure variations at the base of the ice shell due to turbulent motions in the subsurface ocean may also generate observable seismic noise.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/84437, title ="Vital Signs: Seismology of Icy Ocean Worlds", author = "Vance, Steven D. and Kedar, Sharon", journal = "Astrobiology", volume = "18", number = "1", pages = "37-53", month = "January", year = "2018", doi = "10.1089/ast.2016.1612", issn = "1531-1074", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180122-074827302", note = "© 2018 Mary Ann Liebert, Inc. \n\nSubmitted 27 October 2016; Accepted 7 June 2017. \n\nWe thank for their helpful input Sridhar Anandakrishnan, Bruce Banerdt, Jason Goodman, and Jennifer Jackson. This work was partially supported by strategic research and technology funds from the Jet Propulsion Laboratory, Caltech, and by the Icy Worlds node of NASA's Astrobiology Institute (13-13NAI7_2-0024). The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. \n\nNo competing financial interests exist.", revision_no = "11", abstract = "Ice-covered ocean worlds possess diverse energy sources and associated mechanisms that are capable of driving significant seismic activity, but to date no measurements of their seismic activity have been obtained. Such investigations could reveal the transport properties and radial structures, with possibilities for locating and characterizing trapped liquids that may host life and yielding critical constraints on redox fluxes and thus on habitability. Modeling efforts have examined seismic sources from tectonic fracturing and impacts. Here, we describe other possible seismic sources, their associations with science questions constraining habitability, and the feasibility of implementing such investigations. We argue, by analogy with the Moon, that detectable seismic activity should occur frequently on tidally flexed ocean worlds. Their ices fracture more easily than rocks and dissipate more tidal energy than the <1 GW of the Moon and Mars. Icy ocean worlds also should create less thermal noise due to their greater distance and consequently smaller diurnal temperature variations. They also lack substantial atmospheres (except in the case of Titan) that would create additional noise. Thus, seismic experiments could be less complex and less susceptible to noise than prior or planned planetary seismology investigations of the Moon or Mars.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/83241, title ="Amplification and Attenuation across USArray using Ambient Noise Wavefront Tracking", author = "Bowden, Daniel C. and Tsai, Victor C.", journal = "Journal of Geophysical Research. Solid Earth", volume = "122", number = "12", pages = "10086-10101", month = "December", year = "2017", doi = "10.1002/2017JB014804", issn = "2169-9313", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171115-155052574", note = "© 2017 American Geophysical Union. \n\nReceived 30 JUL 2017; Accepted 9 NOV 2017; Accepted article online 15 NOV 2017. \n\nData from USArray were downloaded from the IRIS Data Management Center at http://www.iris.edu/hq/. The velocity model of Schmandt et al. (2015) was downloaded from IRIS at http://ds.iris.edu/ds/products/emc-us-crustvs-2015/. Amplification and attenuation maps are included as supporting information, and other results are available upon request to dbowden@caltech.edu. This work was supported by EAR-1453263 and EAR-1252191. F.-C. L. was supported by NSF grant CyberSEES-1442665 and the King Abdullah University of Science and Technology (KAUST) under award OCRF-2014-CRG3-2300.", revision_no = "37", abstract = "As seismic traveltime tomography continues to be refined using data from the vast USArray data set, it is advantageous to also exploit the amplitude information carried by seismic waves. We use ambient noise cross correlation to make observations of surface wave amplification and attenuation at shorter periods (8–32 s) than can be observed with only traditional teleseismic earthquake sources. We show that the wavefront tracking approach can be successfully applied to ambient noise correlations, yielding results quite similar to those from earthquake observations at periods of overlap. This consistency indicates that the wavefront tracking approach is viable for use with ambient noise correlations, despite concerns of the inhomogeneous and unknown distribution of noise sources. The resulting amplification and attenuation maps correlate well with known tectonic and crustal structure; at the shortest periods, our amplification and attenuation maps correlate well with surface geology and known sedimentary basins, while our longest period amplitudes are controlled by crustal thickness and begin to probe upper mantle materials. These amplification and attenuation observations are sensitive to crustal materials in different ways than traveltime observations and may be used to better constrain temperature or density variations. We also value them as an independent means of describing the lateral variability of observed Rayleigh wave amplitudes without the need for 3-D tomographic inversions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/82693, title ="Toward automated directivity estimates in earthquake moment tensor inversion", author = "Huang, Hsin-Hua and Aso, Naofumi", journal = "Geophysical Journal International", volume = "211", number = "2", pages = "1062-1076", month = "November", year = "2017", doi = "10.1093/gji/ggx354", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171026-083324515", note = "© 2017 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. \n\nAccepted 2017 August 18. Received 2017 July 13; in original form 2017 April 20. Published: 19 August 2017. \n\nWe thank Zachary Ross, Lingling Ye, and Zhongwen Zhan for helpful discussion. We also thank Carl Tape and an anonymous reviewer for their constructive comments. The seismic data used in this study were obtained from the Broadband Array in Taiwan for Seismology (IESAS 1996), National Research Institute for Earth Science and Disaster Resilience (F-net), and Southern California Earthquake Data Center (SCEDC 2013). This work was partially supported by National Science Foundation grant EAR-1453263 and Ministry of Science and Technology grant 105-2116-M-001-026-MY2. Naofumi Aso was a Japan Society for the Promotion of Science (JSPS) Overseas Research Fellow (2015-146).", revision_no = "8", abstract = "Rapid estimates of earthquake rupture properties are useful for both scientific characterization of earthquakes and emergency response to earthquake hazards. Rupture directivity is a particularly important property to constrain since seismic waves radiated in the direction of rupture can be greatly amplified, and even moderate magnitude earthquakes can sometimes cause serious damage. Knowing the directivity of earthquakes is important for ground shaking prediction and hazard mitigation, and is also useful for discriminating which nodal plane corresponds to the actual fault plane particularly when the event lacks aftershocks or outcropped fault traces. Here, we propose a 3-D multiple-time-window directivity inversion method through direct waveform fitting, with source time functions stretched for each station according to a given directivity. By grid searching for the directivity vector in 3-D space, this method determines not only horizontal but vertical directivity components, provides uncertainty estimates, and has the potential to be automated in real time. Synthetic tests show that the method is stable with respect to noise, picking errors, and site amplification, and is less sensitive to station coverage than other methods. Horizontal directivity can be properly recovered with a minimum azimuthal station coverage of 180°, whereas vertical directivity requires better coverage to resolve. We apply the new method to the M_w 6.0 Nantou, Taiwan earthquake, M_w 7.0 Kumamoto, Japan earthquake, and M_w 4.7 San Jacinto fault trifurcation (SJFT) earthquake in southern California. For the Nantou earthquake, we corroborate previous findings that the earthquake occurred on a shallow east-dipping fault plane rather than a west-dipping one. For the Kumamoto and SJFT earthquakes, the directivity results show good agreement with previous studies and demonstrate that the method captures the general rupture characteristics of large earthquakes involving multiple fault ruptures and applies to earthquakes with magnitudes as small as M_w 4.7.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/84374, title ="Seismologically Observed Spatiotemporal Drainage Activity at Moulins", author = "Aso, Naofumi and Tsai, Victor C.", journal = "Journal of Geophysical Research. Solid Earth", volume = "122", number = "11", pages = "9095-9108", month = "November", year = "2017", doi = "10.1002/2017JB014578", issn = "2169-9313", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180117-155132585", note = "© 2017 American Geophysical Union. \n\nReceived 17 JUN 2017; Accepted 17 OCT 2017; Accepted article online 19 OCT 2017; Published online 11 NOV 2017. \n\nWe thank C. Zdanowicz and L. Copland for the meteorological data at the weather station at the confluence. We acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Canada Research Chairs Program, the California Institute of Technology, the University of British Columbia, and Simon Fraser University. We are grateful to the Polar Continental Shelf Program (PCSP) and Kluane Lake Research Station for logistical support. We are grateful to the Kluane First Nation, Parks Canada, and Yukon Territorial Government for permission to conduct field work. WorldView-1 imagery was obtained through the Polar Geospatial Center, University of Minnesota. This work was partially supported by the National Science Foundation grant EAR-1453263 and JSPS KAKENHI grant JP17H06605. Comments from two anonymous reviewers and the Associate Editor helped to improve the manuscript. Seismic data used in this paper are provided in the supporting information.", revision_no = "20", abstract = "Hydrology is important for glacier dynamics, but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations. Since meltwater drainage generates seismic signals, passive seismic analysis has the potential to be used to monitor these processes. To study continuous seismic radiation from the drainage, we analyze geophone data from six stations deployed at the Kaskawulsh Glacier in Yukon, Canada, during the summer of 2014 using ambient noise cross-correlation techniques. We locate the noise sources by backprojecting the amplitude of the cross correlation to the glacier surface. Most of the ambient noise sequences are found in two clusters, with each cluster located in the vicinity of a moulin identified at the surface. Stronger seismic radiation is observed during the day, consistent with expected variability in melt rates. We demonstrate that the sparse seismic network array with 2 km station separation has the ability to detect moulins within the array with a precision of 50 m. We confirm that seismic activity is correlated with air temperature, and thus, melt, on a diurnal timescale, and precipitation correlates with the activity at longer timescales. Our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations, but quantification of water flow rates still remains a challenge. The cross-correlation backprojection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise, tectonic tremor, and volcanic tremor.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/83747, title ="Was the M_w 7.5 1952 Kern County, California, earthquake induced (or triggered)?", author = "Hough, Susan E. and Tsai, Victor C.", journal = "Journal of Seismology", volume = "21", number = "6", pages = "1613-1621", month = "November", year = "2017", doi = "10.1007/s10950-017-9685-x", issn = "1383-4649", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171208-073841565", note = "© 2017 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. \n\nReceived: 6 May 2017; Accepted: 21 July 2017; Published online: 2 October 2017. \n\nWe thank Elizabeth Cochran, Art McGarr, Steve Hickman, Katherine Kendrick, Gail Atkinson, and two anonymous reviewers for their reviews of this manuscript. We also thank Mariano Garcia-Fernandez for his editorial stewardship of this journal, as well as his patience. Figures 1 and 3 are generated using GMT software (Wessel and Smith 1991). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.", revision_no = "11", abstract = "Several recent studies have presented evidence that significant induced earthquakes occurred in a number of oil-producing regions during the early and mid-twentieth century related to either production or wastewater injection. We consider whether the 21 July 1952 M_w 7.5 Kern County earthquake might have been induced by production in the Wheeler Ridge oil field. The mainshock, which was not preceded by any significant foreshocks, occurred 98 days after the initial production of oil in Eocene strata at depths reaching 3 km, within ~1 km of the White Wolf fault (WWF). Based on this spatial and temporal proximity, we explore a potential causal relationship between the earthquake and oil production. While production would have normally be expected to have reduced pore pressure, inhibiting failure on the WWF, we present an analytical model based on industry stratigraphic data and best estimates of parameters whereby an impermeable splay fault adjacent to the main WWF could plausibly have blocked direct pore pressure effects, allowing the poroelastic stress change associated with production to destabilize the WWF, promoting initial failure. This proof-of-concept model can also account for the 98-day delay between the onset of production and the earthquake. While the earthquake clearly released stored tectonic stress, any initial perturbation on or near a major fault system can trigger a larger rupture. Our proposed mechanism provides an explanation for why significant earthquakes are not commonly induced by production in proximity to major faults.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/82873, title ="Rayleigh-Wave H/V via Noise Cross Correlation in Southern California", author = "Muir, Jack B. and Tsai, Victor C.", journal = "Bulletin of the Seismological Society of America", volume = "107", number = "5", pages = "2021-2027", month = "October", year = "2017", doi = "10.1785/0120170051", issn = "0037-1106", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171102-082548374", note = "© 2017 Seismological Society of America. \n\nManuscript received 13 February 2017; Published Online 25 September 2017. \n\nThe authors would like to acknowledge Daniel Bowden for his thoughts and advice during initial code development. Jack Muir would also like to acknowledge the General Sir John Monash Foundation and the Origin Energy Foundation for their financial support. We would also like to thank the three anonymous reviewers and Associate Editor Eric Chael for their input to the article.", revision_no = "23", abstract = "We study the crustal structure of southern California by inverting horizontal‐to‐vertical (H/V) amplitudes of Rayleigh waves observed in noise cross‐correlation signals. This study constitutes a useful addition to traditional phase‐velocity‐based tomographic inversions due to the localized sensitivity of H/V measurements to the near surface of the measurement station site. The continuous data of 222 permanent broadband stations of the Southern California Seismic Network (SCSN) were used in production of noise cross‐correlation waveforms, resulting in a spatially dense set of measurements for the southern California region in the 1–15 s period band. The fine interstation spacing of the SCSN allows retrieval of high signal‐to‐noise ratio Rayleigh waves at periods as low as 1 s, significantly improving the vertical resolution of the resulting tomographic image, compared to previous studies with minimum periods of 5–10 s. In addition, horizontal resolution is naturally improved by increased station density. Tectonic subregions including the Los Angeles basin and Salton trough are clearly visible due to their high short‐period H/V ratios, whereas the Transverse and Peninsular Ranges exhibit low H/V at all periods.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/79266, title ="Explaining Extreme Ground Motion in Osaka Basin during the 2011 Tohoku Earthquake", author = "Tsai, Victor C. and Bowden, Daniel C.", journal = "Geophysical Research Letters", volume = "44", number = "14", pages = "7239-7244", month = "July", year = "2017", doi = "10.1002/2017GL074120", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170721-073805550", note = "© 2017 American Geophysical Union. \n\nReceived 10 MAY 2017; Accepted 18 JUL 2017; Accepted article online 20 JUL 2017; Published online 30 JUL 2017. \n\nWe thank T. Furumura and two anonymous reviewers for comments that helped improve the manuscript. The KiK-net data used in this study were obtained from the National Research Institute for Earth Science and Disaster Prevention (NIED) data centers, http://www.kyoshin.bosai.go.jp/kyoshin/. This work was partially supported by NSF EAR-1453263.", revision_no = "24", abstract = "Despite being 770 km away from the epicenter, observed ground motions due to the Tohoku earthquake in the Osaka Basin were unexpectedly large, with an amplification of more than a factor of 20 compared to immediately outside the basin, and including 2.7 m peak-to-peak roof displacements at one high-rise building. The local ground motions exceeded expectations based on standard computations of site response by a factor of 3, predicted frequencies of peak acceleration were off by at least 50%, and such discrepancies have not yet been explained quantitatively. Here we show that utilizing semianalytic theory for surface-wave amplification, we are able to accurately predict both the amplitudes and frequencies of large ground amplification in the Osaka Basin using only knowledge of the local one-dimensional structure. Comparison between this simple prediction and observed amplification was not expected to be so favorable and suggests that simple one-dimensional surface-wave site amplification factors can be useful in the absence of full three-dimensional wave propagation simulations. Such surface-wave amplification factors can be included in addition to the standard measures of site-specific site amplification and should help explain strong ground motion variability in future large earthquakes that shake Osaka Basin and elsewhere in the world.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/81262, title ="Perturbational and nonperturbational inversion of Rayleigh-wave velocities", author = "Haney, Matthew M. and Tsai, Victor C.", journal = "Geophysics", volume = "82", number = "3", pages = "F15-F28", month = "May", year = "2017", doi = "10.1190/GEO2016-0397.1", issn = "0016-8033", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170908-092450206", note = "© 2017 Society of Exploration Geophysicists. \n\nManuscript received by the Editor 27 July 2016; published online 03 April 2017. \n\nPeer-reviewed code related to this article can be found at http://software.seg.org/2017/0003. \n\nComments by P. Dawson (USGS), E. Muyzert, and an anonymous reviewer have helped to improve this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.", revision_no = "8", abstract = "The inversion of Rayleigh-wave dispersion curves is a classic geophysical inverse problem. We have developed a set of MATLAB codes that performs forward modeling and inversion of Rayleigh-wave phase or group velocity measurements. We describe two different methods of inversion: a perturbational method based on finite elements and a nonperturbational method based on the recently developed Dix-type relation for Rayleigh waves. In practice, the nonperturbational method can be used to provide a good starting model that can be iteratively improved with the perturbational method. Although the perturbational method is well-known, we solve the forward problem using an eigenvalue/eigenvector solver instead of the conventional approach of root finding. Features of the codes include the ability to handle any mix of phase or group velocity measurements, combinations of modes of any order, the presence of a surface water layer, computation of partial derivatives due to changes in material properties and layer boundaries, and the implementation of an automatic grid of layers that is optimally suited for the depth sensitivity of Rayleigh waves.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/73654, title ="Seismic array constraints on reach-scale bedload transport", author = "Schmandt, B. and Gaeuman, D.", journal = "Geology", volume = "45", number = "4", pages = "299-302", month = "April", year = "2017", doi = "10.1130/G38639.1", issn = "0091-7613", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170124-083754227", note = "© 2017 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. \n\nManuscript received 26 September 2016; Revised manuscript received 27 November 2016; Manuscript accepted 4 December 2016. \n\nSeismic data will be openly available through the Incorporated Research Institutions for Seismology Data Management Center (http://ds.iris.edu/ds/nodes/dmc/) beginning in May 2017. Trinity River Restoration Program water and sediment transport data are openly available (http://www.trrp.net). J. Turowski and T. Batholomaus provided helpful reviews. Support from National Science Foundation grant 1554908 (to Schmandt) and grant 1453263 (to Tsai) is acknowledged.", revision_no = "16", abstract = "Measurements and mechanical models of heterogeneous bedload transport in rivers remain basic challenges for studies of landscape evolution and watershed management. A 700 m reach of the Trinity River (northern California, USA), a large gravel-bed river, was instrumented with an array of 76 seismographs during a dam-controlled flood and gravel augmentation to investigate the potential for out-of-stream monitoring. The temporal response to gravel augmentation during constant discharge provides strong evidence of seismic sensitivity to bedload transport and aids in identification of the seismic frequencies most sensitive to bedload in the study area. Following gravel augmentations, the seismic array reveals a period of enhanced transport that spans most or all of the reach for ∼7–10 h. Neither the duration nor the downstream extent of enhanced transport would have been constrained without the seismic array. Sensitivity to along-stream transport variations is further demonstrated by seismic amplitudes that decrease between the upper and lower halves of the reach consistent with decreased bedload flux constrained by time-lapse bathymetry. Insight into the magnitude of impact energy that reaches the bed is also gained from the seismic array. Observed peak seismic power is ∼1%–5% of that predicted by a model of saltation over exposed bedrock. Our results suggest that dissipation of impact energy due to cover effects needs to be considered to seismically constrain bedload transport rates, and that noninvasive constraints from seismology can be used to test and refine mechanical models of bedload transport.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/83985, title ="Tidal modulation of ice shelf buttressing stresses", author = "Robel, Alexander A. and Tsai, Victor C.", journal = "Annals of Glaciology", volume = "58", number = "74", pages = "12-20", month = "April", year = "2017", doi = "10.1017/aog.2017.22", issn = "0260-3055", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171220-130521553", note = "© The Author(s) 2017. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. \n\nPublished online: 04 September 2017. \n\nWe thank Doug MacAyeal, Kelly Brunt, Hilmar Gudmundsson and Olga Sergienko for helpful conversations during the completion of this work. We also thank Bob Bindschadler and Hilmar Gudmundsson for providing GPS data. Source code and documentation of the models used in this study are available as public repositories on GitHub: https://github.com/aarobel/. AR has been supported by the NOAA Climate & Global Change and Caltech Stanback postdoctoral fellowships during the completion of this work. BM was supported by a NSF Earth Sciences Postdoctoral Fellowship award EAR-1452587. This work was also supported by NSF grant EAR-1453263.", revision_no = "9", abstract = "Ocean tides influence the flow of marine-terminating glaciers. Observations indicate that the large fortnightly variations in ice flow at Rutford Ice Stream in West Antarctica originate in the floating ice shelf. We show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow. These nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf. Using a simple viscoelastic model, we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow, including the period, phase and amplitude of flow variations observed at Rutford and Bindschadler Ice Streams.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/74405, title ="Earthquake ground motion amplification for surface waves", author = "Bowden, Daniel C. and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "44", number = "1", pages = "121-127", month = "January", year = "2017", doi = "10.1002/2016GL071885", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170217-132237004", note = "© 2016 American Geophysical Union. \n\nReceived 7 NOV 2016; Accepted 13 DEC 2016; Accepted article online 15 DEC 2016; Published online 5 JAN 2017. \n\nThe velocity model used for Southern California is the SCEC CVM-S4.26, accessed from https://scec.usc.edu/scecpedia, on 25 February 2016, as part of the Unified Community Velocity Model package version 15.10.0. Seismograms are provided by the Caltech/USGS Southern California Seismic Network at http://scedc.caltech.edu. Surface-wave eigenfunctions are generated through R. Herrmann's Computer Programs in Seismology (available at http://www.eas.slu.edu/eqc/eqccps.html). Vertical transfer functions were validated against C. Mueller's NRATTLE script, provided as part of the SMSIM package by D. Boore (available at http://pubs.er.usgs.gov/publication/ofr00509). The authors thank Jian Shi, Fan-Chi Lin, Rob Clayton, and Raul Castro for their helpful discussion. We also thank Francisco Sánchez-Sesma and one anonymous reviewer for their helpful feedback in preparation of the manuscript. This work was supported by EAR-1252191, EAR-1453263, and SCEC-15035.", revision_no = "17", abstract = "Surface waves from earthquakes are known to cause strong damage, especially for larger structures such as skyscrapers and bridges. However, common practice in characterizing seismic hazard at a specific site considers the effect of near-surface geology on only vertically propagating body waves. Here we show that surface waves have a unique and different frequency-dependent response to known geologic structure and that this amplification can be analytically calculated in a manner similar to current hazard practices. Applying this framework to amplification in the Los Angeles Basin, we find that peak ground accelerations for certain large regional earthquakes are underpredicted if surface waves are not properly accounted for and that the frequency of strongest ground motion amplification can be significantly different. Including surface-wave amplification in hazards calculations is therefore essential for accurate predictions of strong ground motion for future San Andreas Fault ruptures.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/69741, title ="Evidence for non-self-similarity of microearthquakes recorded at a Taiwan borehole seismometer array", author = "Lin, Yen-Yu and Ma, Kuo-Fong", journal = "Geophysical Journal International", volume = "206", number = "2", pages = "757-773", month = "August", year = "2016", doi = "10.1093/gji/ggw172", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160818-075652559", note = "© 2016 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. \n\nAccepted 2016 April 25. Received 2016 April 22; in original form 2015 September 4. \n\nWe thank Dr Volker Oye at NORSAR, Norway, providing the Microseismic Monitoring (MIMO) for our seismic clusters location. We appreciate the helpful comments from the Associated Editor, two referees and Prof Luis Rivera, who helped us to improve this manuscript. Support from the Graduate Students Study Abroad Program 101-2917-I-008-001- of the National Science Council (NSC) contributed to this international collaboration. This research was supported by the Taiwan Earthquake Research Center (TEC) funded through the Ministry of Science and Technology (MoST) with grant number 104-2116-M-008-011. The TEC contribution number for this paper is 00119.", revision_no = "10", abstract = "We investigate the relationship between seismic moment M_0 and source duration t_w of microearthquakes by using high-quality seismic data recorded with a vertical borehole array installed in central Taiwan. We apply a waveform cross-correlation method to the three-component records and identify several event clusters with high waveform similarity, with event magnitudes ranging from 0.3 to 2.0. Three clusters—Clusters A, B and C—contain 11, 8 and 6 events with similar waveforms, respectively. To determine how M_0 scales with t_w, we remove path effects by using a path-averaged Q. The results indicate a nearly constant t_w for events within each cluster, regardless of M_0, with mean values of t_w being 0.058, 0.056 and 0.034 s for Clusters A, B and C, respectively. Constant t_w, independent of M_0, violates the commonly used scaling relation t_w ∝ M^(1/3)_0. This constant duration may arise either because all events in a cluster are hosted on the same isolated seismogenic patch, or because the events are driven by external factors of constant duration, such as fluid injections into the fault zone. It may also be related to the earthquake nucleation size.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70201, title ="Offshore Southern California lithospheric velocity structure from noise cross-correlation functions", author = "Bowden, D. C. and Kohler, M. D.", journal = "Journal of Geophysical Research: Solid Earth", volume = "121", number = "5", pages = "3415-3427", month = "May", year = "2016", doi = "10.1002/2016JB012919", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160907-162036144", note = "© 2016 American Geophysical Union.\n\nReceived 16 FEB 2016, Accepted 17 APR 2016, Accepted article online 20 APR 2016, Published online 3 MAY 2016.\n\nThe authors thank Fan-Chi Lin for assistance with the initial signal processing, tilt and DPG corrections, and dispersion measurments; Yiran Ma for assistance with the 2-D inversions; and Rob Clayton, Asaf Inbal, Joann Stock, Simon Klemperer, and Sampath Rathnayaka for helpful discussions. The final model is available by contacting the corresponding author at dbowden@caltech.edu. SCSN data were provided by the Caltech/USGS Southern California Seismic Network at http://scedc.caltech.edu/ [California Institute of Technology, Caltech, 1926]. The OBS and DPG waveform data from the ALBACORE array are available from the IRIS Data Management Center at http://www.iris.edu/hq/. The IRIS Data Management System is funded through the National Science Foundation and specifically the GEO Directorate through the Instrumentation and Facilities Program of the National Science Foundation under Cooperative Agreement EAR-1063471. The OBS deployment was made possible with instruments and logistical support of the U.S. National Ocean Bottom Seismic Instrumentation Pool (OBSIP) at Scripps Institute of Oceanography; in particular, thanks go to Jeff Babcock, Ernie Aaron, Phil Thai, and Mark Gibaud. The OBS deployment and recovery cruises were made possible with the equipment and logistical support of the University-National Oceanographic Laboratory System (UNOLS) vessel fleet and staff support at Scripps with particular thanks to Jon Meyer, Brian Rowe, and Meghan Donohue. Kohler and Weeraratne thank Captain Curl and the crew of R/V Melville for assistance during the 2010 OBS deployment cruise and Captain Vullo and the crew of R/V New Horizon during the 2011 OBS recovery cruise. This work was supported by the National Science Foundation (grant OCE-0825254) and by USGS grant G14AP00074.", revision_no = "14", abstract = "A new shear wave velocity model offshore Southern California is presented that images plate boundary deformation including both thickening and thinning of the crustal and mantle lithosphere at the westernmost edge of the North American continent. The Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE) ocean bottom seismometer array, together with 65 stations of the onshore Southern California Seismic Network, is used to measure ambient noise correlation functions and Rayleigh wave dispersion curves which are inverted for 3-D shear wave velocities. The resulting velocity model defines the transition from continental lithosphere to oceanic, illuminating the complex history and deformation in the region. A transition to the present-day strike-slip regime between the Pacific and North American Plates resulted in broad deformation and capture of the now >200\u2009km wide continental shelf. Our velocity model suggests the persistence of the uppermost mantle volcanic processes associated with East Pacific Rise spreading adjacent to the Patton Escarpment, which marks the former subduction of Farallon Plate underneath North America. The most prominent of these seismic structures is a low-velocity anomaly underlying the San Juan Seamount, suggesting ponding of magma at the base of the crust, resulting in thickening and ongoing adjustment of the lithosphere due to the localized loading. The velocity model also provides a robust framework for future earthquake location determinations and ground-shaking simulations for risk estimates.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/69059, title ="Subseasonal changes observed in subglacial channel pressure, size, and sediment transport", author = "Gimbert, Florent and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "43", number = "8", pages = "3786-3794", month = "April", year = "2016", doi = "10.1002/2016GL068337", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160715-111120593", note = "© 2016 American Geophysical Union. \n\nReceived 8 DEC 2015; Accepted 5 APR 2016; Accepted article online 7 APR 2016; Published online 21 APR 2016. \n\nThis study was funded by NSF grant EAR-1453263. We thank Flavien Beaud and an anonymous reviewer for thorough reviews that improved the manuscript. We also thank Michael Lamb, Olivier Gagliardini, Jean-Philippe Avouac, Gael Durand and Adrien Gilbert for fruitful discussions.", revision_no = "18", abstract = "Water that pressurizes the base of glaciers and ice sheets enhances glacier velocities and modulates glacial erosion. Predicting ice flow and erosion therefore requires knowledge of subglacial channel evolution, which remains observationally limited. Here we demonstrate that detailed analysis of seismic ground motion caused by subglacial water flow at Mendenhall Glacier (Alaska) allows for continuous measurement of daily to subseasonal changes in basal water pressure gradient, channel size, and sediment transport. We observe intermittent subglacial water pressure gradient changes during the melt season, at odds with common assumptions of slowly varying, low-pressure channels. These observations indicate that changes in channel size do not keep pace with changes in discharge. This behavior strongly affects glacier dynamics and subglacial channel erosion at Mendenhall Glacier, where episodic periods of high water pressure gradients enhance glacier surface velocity and channel sediment transport by up to 30% and 50%, respectively. We expect the application of this framework to future seismic observations acquired at glaciers worldwide to improve our understanding of subglacial processes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/64550, title ="High-resolution probing of inner core structure with seismic interferometry", author = "Huang, Hsin-Hua and Lin, Fan-Chi", journal = "Geophysical Research Letters", volume = "42", number = "24", pages = "10622-10630", month = "December", year = "2015", doi = "10.1002/2015GL066390", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160218-105113138", note = "© 2015 American Geophysical Union. \n\nReceived 29 SEP 2015; Accepted 5 DEC 2015; Accepted article online 9 DEC 2015; Published online 23 DEC 2015. \n\nWe thank Alex Song for helpful discussion. We also thank H. Tkalcic and an anonymous reviewer for their constructive comments. All waveform data used in this study can be downloaded from the IRIS Data Management Center. This work was supported by National Science Foundation grants EAR-1316348 and CyberSEES-1442665, the King Abdullah University of Science and Technology (KAUST) under award OCRF-2014-CRG3-2300, and the Ministry of Science and Technology, Taiwan, grant 104-2917-I-564-052.", revision_no = "15", abstract = "Increasing complexity of Earth's inner core has been revealed in recent decades as the global distribution of seismic stations has improved. The uneven distribution of earthquakes, however, still causes a biased geographical sampling of the inner core. Recent developments in seismic interferometry, which allow for the retrieval of core-sensitive body waves propagating between two receivers, can significantly improve ray path coverage of the inner core. In this study, we apply such earthquake coda interferometry to 1846 USArray stations deployed across the U.S. from 2004 through 2013. Clear inner core phases PKIKP^2 and PKIIKP^2 are observed across the entire array. Spatial analysis of the differential travel time residuals between the two phases reveals significant short-wavelength variation and implies the existence of strong structural variability in the deep Earth. A linear N-S trending anomaly across the middle of the U.S. may reflect an asymmetric quasi-hemispherical structure deep within the inner core with boundaries of 99°W and 88°E.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/64406, title ="Nonperturbational surface-wave inversion: A Dix-type relation for surface waves", author = "Haney, Matthew M. and Tsai, Victor C.", journal = "Geophysics", volume = "80", number = "6", pages = "EN167-EN177", month = "November", year = "2015", doi = "10.1190/GEO2014-0612.1", issn = "0016-8033", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160211-082242072", note = "© 2015 Society of Exploration Geophysicists. \n\nManuscript received by the Editor 28 December 2014; revised manuscript received 28 April 2015; published online 16 October 2015. \n\nThe authors wish to thank F.-C. Lin for providing access to the phase velocity maps of the western United States. They also thank assistant editor V. Socco, associate editor G. Tsoflias, P. Dawson, and three anonymous reviewers for their helpful comments.", revision_no = "10", abstract = "We extend the approach underlying the well-known Dix equation in reflection seismology to surface waves. Within the context of surface wave inversion, the Dix-type relation we derive for surface waves allows accurate depth profiles of shear-wave velocity to be constructed directly from phase velocity data, in contrast to perturbational methods. The depth profiles can subsequently be used as an initial model for nonlinear inversion. We provide examples of the Dix-type relation for under-parameterized and over-parameterized cases. In the under-parameterized case, we use the theory to estimate crustal thickness, crustal shear-wave velocity, and mantle shear-wave velocity across the Western U.S. from phase velocity maps measured at 8-, 20-, and 40-s periods. By adopting a thin-layer formalism and an over-parameterized model, we show how a regularized inversion based on the Dix-type relation yields smooth depth profiles of shear-wave velocity. In the process, we quantitatively demonstrate the depth sensitivity of surface-wave phase velocity as a function of frequency and the accuracy of the Dix-type relation. We apply the over-parameterized approach to a near-surface data set within the frequency band from 5 to 40 Hz and find overall agreement between the inverted model and the result of full nonlinear inversion.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/58797, title ="Predicting short-period, wind-wave-generated seismic noise in coastal regions", author = "Gimbert, Florent and Tsai, Victor C.", journal = "Earth and Planetary Science Letters", volume = "426", pages = "280-292", month = "September", year = "2015", doi = "10.1016/j.epsl.2015.06.017", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150707-153131318", note = "© 2015 Elsevier B.V. \n\nReceived 19 March 2015; Received in revised form 5 June 2015; Accepted 6 June 2015; Available online 7 July 2015. \n\nThis study was partially funded by Stanback Discovery Fund for Global Environmental Science and NSF grant EAR-1453263. We thank two anonymous reviewers for helpful reviews.", revision_no = "22", abstract = "Substantial effort has recently been made to predict seismic energy caused by ocean waves in the 4–10 s period range. However, little work has been devoted to predict shorter period seismic waves recorded in coastal regions. Here we present an analytical framework that relates the signature of seismic noise recorded at 0.6–2 s periods (0.5–1.5 Hz frequencies) in coastal regions with deep-ocean wave properties. Constraints on key model parameters such as seismic attenuation and ocean wave directionality are provided by jointly analyzing ocean-floor acoustic noise and seismic noise measurements. We show that 0.6–2 s seismic noise can be consistently predicted over the entire year. The seismic noise recorded in this period range is mostly caused by local wind-waves, i.e. by wind-waves occurring within about 2000 km of the seismic station. Our analysis also shows that the fraction of ocean waves traveling in nearly opposite directions is orders of magnitude smaller than previously suggested for wind-waves, does not depend strongly on wind speed as previously proposed, and instead may depend weakly on the heterogeneity of the wind field. This study suggests that wind-wave conditions can be studied in detail from seismic observations, including under specific conditions such as in the presence of sea ice.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/58593, title ="Time Scale for Rapid Draining of a Surficial Lake Into the Greenland Ice Sheet", author = "Rice, James R. and Tsai, Victor C.", journal = "Journal of Applied Mechanics", volume = "82", number = "7", pages = "Art. No. 071001", month = "July", year = "2015", doi = "10.1115/1.4030325", issn = "0021-8936", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150625-080038328", note = "© 2015 ASME. \n\nContributed by the Applied Mechanics Division of ASME for publication in the Journal of Applied Mechanics. Manuscript received February 2, 2015; final manuscript received March 11, 2015; published online June 3, 2015. \n\nThis research was supported by the Harvard University School of Engineering and Applied Sciences Blue Hills Hydrology Endowment (MCF), the National Science Foundation, Office of Polar Programs, Award Nos. ANT-0739444 (June 2008 to May 2012) and 13,41499 (March 2014 to February 2017) to Harvard University.", revision_no = "9", abstract = "A 2008 report by Das et al. documented the rapid drainage during summer 2006 of a supraglacial lake, of approximately 44×10^6 m^3, into the Greenland ice sheet over a time scale moderately longer than 1\u2009hr. The lake had been instrumented to record the time-dependent fall of water level and the uplift of the ice nearby. Liquid water, denser than ice, was presumed to have descended through the sheet along a crevasse system and spread along the bed as a hydraulic facture. The event led two of the present authors to initiate modeling studies on such natural hydraulic fractures. Building on results of those studies, we attempt to better explain the time evolution of such a drainage event. We find that the estimated time has a strong dependence on how much a pre-existing crack/crevasse system, acting as a feeder channel to the bed, has opened by slow creep prior to the time at which a basal hydraulic fracture nucleates. We quantify the process and identify appropriate parameter ranges, particularly of the average temperature of the ice beneath the lake (important for the slow creep opening of the crevasse). We show that average ice temperatures 5–7\u2009\u2009°C below melting allow such rapid drainage on a time scale which agrees well with the 2006 observations.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/59260, title ="Modeling of subglacial hydrological development following rapid supraglacial lake drainage", author = "Dow, C. F. and Kulessa, B.", journal = "Journal of Geophysical Research. Earth Surface", volume = "120", number = "6", pages = "1127-1147", month = "June", year = "2015", doi = "10.1002/2014JF003333", issn = "2169-9003", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150806-081044372", note = "© 2015 American Geophysical Union. \n\nReceived 8 SEP 2014; Accepted 8 MAY 2015; Accepted article online 14 MAY 2015; Published online 30 JUN 2015. \n\nFor further information on the modeling methodology see the supporting information and/or contact C.F. Dow. This project was funded with NERC grant NE/G007195/1 and the Greenland Analogue Project. C.F.D. was funded by a NERC doctoral scholarship and a NASA Postdoctoral Program fellowship at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities. The authors would like to thank Mauro Werder and Ian Hewitt for helpful discussions about the modeling approach. Three anonymous reviewers and the Associate Editor are thanked for their helpful suggestions that have improved this manuscript.", revision_no = "16", abstract = "The rapid drainage of supraglacial lakes injects substantial volumes of water to the bed of the Greenland ice sheet over short timescales. The effect of these water pulses on the development of basal hydrological systems is largely unknown. To address this, we develop a lake drainage model incorporating both (1) a subglacial radial flux element driven by elastic hydraulic jacking and (2) downstream drainage through a linked channelized and distributed system. Here we present the model and examine whether substantial, efficient subglacial channels can form during or following lake drainage events and their effect on the water pressure in the surrounding distributed system. We force the model with field data from a lake drainage site, 70 km from the terminus of Russell Glacier in West Greenland. The model outputs suggest that efficient subglacial channels do not readily form in the vicinity of the lake during rapid drainage and instead water is evacuated primarily by a transient turbulent sheet and the distributed system. Following lake drainage, channels grow but are not large enough to reduce the water pressure in the surrounding distributed system, unless preexisting channels are present throughout the domain. Our results have implications for the analysis of subglacial hydrological systems in regions where rapid lake drainage provides the primary mechanism for surface-to-bed connections.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/57198, title ="The Yellowstone magmatic system from the mantle plume to the upper crust", author = "Huang, Hsin-Hua and Lin, Fan-Chi", journal = "Science", volume = "348", number = "6236", pages = "773-776", month = "May", year = "2015", doi = "10.1126/science.aaa5648 ", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150504-121706460", note = "© 2015 American Association for the Advancement of Science. \n\n20 January 2015; accepted 1 April 2015. Published online 23 April 2015. \n \nData were collected from the Yellowstone seismograph network operated by the University of Utah, the NSF funded EarthScope project, and a temporary seismic array operated by Stanford University. All waveform data used in this project can be obtained through the IRIS Data Management Center. This study was supported by the University of Utah, the University of Utah Seismograph Stations, the U.S. National Science Foundation in support of the EarthScope Transportable Array, Cyber-SEES-1442665, EAR-1252191, and the Brinson Foundation and Carrico funds.", revision_no = "26", abstract = "The Yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world. An understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk. Using a joint local and teleseismic earthquake P-wave seismic inversion, we unveil a basaltic lower-crustal magma body that provides a magmatic link between the Yellowstone mantle plume and the previously imaged upper-crustal magma reservoir. This lower-crustal magma body has a volume of 46,000 km^3, ~4.5 times larger than the upper-crustal magma reservoir, and contains a melt fraction of ~2%. These estimates are critical to understanding the evolution of bimodal basaltic-rhyolitic volcanism, explaining the magnitude of CO_2 discharge, and constraining dynamic models of the magmatic system for volcanic hazard assessment.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/56907, title ="Site amplification, attenuation, and scattering from noise correlation amplitudes across a dense array in Long Beach, CA", author = "Bowden, D. C. and Tsai, V. C.", journal = "Geophysical Research Letters", volume = "42", number = "5", pages = "1360-1367", month = "March", year = "2015", doi = "10.1002/2014GL062662", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150423-094707871", note = "© 2015 American Geophysical Union. \n\nReceived 2 December 2014; accepted 27 January 2015; accepted article online 2 February 2015; published online 6 March 2015. \n\nThe authors gratefully acknowledge Dan Hollis at NodalSeismic LLC, and Signal Hill Petroleum, Inc., for permitting us to use the Long Beach data. We thank Dunzhu Li for scripts and advice in handling the large quantity of cross correlations, and Rob Clayton and Asaf Inbal for helpful discussion. We are also thankful for the constructive and helpful comments from Jesse Lawrence and an anonymous reviewer. This project is supported by NSF grants EAR-1252191, EAR-1214912, and CyberSEES-1442665. Fan-Chi Lin also acknowledges the financial support from Signal Hill Petroleum for this research. \n\nThe Editor thanks two anonymous reviewers for their assistance in evaluating this paper.", revision_no = "12", abstract = "For accurate seismic hazard evaluation, both the spatial and frequency-dependent variabilities in the amplitudes of earthquake ground motions are needed. While this information is rarely fully available due to the paucity of relevant seismic data, dense arrays like the 5200-geophone array in Long Beach, California provide the opportunity to study this amplitude variability. Here we show that ambient noise correlation amplitudes from the Long Beach array can be used to directly determine frequency-dependent site amplification factors. We analyze Rayleigh-wavefield amplitude gradients from ambient noise correlations that are processed so that relative amplitudes satisfy the wave equation and are therefore meaningful. Ultimately, we construct maps of site amplification across Long Beach at frequencies of 0.67, 1.0, and 2.0\u2009Hz. These maps correlate well with local structure, notably the Newport-Inglewood Fault and also to known velocity structure. Through this process, we also obtain constraints on average attenuation structure and local scattering.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/55660, title ="Seismologically determined bedload flux during the typhoon season", author = "Chao, Wei-An and Wu, Yih-Min", journal = "Scientific Reports", volume = "5", pages = "Art. No. 8261", month = "February", year = "2015", doi = "10.1038/srep08261 ", issn = "2045-2322", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150310-084646630", note = "© 2015 Nature Publishing Group. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder in order to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. \n\nReceived 14 September 2014; Accepted 13 January 2015; Published 5 February 2015. \n\nThis research has been supported by the National Science Council of the Republic of China (NSC 99-2627-M-002-015). The authors acknowledge the Central Geological Survey (CGS) and Water Resource Agency (WRA), Taiwan, for providing the fluvial data, and the Central Weather Bureau (CWB) and Academia Sinica, Taiwan, for providing the broadband seismic data. The meteorological data was provided by the CWB. The software package GMT (Generic Mapping Tools, http://gmt.soest.hawaii.edu/) was used in making some the figures in this paper. \n\nAuthor contributions: W.A.C. performed the river seismic noise analysis and the inversion of bedload flux. Y.M.W. and L.Z. helped to co-ordinate the deployment of seismic array. W.A.C. and C.H.C. deployed and maintained the seismic array. V.C.T. assisted in implementing the seismic impact forward model. All of the authors contributed to the data acquisition and interpretation, and the writing of this paper.", revision_no = "17", abstract = "Continuous seismic records near river channels can be used to quantify the energy induced by river sediment transport. During the 2011 typhoon season, we deployed a seismic array along the Chishan River in the mountain area of southern Taiwan, where there is strong variability in water discharge and high sedimentation rates. We observe hysteresis in the high-frequency (5–15\u2005Hz) seismic noise level relative to the associated hydrological parameters. In addition, our seismic noise analysis reveals an asymmetry and a high coherence in noise cross-correlation functions for several station pairs during the typhoon passage, which corresponds to sediment particles and turbulent flows impacting along the riverbed where the river bends sharply. Based on spectral characteristics of the seismic records, we also detected 20 landslide/debris flow events, which we use to estimate the sediment supply. Comparison of sediment flux between seismologically determined bedload and derived suspended load indicates temporal changes in the sediment flux ratio, which imply a complex transition process from the bedload regime to the suspension regime between typhoon passage and off-typhoon periods. Our study demonstrates the possibility of seismologically monitoring river bedload transport, thus providing valuable additional information for studying fluvial bedrock erosion and mountain landscape evolution.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/55723, title ="An improved model for tidally modulated grounding-line migration", author = "Tsai, Victor C. and Gudmundsson, G. Hilmar", journal = "Journal of Glaciology", volume = "61", number = "226", pages = "216-222", month = "January", year = "2015", doi = "10.3189/2015JoG14J152", issn = "0022-1430", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150312-074943389", note = "© 2015 IGS.\n\nMS received 12 August 2014 and accepted in revised form 6 December 2014.\n\nWe thank Roiy Sayag and two anonymous reviewers for constructive reviews, and the California Institute of Technology for supporting G.H.G.’s visit. This research was carried out at the California Institute of Technology and the Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration and partly funded through the President’s and Director’s Fund Program.", revision_no = "13", abstract = "Understanding grounding-line dynamics is necessary for predictions of long-term ice-sheet stability. However, despite growing observations of the tidal influence on grounding-line migration, this short-timescale migration is poorly understood, with most modeling attempts assuming beam theory to calculate displacements. Here we present an improved model of tidal grounding-line migration that treats migration as an elastic fracture problem, forced by the additional ocean water pressure from the tide. This new model predicts that the grounding line cannot be assumed to be in hydrostatic equilibrium and, furthermore, that migration is inherently asymmetric and nonlinear, with migration distances that are not proportional to the tidal load. Specifically, for constant surface slope, the grounding line migrates upstream approximately ten times further as the tide rises from mean sea level to high tide than it migrates downstream as the tide falls from mean sea level to low tide, and migration distances are substantially larger than simple flotation arguments suggest. Numerical tests also show that the dependence of migration distance on elastic moduli and ice-sheet thickness are inconsistent with predictions of beam theory for a range of realistic values. Finally, applying the new model to observations in Antarctica results in new estimates of bed slopes, though these estimates remain uncertain due to imperfect knowledge of the relevant rheological parameters.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/55701, title ="Marine ice-sheet profiles and stability under Coulomb basal conditions", author = "Tsai, Victor C. and Stewart, Andrew L.", journal = "Journal of Glaciology", volume = "61", number = "226", pages = "205-215", month = "January", year = "2015", doi = "10.3189/2015JoG14J221", issn = "0022-1430", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150311-105436730", note = "© 2015 IGS.\n\nMS received 17 November 2014 and accepted in revised form 14 February 2015. \n\nWe thank G.H. Gudmundsson for useful discussions, and two anonymous reviewers for helpful suggestions. This research was carried out at the Jet Propulsion Laboratory and the California Institute of Technology under a contract with the National Aeronautics and Space Administration and funded through the President’s and Director’s Fund Program. Partial support was also provided by the Stanback Discovery Fund for Global Environmental Science.", revision_no = "20", abstract = "The behavior of marine-terminating ice sheets, such as the West Antarctic ice sheet, is of\ninterest due to the possibility of rapid grounding-line retreat and consequent catastrophic loss of ice.\nCritical to modeling this behavior is a choice of basal rheology, where the most popular approach is to\nrelate the ice-sheet velocity to a power-law function of basal stress. Recent experiments, however,\nsuggest that near-grounding line tills exhibit Coulomb friction behavior. Here we address how Coulomb\nconditions modify ice-sheet profiles and stability criteria. The basal rheology necessarily transitions to\nCoulomb friction near the grounding line, due to low effective stresses, leading to changes in ice-sheet\nproperties within a narrow boundary layer. Ice-sheet profiles ‘taper off’ towards a flatter upper surface,\ncompared with the power-law case, and basal stresses vanish at the grounding line, consistent with\nobservations. In the Coulomb case, the grounding-line ice flux also depends more strongly on flotation\nice thickness, which implies that ice sheets are more sensitive to climate perturbations. Furthermore,\nwith Coulomb friction, the ice sheet grounds stably in shallower water than with a power-law rheology.\nThis implies that smaller perturbations are required to push the grounding line into regions of negative\nbed slope, where it would become unstable. These results have important implications for ice-sheet\nstability in a warming climate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46343, title ="Modeling the elastic transmission of tidal stresses to great distances inland in channelized ice streams", author = "Thompson, J. and Simons, M.", journal = "Cryosphere", volume = "8", number = "6", pages = "2007-2029", month = "November", year = "2014", doi = "10.5194/tc-8-2007-2014 ", issn = "1994-0416", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140618-141557404", note = "© 2014 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. \n\nReceived: 21 February 2014 – Published in The Cryosphere Discuss.: 25 April 2014; Revised: 18 September 2014 – Accepted: 22 September 2014 – Published: 5 November 2014. \n\nThe authors would like to thank the two anonymous reviewers of this manuscript and editor Oliver Gagliardini for their constructive comments. We would also like to thank J. N. Bassis and Martin Lüthi for their constructive review of an earlier version of this manuscript. Part of this research was carried out at the Jet Propulsion Laboratory and the California Institute of Technology under a contract with the National Aeronautics and Space Administration and funded through the President’s and Director’s Fund Program.", revision_no = "29", abstract = "Geodetic surveys suggest that ocean tides can modulate the motion of Antarctic ice streams, even at stations many tens of kilometers inland from the grounding line. These surveys suggest that ocean tidal stresses can perturb ice stream motion at distances about an order of magnitude farther inland than tidal flexure of the ice stream alone. Recent models exploring the role of tidal perturbations in basal shear stress are primarily one- or two-dimensional, with the impact of the ice stream margins either ignored or parameterized. Here, we use two- and three-dimensional finite-element modeling to investigate transmission of tidal stresses in ice streams and the impact of considering more realistic, three-dimensional ice stream geometries. Using Rutford Ice Stream as a real-world comparison, we demonstrate that the assumption that elastic tidal stresses in ice streams propagate large distances inland fails for channelized glaciers due to an intrinsic, exponential decay in the stress caused by resistance at the ice stream margins. This behavior is independent of basal conditions beneath the ice stream and cannot be fit to observations using either elastic or nonlinear viscoelastic rheologies without nearly complete decoupling of the ice stream from its lateral margins. Our results suggest that a mechanism external to the ice stream is necessary to explain the tidal modulation of stresses far upstream of the grounding line for narrow ice streams. We propose a hydrologic model based on time-dependent variability in till strength to explain transmission of tidal stresses inland of the grounding line. This conceptual model can reproduce observations from Rutford Ice Stream.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/53971, title ="Cooling magma model for deep volcanic long-period earthquakes", author = "Aso, Naofumi and Tsai, Victor C.", journal = "Journal of Geophysical Research. Solid Earth", volume = "119", number = "11", pages = "8442-8456", month = "November", year = "2014", doi = "10.1002/2014JB011180 ", issn = "2169-9313", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150122-080316714", note = "© 2014 American Geophysical Union. \n\nReceived 8 APR 2014; Accepted 9 OCT 2014; Accepted article online 16 OCT 2014; Published online 28 NOV 2014. \n\nData related to numerical calculations can be found in the appendices. This work was supported by JSPS (12 J09135). Satoshi Ide and Atsuko Namiki are thanked for discussions throughout this work. Comments from Haruhisa Nakamichi, Mike West, and the Associate Editor helped to improve the manuscript.", revision_no = "13", abstract = "Deep long-period events (DLP events) or deep low-frequency earthquakes (deep LFEs) are deep earthquakes that radiate low-frequency seismic waves. While tectonic deep LFEs on plate boundaries are thought to be slip events, there have only been a limited number of studies on the physical mechanism of volcanic DLP events around the Moho (crust-mantle boundary) beneath volcanoes. One reasonable mechanism capable of producing their initial fractures is the effect of thermal stresses. Since ascending magma diapirs tend to stagnate near the Moho, where the vertical gradient of density is high, we suggest that cooling magma may play an important role in volcanic DLP event occurrence. Assuming an initial thermal perturbation of 400°C within a tabular magma of half width 41\u2009m or a cylindrical magma of 74\u2009m radius, thermal strain rates within the intruded magma are higher than tectonic strain rates of ~\u200910^(−14)\u2009s^(−1) and produce a total strain of 2\u2009×\u200910^(−4). Shear brittle fractures generated by the thermal strains can produce a compensated linear vector dipole mechanism as observed and potentially also explain the harmonic seismic waveforms from an excited resonance. In our model, we predict correlation between the particular shape of the cluster and the orientation of focal mechanisms, which is partly supported by observations of Aso and Ide (2014). To assess the generality of our cooling magma model as a cause for volcanic DLP events, additional work on relocations and focal mechanisms is essential and would be important to understanding the physical processes causing volcanic DLP events.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/53200, title ="A physical model for seismic noise generation by turbulent flow in rivers", author = "Gimbert, Florent and Tsai, Victor C.", journal = "Journal of Geophysical Research. Earth Surface", volume = "119", number = "10", pages = "2209-2238", month = "October", year = "2014", doi = "10.1002/2014JF003201", issn = "2169-9003", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150106-082010292", note = "© 2014 American Geophysical Union. \n\nReceived 5 MAY 2014; Accepted 15 SEP 2014; Accepted article online 18 SEP 2014; Published online 16 OCT 2014. \n\nWe thank B. Schmandt for providing data and J.G. Venditti as well as J.-P. Avouac for interesting discussions. We thank two anonymous reviewers and D. Roth for helpful reviews. M.P.L acknowledges funding from the Terrestrial Hazards Observations and Reports Programs at Caltech and NSF grant EAR-1349115. F.G. was partially supported by funding from the Tectonics Observatory.", revision_no = "13", abstract = "Previous studies suggest that the seismic noise induced by rivers may be used to infer river transport properties, and previous theoretical work showed that bedload sediment flux can be inverted from seismic data. However, the lack of a theoretical framework relating water flow to seismic noise prevents these studies from providing accurate bedload fluxes and quantitative information on flow processes. Here we propose a forward model of seismic noise caused by turbulent flow. In agreement with previous observations, modeled turbulent flow-induced noise operates at lower frequencies than bedload-induced noise. Moreover, the differences in the spectral signatures of turbulent flow-induced and bedload-induced forces at the riverbed are significant enough that these two processes can be characterized independently using seismic records acquired at various distances from the river. In cases with isolated turbulent flow noise, we suggest that riverbed stress can be inverted. Finally, we validate our model by comparing predictions to previously reported observations. We show that our model captures the spectral peak located around 6–7\u2009Hz and previously attributed to water flow at Hance Rapids in the Colorado River (United States); we also show that turbulent flow causes a significant part of the seismic noise recorded at the Trisuli River in Nepal, which reveals that the hysteresis curve previously reported there does not solely include bedload, but is also largely influenced by turbulent flow-induced noise. We expect the framework presented here to be useful to invert realistic bedload fluxes by enabling the removal of the turbulent flow contribution from seismic data.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52230, title ="Green’s Functions for Surface Waves in a Generic Velocity Structure", author = "Tsai, Victor C. and Atiganyanun, Sarun", journal = "Bulletin of the Seismological Society of America", volume = "104", number = "5", pages = "2573-2578", month = "October", year = "2014", doi = "10.1785/0120140121", issn = "0037-1106", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141201-131138693", note = "© 2014 Seismological Society of America. \n\nManuscript received 1 May 2014; Published Online 9 September 2014. \n\nThe authors thank M. Haney and an anonymous reviewer for constructive comments, and the Summer Undergraduate Research Fellowship Program at Caltech and J. Edward Richter for funding S.A.", revision_no = "11", abstract = "Methodologies for calculating surface‐wave velocities and the associated displacement/stress eigenfunctions and Green’s functions have been well established for many decades. However, to our knowledge, no one has ever documented a quantitative evaluation of these properties for commonly used empirical scalings. For example, it is currently not possible to take a given power‐law dependence of shear‐wave velocity on depth and look up the corresponding dependence of phase velocity on frequency, or Green’s function surface displacement. We address this gap in the literature and here provide explicit quantitatively accurate expressions for phase velocities and Green’s function amplitudes for a few commonly used empirical formulas for near‐surface velocity structure. These exact expressions are found to be immediately useful in applications that use shallow phase velocities and also in applications that interpret seismic amplitudes or amplitude ratios from near‐surface processes such as fluvial transport, icequakes, landslides, and volcanic tremor.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46271, title ="3-D crustal structure of the western United States: application of Rayleigh-wave ellipticity extracted from noise cross-correlations", author = "Lin, Fan-Chi and Tsai, Victor C.", journal = "Geophysical Journal International", volume = "198", number = "2", pages = "656-670", month = "August", year = "2014", doi = "10.1093/gji/ggu160", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140613-160117253", note = "© 2014 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. \n\nFirst published online: May 31, 2014. \n\nThe data used in this research were obtained from the IRIS Data\nManagement Center and originate predominantly from the Transportable Array component of USArray. The authors are grateful to the editor Yehuda Ben-Zion, Vedran Lekic and an anonymous reviewer for comments that helped improve this paper. This research\nwas supported by a grant from the U.S. National Science Foundation,\ngrant EAR-1252191.", revision_no = "20", abstract = "We present a new 3-D seismic model of the western United States crust derived from a joint\ninversion of Rayleigh-wave phase velocity and ellipticity measurements using periods from\n8 to 100 s. Improved constraints on upper-crustal structure result from use of short-period\nRayleigh-wave ellipticity, or Rayleigh-wave H/V (horizontal to vertical) amplitude ratios,\nmeasurements determined using multicomponent ambient noise cross-correlations. To retain\nthe amplitude ratio information between vertical and horizontal components, for each station,\nwe perform daily noise pre-processing (temporal normalization and spectrum whitening) simultaneously\nfor all three components. For each station pair, amplitude measurements between\ncross-correlations of different components (radial–radial, radial–vertical, vertical–radial and\nvertical–vertical) are then used to determine the Rayleigh-wave H/V ratios at the two station\nlocations. We use all EarthScope/USArray Tranportable Array data available between 2007\nJanuary and 2011 June to determine the Rayleigh-wave H/V ratios and their uncertainties at all\nstation locations and construct new Rayleigh-wave H/V ratio maps in the western United States\nbetween periods of 8 and 24 s. Combined with previous longer period earthquake Rayleigh-wave\nH/V ratio measurements and Rayleigh-wave phase velocity measurements from both\nambient noise and earthquakes, we invert for a new 3-D crustal and upper-mantle model in the\nwestern United States. Correlation between the inverted model and known geological features\nat all depths suggests good resolution in five crustal layers. Use of short-period Rayleigh-wave\nH/V ratio measurements based on noise cross-correlation enables resolution of distinct near\nsurface features such as the Columbia River Basalt flows, which overlie a thick sedimentary\nbasin.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44634, title ="Ambient noise correlation on the Amery Ice Shelf, East Antarctica", author = "Zhan, Zhongwen and Tsai, Victor C.", journal = "Geophysical Journal International", volume = "196", number = "3", pages = "1796-1802", month = "March", year = "2014", doi = "10.1093/gji/ggt488", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140403-102823691", note = "© 2013 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. \n\nAccepted 2013 November 28. Received 2013 November 27; in original form 2013 August 16. First published online: December 20, 2013. \n\nWe thank Fan-Chi Lin, Robert Clayton and Mark Simons for helpful discussions. We thank the editor Michael Ritzwoller and two anonymous reviewers for their comments that improved the manuscript. The seismic data is from the Incorporated Research Institutions for Seismology (IRIS) and David Heeszel and Helen Fricker helped with the seismic data. This work is supported in part by the Keck Institute for Space Studies (JMJ).", revision_no = "17", abstract = "The structure of ice shelves is important for modelling the dynamics of ice flux from the continents to the oceans. While other, more traditional techniques provide many constraints, passive imaging with seismic noise is a complementary tool for studying and monitoring ice shelves. As a proof of concept, here we study noise cross-correlations and autocorrelations on the Amery Ice Shelf, East Antarctica. We find that the noise field on the ice shelf is dominated by energy trapped in a low-velocity waveguide caused by the water layer below the ice. Within this interpretation, we explain spectral ratios of the noise cross-correlations as P-wave resonances in the water layer, and obtain an independent estimate of the water-column thickness, consistent with other measurements. For stations with noise dominated by elastic waves, noise autocorrelations also provide similar results. High-frequency noise correlations also require a 50-m firn layer near the surface with P-wave velocity as low as 1 km s^(−1). Our study may also provide insight for future planetary missions that involve seismic exploration of icy satellites such as Titan and Europa. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44293, title ="Rupture complexity of the 1994 Bolivia and 2013 Sea of Okhotsk deep earthquakes", author = "Zhan, Zhongwen and Kanamori, Hiroo", journal = "Earth and Planetary Science Letters", volume = "385", pages = "89-96", month = "January", year = "2014", doi = "10.1016/j.epsl.2013.10.028", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140313-091602304", note = "© 2013 Elsevier B. V. Received 14 October 2013. Accepted 19 October 2013. Available online 8 November 2013. Editor: Y. Ricard. We thank Z. Duputel, Q. Zhang and Y. Huang for discussion. The Incorporated Research Institutions for Seismology (IRIS) provided the seismic data. This work is supported by the Tectonic Observatory at California Institute of Technology through GPS.TO2-4.2-Grant.Moore TO2 and is a contribution (#10095) to the Seismological Laboratory, California Institute of Technology.", revision_no = "17", abstract = "The physical mechanism of deep earthquakes (depth >300 km) remains enigmatic, partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations. The two largest deep earthquakes, the recent Great 2013 Sea of Okhotsk earthquake (M\u20098.3, depth 607 km) and the Great 1994 Bolivia earthquake (M\u20098.3, depth 637 km), together provide a unique opportunity to compare their rupture patterns in detail. Here we extend a travel-time sub-event location method to perform full teleseismic P-waveform inversion. This new method allows us to explain the observed broadband records with a set of sub-events whose model parameters are robustly constrained without smoothing. We find that while the Okhotsk event is mostly unilateral, rupturing 90 km along strike with a velocity over 4 km/s, the Bolivia earthquake ruptured about half this distance at a slow velocity (about 1.5 km/s) and displayed a major change in rupture direction. We explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states. Phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42332, title ="Multiple fluvial processes detected by riverside seismic and infrasound monitoring of a controlled flood in the Grand Canyon", author = "Schmandt, Brandon and Aster, Richard C.", journal = "Geophysical Research Letters", volume = "40", number = "18", pages = "4858-4863", month = "September", year = "2013", doi = "10.1002/grl.50953", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131108-093414888", note = "© 2013 American Geophysical Union. \n\nReceived 28 July 2013; revised 6 September 2013; accepted 9 September 2013; published 18 September 2013. \n\nWe thank M. Hahn and R. Newton from the National Park Service, and J. Schmidt, C. Fritzinger, N. Voichick, and R. Tusso from the Grand Canyon Monitoring and Research Center for making it possible to use this exceptional science opportunity. The IRIS PASSCAL Instrument Center, and particularly P. Miller and G. Slad are thanked for instrument and data handling support. E. Aster assisted with fieldwork. Mike Lamb provided informal feedback. Two anonymous reviewers helped improve the manuscript. B.S. acknowledges Caltech for time to conduct fieldwork and\nsupport for this research from the University of New Mexico.\n[24] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.", revision_no = "20", abstract = "As rivers transport water and sediment across Earth's surface, they radiate elastic and acoustic waves. We use seismic and infrasound observations during a controlled flood experiment (CFE) in the Grand Canyon to show that three types of fluvial processes can be monitored from outside the channel. First, bed-load transport under conditions of evolving bed mobility is identified as the dominant seismic source between 15 and 45 Hz. Two lower-frequency seismic bands also excited by the CFE exhibited greater power increases and are consistent with source processes related to fluid rather than sediment transport. The second fluvial seismic source is inferred to be fluid tractions on the rough riverbed, which drive the maximum seismic power increase at 0.73 Hz, but do not excite infrasound. Waves at the fluid-air interface are suggested as a third source, which generates a common 6–7 Hz peak in seismic and infrasound responses to the CFE.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42343, title ="Seismic interferometry with antipodal station pairs", author = "Lin, Fan-Chi and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "40", number = "17", pages = "4609-4613", month = "September", year = "2013", doi = "10.1002/grl.50907 ", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131108-135759821", note = "© 2013 American Geophysical Union. \n\nReceived 25 June 2013; revised 19 August 2013; accepted 24 August 2013; published 11 September 2013. \n\nThe authors thank Xiaodong Song, Vernon F.\nCormier, Rob W. Clayton, Donald V. Helmberger, and Zacharie Duputel for\ncomments that helped to improve this paper. All data used in this paper were\nobtained from the IRIS-DMC. This work has been supported by the NSF grant\nEAR-1316348. \n\nThe Editor thanks Xiaodong Song and an anonymous reviewer for\ntheir assistance in evaluating this paper.", revision_no = "28", abstract = "In this study, we analyze continuous data from all Global Seismographic Network stations between year 2000 and 2009 and demonstrate that several body wave phases (e.g., PP, PcPPKP, SKSP, and PPS) propagating between nearly antipodal station pairs can be clearly observed without array stacking using the noise/coda cross-correlation method. Based on temporal correlations with global seismicity, we show that the observed body waves are clearly earthquake related. Moreover, based on single-earthquake analysis, we show that the earthquake coda energy observed between ~10,000 and 30,000\u2009s after a large earthquake contributes the majority of the signal. We refine our method based on these observations and show that the signal can be significantly improved by selecting only earthquake coda times. With our improved processing, the PKIKP phase, which does not benefit from the focusing effect near the antipode, can now also clearly be observed for long-distance station pairs.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41411, title ="Spurious velocity changes caused by temporal variations in ambient noise frequency content", author = "Zhan, Zhongwen and Tsai, Victor C.", journal = "Geophysical Journal International", volume = "194", number = "3", pages = "1574-1581", month = "September", year = "2013", doi = "10.1093/gji/ggt170", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130919-113717281", note = "© The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.\n\nAccepted 2013 April 23; Received 2013 April 22; In original form 2013 February 18.\n\nWe thank Fan-Chi Lin and Dongzhou Zhang for helpful discussions. We thank the editorMichael Ritzwoller and two anonymous reviewers for their comments that improved the manuscript. The facilities of the Southern California Earthquake Data Center (SCEDC), and the Southern California Seismic Network (SCSN), were used for access to waveforms data required in this study. This work is partially\nsupported by the NSF/EAR-0838247 and Southern California\nEarthquake Center (12064).", revision_no = "16", abstract = "Ambient seismic noise cross-correlations are now being used to detect temporal variations of seismic velocity, which are typically on the order of 0.1 per cent. At this small level, temporal variations in the properties of noise sources can cause apparent velocity changes. For example, the spatial distribution and frequency content of ambient noise have seasonal variations due to the seasonal hemispherical shift of storms. Here, we show that if the stretching method is used to measure time-shifts, then the temporal variability of noise frequency content causes apparent velocity changes due to the changes in both amplitude and phase spectra caused by waveform stretching. With realistic seasonal variations of frequency content in the Los Angeles Basin, our numerical tests produce about 0.05 per cent apparent velocity change, comparable to what Meier et al. observed in the Los Angeles Basin. We find that the apparent velocity change from waveform stretching depends on time windows and station-pair distances, and hence it is important to test a range of these parameters to diagnose the stretching bias. Better understanding of spatiotemporal noise source properties is critical for more accurate and reliable passive monitoring.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41960, title ="Using centroid time-delays to characterize source durations and identify earthquakes with unique characteristics", author = "Duputel, Zacharie and Tsai, Victor C.", journal = "Earth and Planetary Science Letters", volume = "374", pages = "92-100", month = "July", year = "2013", doi = "10.1016/j.epsl.2013.05.024 ", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131017-080058538", note = "© 2013 Elsevier B.V. \n\nReceived 19 March 2013; Received in revised form 13 May 2013; Accepted 14 May 2013; Editor: P. Shearer. Available online 13 June 2013. \n\nWe thank Zhongwen Zhan, Gavin Hayes, Göran Ekström and an anonymous reviewer for their helpful suggestions. This work uses Federation of Digital Seismic Networks (FDSN) seismic data, CMT solutions from the Global CMT catalog and event origin times from the USGS PDE catalog. The Incorporated Research Institutions for Seismology (IRIS) Data Management System (DMS) was used to access the data. This work made use of the Matplotlib python library created by John D. Hunter.", revision_no = "15", abstract = "The relationship between M_0 and the rupture duration is often difficult to establish. This is particularly true for large earthquakes for which the moment rate functions (MRF) generally have complicated shapes, and the estimated durations can vary considerably depending on the methodology used to evaluate the MRF. In this work, we show that the centroid time-delay (τ_c) provides an alternative estimate of the source duration. Inverted MRFs often end gradually, making the end of coseismic rupture difficult to detect. In such cases, when the rupture duration is not well defined, the time-delay τ_c is a useful quantity to represent the first-order temporal characteristics of the rupture process. Variations in stress parameter Δσ can be investigated by assuming a standard scaling relationship between the seismic moment M0M0 and τ_c .This simple scaling relationship can also be used to identify unusual earthquakes, with unique source properties, such as events involving complicated rupture processes or earthquakes characterized by unusual rupture velocities, stress drops or aspect ratios.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39122, title ="Extracting seismic core phases with array interferometry", author = "Lin, Fan-Chi and Tsai, Victor C.", journal = "Geophysical Research Letters", volume = "40", number = "6", pages = "1049-1053", month = "March", year = "2013", doi = "10.1002/grl.50237", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130627-095929812", note = "© 2013 American Geophysical Union. \n\nReceived 17 December 2012; revised 5 February 2013; accepted 6 February 2013; published 26 March 2013. \n\nThe authors thank Michael Ritzwoller, Xiaodong Song, and an anonymous reviewer for comments that helped to improve this paper. The facilities of the IRIS Data Management Center\nand the New Zealand GeoNet Project provided the waveform data used in this study.", revision_no = "21", abstract = "Seismic body waves that sample Earth's core are indispensable for studying the most remote regions of the planet. Traditional core phase studies rely on well-defined earthquake signals, which are spatially and temporally limited. We show that, by stacking ambient-noise cross-correlations between USArray seismometers, body wave phases reflected off the outer core (ScS), and twice refracted through the inner core (PKIKP^2) can be clearly extracted. Temporal correlation between the amplitude of these core phases and global seismicity suggests that the signals originate from distant earthquakes and emerge due to array interferometry. Similar results from a seismic array in New Zealand demonstrate that our approach is applicable in other regions and with fewer station pairs. Extraction of core phases by interferometry can significantly improve the spatial sampling of the deep Earth because the technique can be applied anywhere broadband seismic arrays exist.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/37614, title ="Locating a scatterer in the active volcanic area of Southern Peru from ambient noise cross-correlation", author = "Ma, Yiran and Clayton, Robert W.", journal = "Geophysical Journal International", volume = "192", number = "3", pages = "1332-1341", month = "March", year = "2013", doi = "10.1093/gji/ggs103 ", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130325-143244581", note = "© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. \n\nAccepted 2012 December 5. Received 2012 November 3; in original form 2012 July 17. First published online: January 10, 2013. \n\nWe thank Fan-Chi Lin at Caltech for providing many useful suggestions and thank Dunzhu Li at Caltech for providing the finite difference code. We also thank two anonymous reviewers for their helpful comments. Contribution number 212 from the Tectonics Observatory at Caltech. This work is supported by NSF (EAR-1045683) and the Gordon and Betty Moore Foundation.", revision_no = "23", abstract = "We report on a strong scatterer of seismic energy in the 5–10 s period range located in the volcanic arc of Southern Peru. It is superficially like an active noise source in that it produces a continuous signal that arrives earlier than the inter-station surface wave in the noise cross-correlations. However, it is clearly determined to be a scatterer based on the coda arrivals observed in the cross-correlations, and the fact that it scatters waves from earthquake sources. We model the scatterer as a cylinder approximately 5 km in diameter with a shear wave velocity 30\u2009per\u2009cent lower than the background velocity. It is likely to exist at the depth of 5–10 km, and is located at 71.6°W/16.1°S with an error of 10 km, which is near the inactive volcano Nevado Chachani and the active volcano El Misti which recently erupted in 1985. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38666, title ="Estimating the effect of Earth elasticity and variable water density on tsunami speeds", author = "Tsai, Victor C. and Ampuero, Jean-Paul", journal = "Geophysical Research Letters", volume = "40", number = "3", pages = "492-496", month = "February", year = "2013", doi = "10.1002/grl.50147", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130524-103956058", note = "© 2013 American Geophysical Union.\n\nReceived 27 November 2012; revised 25 December 2012; accepted 7 January 2013; published 13 February 2013.\n\nThe authors thank an anonymous reviewer for the comments.", revision_no = "15", abstract = "The speed of tsunami waves is typically calculated using the shallow-water approximation over a rigid-body Earth. Recent comparisons of tsunami arrival times from the 11 March 2011 tsunami suggest, however, that the standard formulation has errors around the 1% level, and it has been suggested that the elasticity of the Earth can explain the discrepancy. While previous work has indeed shown that such elastic deformation can modify tsunami speeds, the effect has been neglected partly due to the difficulty in understanding how large this elastic effect is. Here, we remedy this by providing a new derivation and expression for how to incorporate the first-order effect that solid Earth elasticity and ocean water compressibility have on tsunami speeds. This result is shown to agree approximately with previous theory and helps to explain observed timing discrepancies from the 11 March 2011 tsunami. The dispersive elastic correction and the non-dispersive compressibility correction together may account for the majority of the observed discrepancy.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/35997, title ="Anomalously steep dips of earthquakes in the 2011 Tohoku-Oki source region and possible explanations", author = "Zhan, Zhongwen and Helmberger, Don", journal = "Earth and Planetary Science Letters", volume = "353-35", pages = "121-133", month = "November", year = "2012", doi = "10.1016/j.epsl.2012.07.038", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121214-153612362", note = "© 2012 Elsevier B.V. \n\nReceived 19 May 2012. Received in revised form 26 July 2012. Accepted 27 July 2012. Editor: P.Shearer. Available online 7 September 2012. \n\nWe thank Seiichi Miura, Narumi Takahashi, Aki Ito and Ryota Hino for providing their velocity models or earthquake catalog. We thank Robert Graves, another anonymous USGS internal reviewer and two anonymous reviewers for their comments that improved the manuscript. The Incorporated Research Institutions for Seismology (IRIS) provided the seismic data. All figures are made with GMT. This work is supported by the National Science Foundation through grant number EAR-1142020. Contribution #10080 of the Tectonic Observatory, California Institute of Technology.", revision_no = "23", abstract = "The 2011 M_w 9.1 Tohoku-Oki earthquake had unusually large slip (over 50 m) concentrated in a relatively small region, with local stress drop inferred to be 5–10 times larger than that found for typical megathrust earthquakes. Here we conduct a detailed analysis of foreshocks and aftershocks (M_w 5.5–7.5) sampling this megathrust zone for possible clues regarding such differences in seismic excitation. We find that events occurring in the region that experienced large slip during the M_w 9.1 event had steeper dip angles (by 5–10°) than the surrounding plate interface. This discrepancy cannot be explained by a single smooth plate interface. We provide three possible explanations. In Model I, the oceanic plate undergoes two sharp breaks in slope, which were not imaged well in previous seismic surveys. These break-points may have acted as strong seismic barriers in previous seismic ruptures, but may have failed in and contributed to the complex rupture pattern of the Tohoku-Oki earthquake. In Model II, the discrepancy of dip angles is caused by a rough plate interface, which in turn may be the underlying cause for the overall strong coupling and concentrated energy-release. In Model III, the earthquakes with steeper dip angles did not occur on the plate interface, but on nearby steeper subfaults. Since the differences in dip angle are only 5–10°, this last explanation would imply that the main fault has about the same strength as the nearby subfaults, rather than much weaker. A relatively uniform fault zone with both the main fault and the subfaults inside is consistent with Model III. Higher resolution source locations and improved models of the velocity structure of the megathrust fault zone are necessary to resolve these issues.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/35738, title ="The 2012 Sumatra great earthquake sequence", author = "Duputel, Zacharie and Kanamori, Hiroo", journal = "Earth and Planetary Science Letters", volume = "351", pages = "247-257", month = "October", year = "2012", doi = "10.1016/j.epsl.2012.07.017", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121130-081922437", note = "© 2012 Elsevier B.V.\n\nAccepted 15 July 2012. Available online 1 September 2012. \nEditor: P. Shearer.\n\nWe thank Shengji Wei, Don Helmberger, Thorne Lay and two anonymous reviewers for helpful discussions. This work uses Federation of Digital Seismic Networks (FDSN) seismic data and CMT solutions from the Global CMT catalog. The Incorporated Research Institutions for Seismology (IRIS) Data Management System (DMS) was used to access the data. This work made use of the Matplotlib python library, of the Basemap toolkit and of the neighbourhood algorithm sampler developed by Malcolm Sambridge. Lingsen Meng and Jean-Paul Ampuero were supported by NSF Grant EAR-1015704.", revision_no = "23", abstract = "The equatorial Indian Ocean is a well known place of active intraplate deformation defying the conventional view of rigid plates separated by narrow boundaries where deformation is confined. On 11 April 2012, this region was hit in a couple of hours by two of the largest strike-slip earthquakes ever recorded (moment magnitudes Mw=8.6 and 8.2). Broadband seismological observations of the Mw=8.6 mainshock indicate a large centroid depth (∼30 km) and remarkable rupture complexity. Detailed study of the surface-wave directivity and moment rate functions clearly indicates the partition of the rupture into at least two distinct subevents. To account for these observations, we developed a procedure to invert for multiple-point-source parameters. The optimum source model at long period consists of two point sources separated by about 209 km with magnitudes Mw=8.5 and 8.3. To explain the remaining discrepancies between predicted and observed surface waves, we can refine this model by adding directivity along the WNW–ESE axis. However, we do not exclude more complicated models. To analyze the Mw=8.2 aftershock, we removed the perturbation due to large surface-wave arrivals of the Mw=8.6 mainshock by subtracting the corresponding synthetics computed for the two-subevent model. Analysis of the surface-wave amplitudes suggests that the Mw=8.2 aftershock had a large centroid depth between 30 km and 40 km. This major earthquake sequence brings a new perspective to the seismotectonics of the equatorial Indian Ocean and reveals active deep lithospheric deformation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/33251, title ="Earthquake in a Maze: Compressional Rupture Branching During the 2012 M_w 8.6 Sumatra Earthquake", author = "Meng, L. and Ampuero, J.-P.", journal = "Science", volume = "337", number = "6095", pages = "724-726", month = "August", year = "2012", doi = "10.1126/science.1224030", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120816-082822417", note = "© 2012 American Association for the Advancement of Science. Received 30 April 2012; accepted 5 July 2012.\nPublished Online July 19 2012.\nThis research was supported by NSF\ngrant EAR-1015704, by the Gordon and Betty Moore\nFoundation, and by the Southern California Earthquake Center\n(SCEC), which is funded by NSF Cooperative Agreement\nEAR-0106924 and USGS Cooperative Agreement 02HQAG0008.\nThe Japanese Hi-net (www.hinet.bosai.go.jp) and the European\nORFEUS (www.orfeus-eu.org) data centers were used to\naccess the broadband seismograms. The magnetic anomalies\nare from the EMAG2 database available at the National\nGeophysical Data Center (www.ngdc.noaa.gov). The satellite\ngravity anomaly data are from the UCSD TOPEX v. 18.1\ndatabase. The multibeam bathymetry from the KNOX06RR\ncruise is available at Marine Geoscience Data System\n(www.marine-geo.org). We thank R.-C. Lien and B. Ma\nfor providing the Roger Revelle (RR1201) multibeam data\nfrom the DYNAMO cruise. We thank H. Kanamori for\nvaluable discussions about this event. This paper is Caltech\nTectonics Observatory contribution 215, Caltech Seismo Lab\ncontribution 10078, and SCEC contribution 1656.", revision_no = "30", abstract = "Seismological observations of the 2012 moment magnitude 8.6 Sumatra earthquake reveal unprecedented complexity of dynamic rupture. The surprisingly large magnitude results from the combination of deep extent, high stress drop, and rupture of multiple faults. Back-projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system, with relatively slow rupture speed. The east-southeast–west-northwest ruptures add a new dimension to the seismotectonics of the Wharton Basin, which was previously thought to be controlled by north-south strike-slip faulting. The rupture turned twice into the compressive quadrant, against the preferred branching direction predicted by dynamic Coulomb stress calculations. Orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure-insensitive strength of the deep oceanic lithosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/32651, title ="Joint inversion of Rayleigh wave phase velocity and ellipticity using USArray: Constraining velocity and density structure in the upper crust", author = "Lin, Fan-Chi and Schmandt, Brandon", journal = "Geophysical Research Letters", volume = "39", number = "12", pages = "Art. No. L12303", month = "June", year = "2012", doi = "10.1029/2012GL052196 ", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120723-130421973", note = "© 2012 American Geophysical Union. \n\nReceived 27 April 2012; accepted 23 May 2012; published 21 June 2012. \n\nInstruments (data) used in this study were made available through EarthScope (EAR-0323309), supported by the National Science Foundation. The facilities of the IRIS Data Management System (EAR-0552316) were used for access the waveform and metadata required in this study. The authors are grateful to Donald Forsyth and an anonymous reviewer for comments that helped to improve this paper. This research was supported by the Director’s Post-Doctoral Fellowship of the Seismological Laboratory at the California Institute of Technology and the Gordon and Betty Moore Foundation. The Editor thanks the two anonymous reviewers for assisting in the evaluation of this paper.", revision_no = "25", abstract = "Rayleigh wave ellipticity, or H/V ratio, observed on the surface is particularly sensitive to shallow earth structure. In this study, we jointly invert measurements of Rayleigh wave H/V ratio and phase velocity between 24–100 and 8–100 sec period, respectively, for crust and upper mantle structure beneath more than 1000 USArray stations covering the western United States. Upper crustal structure, in particular, is better constrained by the joint inversion compared to inversions based on phase velocities alone. In addition to imaging Vs structure, we show that the joint inversion can be used to constrain Vp/Vs and density in the upper crust. New images of uppermost crustal structure (<3 km depth) are in excellent agreement with known surface features, with pronounced low Vs, low density, and high Vp/Vs anomalies imaged in the locations of several major sedimentary basins including the Williston, Powder River, Green River, Denver, and San Juan basins. These results demonstrate not only the consistency of broadband H/V ratios and phase velocity measurements, but also that their complementary sensitivities have the potential to resolve density and Vp/Vs variations. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/32368, title ="The local amplification of surface waves: A new observable to constrain elastic velocities, density, and anelastic attenuation", author = "Lin, Fan-Chi and Tsai, Victor C.", journal = "Journal of Geophysical Research B", volume = "117", number = "B6", pages = "Art. No. B06302", month = "June", year = "2012", doi = "10.1029/2012JB009208", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120711-153922657", note = "© 2012 American Geophysical Union. \n\nReceived 7 February 2012; revised 19 April 2012; accepted 22 April 2012; published 5 June 2012. \n\nThe authors wish to thank two anonymous reviewers for constructive comments and B. Schmandt and R. W. Clayton for helpful discussions. The facilities of the IRIS Data Management System, specifically the IRIS Data Management Center, were used to access the waveform and metadata required in this study. The IRIS DMS is funded through the National Science Foundation and specifically the GEO Directorate through the Instrumentation and Facilities Program of the National Science Foundation under Cooperative Agreement EAR-0552316. This research was supported by the Director’s Post Doctoral Fellowship of the Seismological Laboratory at the California Institute of Technology and by NSF grants EAR-0711526 and EAR-0844097 at the University of Colorado Boulder.", revision_no = "16", abstract = "The deployment of USArray across the continental U.S. has prompted developments within surface wave tomography to exploit this unprecedented data set. Here, we present a\nmethod to measure a new surface wave observable: broadband surface wave amplification that provides new and unique constraints on elastic velocities and density within the\ncrust and upper mantle. The method, similar to its phase velocity counterpart referred to as Helmholtz tomography, initiates by constructing phase travel time and amplitude maps across the array for each period and earthquake. Spatial differential operators are then applied to evaluate the amplitude variation, as well as the effect of focusing/defocusing.\nBased on the 2-D damped wave equation, the amplitude variation corrected for focusing/defocusing is linked directly to both local amplification and intrinsic attenuation, which are separated by examining waves propagating in opposite directions. We apply the method to\nteleseismic Rayleigh waves observed across USArray between periods of 24 and 100 s and show that the observed amplification maps are strongly correlated with known geological features. Small-scale attenuation measurements are contaminated by wavefield complexities, but larger-scale anelastic attenuation is estimated reliably. The observed amplification maps compare well with predictions based on recent 3-D shear velocity\nmodels of the western U.S. that were produced from ambient noise and earthquake data.\nNotably, predictions based on models with different prescribed density structures demonstrate the potential for using estimates of local amplification to constrain not only 3-D velocity structure but also density.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/31612, title ="Modeling Turbulent Hydraulic Fracture Near a Free Surface", author = "Tsai, Victor C. and Rice, James R.", journal = "Journal of Applied Mechanics", volume = "79", number = "3", pages = "Art. No. 031003", month = "May", year = "2012", doi = "10.1115/1.4005879 ", issn = "0021-8936", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120523-104717621", note = "© 2012 ASME. \n\nReceived 19 September 2011; revised 11 January 2012; accepted manuscript posted 6 February 2012; published 4 April 2012. \n\nThis research was supported by National Science Foundation OPP Grant No. ANT-0739444 to Harvard University and a Mendenhall Postdoctoral Fellowship of the United States Geological Survey to VCT.", revision_no = "15", abstract = "Motivated by observations of the subglacial drainage of water, we consider a hydraulic fracture problem in which the crack grows parallel to a free surface, subject to fully turbulent fluid flow. Using a hybrid Chebyshev/series-minimization numerical approach, we solve for the pressure profile, crack opening displacement, and crack growth rate for a crack that begins relatively short but eventually becomes long compared with the distance to the free surface. We plot nondimensionalized results for a variety of different times, corresponding with different fracture lengths, and find substantial differences when free-surface effects are important.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/29501, title ="A physical model for seismic noise generation from sediment transport in rivers", author = "Tsai, Victor C. and Minchew, Brent", journal = "Geophysical Research Letters", volume = "39", number = "L2", pages = "Art. No. L02404", month = "January", year = "2012", doi = "10.1029/2011GL050255", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120228-092625703", note = "© 2012 by the American Geophysical Union.\n\nReceived 3 November 2011; revised 28 December 2011; accepted 28 December 2011; published 28 January 2012.\n\nThe authors thank J.-P. Avouac, L. Bollinger, and J. Lavé for helpful comments, and thank J. Johnson and L. Sklar for thoughtful reviews. This research was partially supported by NSF grant EAR0922199 to MPL. \n\nThe Editor thanks Joel Johnson and Leonard Sklar for their assistance in evaluating this paper.", revision_no = "23", abstract = "Measuring sediment flux in rivers remains a significant problem in studies of landscape evolution. Recent studies suggest that observations of seismic noise near rivers can help provide such measurements, but the lack of models linking observed seismic quantities to sediment flux has prevented the method from being used. Here, we develop a forward model to describe the seismic noise induced by the transport of sediment in rivers. The model provides an expression for the power spectral density (PSD) of the Rayleigh waves generated by impulsive impacts from saltating particles which scales linearly with the number of particles of a given size and the square of the linear momentum. After incorporating expressions for the impact velocity and rate of impacts for fluvially transported sediment, we observe that the seismic noise PSD is strongly dependent on the sediment size, such that good constraints on grain size distribution are needed for reliable estimates of sediment flux based on seismic noise observations. The model predictions for the PSD are consistent with recent measurements and, based on these data, a first attempt at inverting seismic noise for the sediment flux is provided.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28679, title ="Quantifying the influence of sea ice on ocean microseism using observations from the Bering Sea, Alaska", author = "Tsai, Victor C. and McNamara, Daniel E.", journal = "Geophysical Research Letters", volume = "38", number = "22", pages = "Art. No. L22502", month = "November", year = "2011", doi = "10.1029/2011GL049791", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120105-135036867", note = "This paper is not subject to U.S. copyright. Published in 2011 by the American Geophysical Union. \n\nReceived 23 September 2011; revised 21 October 2011; accepted 23 October 2011; published 19 November 2011. \n\nThe authors thank A. T. Ringler, S. O’Neel, F. Walter, P. D. Bromirski, V. Schlindwein, and S. Kedar for helpful comments. We also thank the Alaska Earthquake Information Center and the National Ice Center for providing the data used. This research was supported by the Mendenhall Postdoctoral Fellowship program of the United States\nGeological Survey. The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.", revision_no = "16", abstract = "Microseism is potentially affected by all processes that alter ocean wave heights. Because strong sea ice prevents large ocean waves from forming, sea ice can therefore significantly affect microseism amplitudes. Here we show that this link between sea ice and microseism is not only a robust one but can be quantified. In particular, we show that 75–90% of the variability in microseism power in the Bering Sea can be predicted using a fairly crude model of microseism damping by sea ice. The success of this simple parameterization suggests that an even stronger link can be established between the mechanical strength of sea ice and microseism power, and that microseism can eventually be used to monitor the strength of sea ice, a quantity that is not as easily observed through other means.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28458, title ="Are secular correlations between sunspots, geomagnetic activity, and global temperature significant?", author = "Love, Jeffrey J. and Mursula, Kalevi", journal = "Geophysical Research Letters", volume = "38", number = "21", pages = "Art. No. L21703", month = "November", year = "2011", doi = "10.1029/2011GL049380", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111213-144757573", note = "© 2011 by the American Geophysical Union. \n\nReceived 19 August 2011; accepted 12 October 2011; published 11 November 2011. \n\nWe thank E. W. Cliver, V. Courtillot, C. A. Finn, J. L. Gannon, J. W. Godt, E. J. Rigler, and an anonymous reviewer for reviewing a draft manuscript. K. Mursula acknowledges financial support from the Academy of Finland, projects 128189 and 131350. [17] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper. ", revision_no = "19", abstract = "Recent studies have led to speculation that solar-terrestrial interaction, measured by sunspot number and geomagnetic activity, has played an important role in global temperature change over the past century or so. We treat this possibility as an hypothesis for testing. We examine the statistical significance of cross-correlations between sunspot number, geomagnetic activity, and global surface temperature for the years 1868–2008, solar cycles 11–23. The data contain substantial autocorrelation and nonstationarity, properties that are incompatible with standard measures of cross-correlational significance, but which can be largely removed by averaging over solar cycles and first-difference detrending. Treated data show an expected statistically-significant correlation between sunspot number and geomagnetic activity, Pearson p < 10^(−4), but correlations between global temperature and sunspot number (geomagnetic activity) are not significant, p = 0.9954, (p = 0.8171). In other words, straightforward analysis does not support widely-cited suggestions that these data record a prominent role for solar-terrestrial interaction in global climate change. With respect to the sunspot-number, geomagnetic-activity, and global-temperature data, three alternative hypotheses remain difficult to reject: (1) the role of solar-terrestrial interaction in recent climate change is contained wholly in long-term trends and not in any shorter-term secular variation, or, (2) an anthropogenic signal is hiding correlation between solar-terrestrial variables and global temperature, or, (3) the null hypothesis, recent climate change has not been influenced by solar-terrestrial interaction.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/27770, title ="Constraints on the long-period moment-dip tradeoff for the Tohoku earthquake", author = "Tsai, Victor C. and Hayes, Gavin P.", journal = "Geophysical Research Letters", volume = "38", number = "7", pages = "Art. No. L00G17", month = "October", year = "2011", doi = "10.1029/2011GL049129", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111114-110515606", note = "© 2011 American Geophysical Union. \n\nReceived 1 August 2011; revised 19 September 2011; accepted 22 September 2011; published 25 October 2011. \n\nWe thank L. Rivera for providing scripts and other support, and two anonymous reviewers for comments that lead to improvements in the paper. This work was partially supported by a Mendenhall Postdoctoral Fellowship to VCT. The Editor wishes to thank two anonymous reviewers for their assistance evaluating this paper.", revision_no = "16", abstract = "Since the work of Kanamori and Given (1981), it has been recognized that shallow, pure dip-slip earthquakes excite long-period surface waves such that it is difficult to independently constrain the moment (M_0) and the dip (δ) of the source mechanism, with only the product M_0 sin(2δ) being well constrained. Because of this, it is often assumed that the primary discrepancies between the moments of shallow, thrust earthquakes are due to this moment-dip tradeoff. In this work, we quantify how severe this moment-dip tradeoff is depending on the depth of the earthquake, the station distribution, the closeness of the mechanism to pure dip-slip, and the quality of the data. We find that both long-period Rayleigh and Love wave modes have moment-dip resolving power even for shallow events, especially when stations are close to certain azimuths with respect to mechanism strike and when source depth is well determined. We apply these results to USGS W phase inversions of the recent M9.0 Tohoku, Japan earthquake and estimate the likely uncertainties in dip and moment associated with the moment- dip tradeoff. After discussing some of the important sources of moment and dip error, we suggest two methods for potentially improving this uncertainty.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/27397, title ="Understanding the amplitudes of noise correlation measurements", author = "Tsai, Victor C.", journal = "Journal of Geophysical Research B", volume = "116", number = "B9", pages = "Art. No. B09311", month = "September", year = "2011", doi = "10.1029/2011JB008483", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111025-085851425", note = "© 2011 American Geophysical Union. \n\nReceived 23 April 2011; revised 17 June 2011; accepted 29 June 2011; published 24 September 2011. \n\nThe author thanks R. Snieder, F.‐C. Lin, G. A. Prieto, M. P. Moschetti, L. Ramirez, D. E. McNamara, two anonymous reviewers and the Associate Editor for helpful comments. This research was supported by a Mendenhall postdoctoral fellowship from the U.S. Geological Survey.", revision_no = "19", abstract = "Cross correlation of ambient seismic noise is known to result in time series from which station-station travel-time measurements can be made. Part of the reason that these cross-correlation travel-time measurements are reliable is that there exists a theoretical framework that quantifies how these travel times depend on the features of the ambient noise. However, corresponding theoretical results do not currently exist to describe how the amplitudes of the cross correlation depend on such features. For example, currently it is not possible to take a given distribution of noise sources and calculate the cross correlation amplitudes one would expect from such a distribution. Here, we provide a ray-theoretical framework for calculating cross correlations. This framework differs from previous work in that it explicitly accounts for attenuation as well as the spatial distribution of sources and therefore can address the issue of quantifying amplitudes in noise correlation measurements. After introducing the general framework, we apply it to two specific problems. First, we show that we can quantify the amplitudes of coherency measurements, and find that the decay of coherency with station-station spacing depends crucially on the distribution of noise sources. We suggest that researchers interested in performing attenuation measurements from noise coherency should first determine how the dominant sources of noise are distributed. Second, we show that we can quantify the signal-to-noise ratio of noise correlations more precisely than previous work, and that these signal-to-noise ratios can be estimated for given situations prior to the deployment of seismometers. It is expected that there are applications of the theoretical framework beyond the two specific cases considered, but these applications await future work.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46278, title ="A model for seasonal changes in GPS positions and seismic wave speeds due to thermoelastic and hydrologic variations", author = "Tsai, Victor C.", journal = "Journal of Geophysical Research B", volume = "116", number = "B4", pages = "Art. No. B04404", month = "April", year = "2011", doi = "10.1029/2010JB008156 ", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140616-103542486", note = "This paper is not subject to U.S. copyright.\nPublished in 2011 by the American Geophysical Union.\n\nReceived 8 December 2010; revised 9 February 2011; accepted 10 February 2011; published 19 April 2011.\n\n\nThe author thanks P. A. Johnson and two\nanonymous reviewers for helpful comments. This research was supported\nby the Mendenhall Postdoctoral Fellowship program of the United States\nGeological Survey.", revision_no = "10", abstract = "It is known that GPS time series contain a seasonal variation that is not due to tectonic motions, and it has recently been shown that crustal seismic velocities may also vary seasonally. In order to explain these changes, a number of hypotheses have been given, among which thermoelastic and hydrology-induced stresses and strains are leading candidates. Unfortunately, though, since a general framework does not exist for understanding such seasonal variations, it is currently not possible to quickly evaluate the plausibility of these hypotheses. To fill this gap in the literature, I generalize a two-dimensional thermoelastic strain model to provide an analytic solution for the displacements and wave speed changes due to either thermoelastic stresses or hydrologic loading, which consists of poroelastic stresses and purely elastic stresses. The thermoelastic model assumes a periodic surface temperature, and the hydrologic models similarly assume a periodic near-surface water load. Since all three models are two-dimensional and periodic, they are expected to only approximate any realistic scenario; but the models nonetheless provide a quantitative framework for estimating the effects of thermoelastic and hydrologic variations. Quantitative comparison between the models and observations is further complicated by the large uncertainty in some of the relevant parameters. Despite this uncertainty, though, I find that maximum realistic thermoelastic effects are unlikely to explain a large fraction of the observed annual variation in a typical GPS displacement time series or of the observed annual variations in seismic wave speeds in southern California. Hydrologic loading, on the other hand, may be able to explain a larger fraction of both the annual variations in displacements and seismic wave speeds. Neither model is likely to explain all of the seismic wave speed variations inferred from observations. However, more definitive conclusions cannot be made until the model parameters are better constrained.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46218, title ="Averaging and sampling for magnetic-observatory hourly data", author = "Love, J. J. and Tsai, V. C.", journal = "Annales Geophysicae", volume = "28", number = "11", pages = "2079-2096", month = "November", year = "2010", doi = "10.5194/angeo-28-2079-2010 ", issn = "0992-7689", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140611-145630940", note = "© 2010 Author(s). This work is distributed\nunder the Creative Commons Attribution 3.0 License. \n\nReceived: 21 May 2010 – Revised: 25 October 2010 – Accepted: 5 November 2010 – Published: 12 November 2010.\n\nThe results presented here rely on data collected\nat the Barrow, Chambon la Forêt, Eskdalemuir, and Huancayo\nobservatories. The Barrow observatory is operated by the\nUSGS Geomagnetism Program. We thank the Institut de Physique\ndu Globe de Paris (France), the British Geological Survey (Great\nBritain), and the Instituto Geofisico (Perú) for continuing to support\nthe operation of their observatories. We acknowledge Intermagnet\n(www.intermagnet.org) for facilitating the dissemination of magnetic\nobservatory data and for promoting high standards of magnetic\nobservatory practice. We thank the Edinburgh and Kyoto World\nData Centers for archiving observatory data and making them readily\navailable to the scientific community. We thank the Boulder\nWorld Data Center (NGDC) for use of their observatory yearbooks.\nWe thank P. S. Earle, J. W. Kirchner, K. Mursula, and L. Svalgaard\nfor useful conversation/communication. We thank A. Chulliat,\nC. A. Finn, V. Lesur, and S. Macmillan, for reviewing a draft\nmanuscript.\nTopical Editor I. A. Daglis thanks S. Macmillan, A. Chulliat,\nand another anonymous referee for their help in evaluating this\npaper.", revision_no = "12", abstract = "A time and frequency-domain analysis is made of the effects of averaging and sampling methods used for constructing magnetic-observatory hourly data values. Using 1-min data as a proxy for continuous, geomagnetic variation, we construct synthetic hourly values of two standard types: instantaneous \"spot\" measurements and simple 1-h \"boxcar\" averages. We compare these average-sample types with others: 2-h average, Gaussian, and \"brick-wall\" low-frequency-pass. Hourly spot measurements provide a statistically unbiased representation of the amplitude range of geomagnetic-field variation, but as a representation of continuous field variation over time, they are significantly affected by aliasing, especially at high latitudes. The 1-h, 2-h, and Gaussian average-samples are affected by a combination of amplitude distortion and aliasing. Brick-wall values are not affected by either amplitude distortion or aliasing, but constructing them is, in an operational setting, relatively more difficult than it is for other average-sample types. It is noteworthy that 1-h average-samples, the present standard for observatory hourly data, have properties similar to Gaussian average-samples that have been optimized for a minimum residual sum of amplitude distortion and aliasing. For 1-h average-samples from medium and low-latitude observatories, the average of the combination of amplitude distortion and aliasing is less than the 5.0 nT accuracy standard established by Intermagnet for modern 1-min data. For medium and low-latitude observatories, average differences between monthly means constructed from 1-min data and monthly means constructed from any of the hourly average-sample types considered here are less than the 1.0 nT resolution of standard databases. We recommend that observatories and World Data Centers continue the standard practice of reporting simple 1-h-average hourly values.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46222, title ="A model for turbulent hydraulic fracture and application to crack propagation at glacier beds", author = "Tsai, Victor C. and Rice, James R.", journal = "Journal of Geophysical Research F", volume = "115", number = "F3", pages = "Art. No. F03007", month = "September", year = "2010", doi = "10.1029/2009JF001474", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140612-072458326", note = "© 2010 American Geophysical Union. \n\nReceived 4 August 2009; revised 23 November 2009; accepted 5 March 2010; published 20 July 2010. \n\nThis research was supported by National Science Foundation OPP grant ANT‐0739444. We thank R. Hindmarsh and two anonymous reviewers for helpful comments.", revision_no = "20", abstract = "Glaciological observations of under-flooding suggest that fluid-induced hydraulic fracture of an ice sheet from its bed sometimes occurs quickly, possibly driven by turbulently flowing water in a broad sheet flow. Taking the approximation of a fully turbulent flow into an elastic ice medium with small fracture toughness, we derive an approximate expression for the crack-tip speed, opening displacement and pressure profile. We accomplish this by first showing that a Manning-Strickler channel model for resistance to turbulent flow leads to a mathematical structure somewhat similar to that for resistance to laminar flow of a power law viscous fluid. We then adapt the plane-strain asymptotic crack solution of Desroches et al. (1994) and the power law self-similar solution of Adachi and Detournay (2002) for that case to calculate the desired quantities. The speed of crack growth is shown to scale as the overpressure (in excess of ice overburden) to the power 7/6, inversely as ice elastic modulus to the power 2/3, and as the ratio of crack length to wall roughness scale to the power 1/6. We tentatively apply our model by choosing parameter values thought appropriate for a basal crack driven by the rapid drainage of a surface meltwater lake near the margin of the Greenland Ice Sheet. Making various approximations perhaps relevant to this setting, we estimate fluid inflow rate to the basal fracture and vertical and horizontal surface displacements and find order-of-magnitude agreement with observations by Das et al. (2008) associated with lake drainage. Finally, we discuss how these preliminary estimates could be improved.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46219, title ="The relationship between noise correlation and the Green's function in the presence of degeneracy and the absence of equipartition", author = "Tsai, Victor C.", journal = "Geophysical Journal International", volume = "182", number = "3", pages = "1509-1514", month = "September", year = "2010", doi = "10.1111/j.1365-246X.2010.04693.x ", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140611-150433557", note = "Journal compilation © 2010 RAS.\nNo claim to original US government works.\n\nAccepted 2010 June 5. Received 2010 June 4; in original form 2010 February 23.\n\nThe author thanks Y. Fan, L. Ramirez, S. Hartzell, T. Tada, D. E.\nMcNamara and two anonymous reviewers for helpful comments.\nThis research was supported by the Mendenhall Postdoctoral Fellowship\nprogram of the United States Geological Survey.", revision_no = "10", abstract = "Recent derivations have shown that when noise in a physical system has its energy equipartitioned into the modes of the system, there is a convenient relationship between the cross correlation of time-series recorded at two points and the Green's function of the system. Here, we show that even when energy is not fully equipartitioned and modes are allowed to be degenerate, a similar (though less general) property holds for equations with wave equation structure. This property can be used to understand why certain seismic noise correlation measurements are successful despite known degeneracy and lack of equipartition on the Earth.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46221, title ="An explicit relationship between time-domain noise correlation and spatial autocorrelation (SPAC) results", author = "Tsai, Victor C. and Moschetti, Morgan P.", journal = "Geophysical Journal International", volume = "182", number = "1", pages = "454-460", month = "July", year = "2010", doi = "10.1111/j.1365-246X.2010.04633.x", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140612-071051007", note = "No claim to original US government works.\nJournal compilation © 2010 RAS.\n\n\nAccepted 2010 April 18. Received 2010 April 7; in original form 2009 December 10. The authors would like to thank S. Hartzell, M. W. Asten, W. J.\nStephenson, F. C. Lin, R. Snieder and D. E. McNamara for helpful\ndiscussions, and two anonymous reviewers for helpful comments.\nSeismograms were provided by IRIS. This research was supported\nby the Mendenhall Postdoctoral Fellowship program of the United\nStates Geological Survey", revision_no = "13", abstract = "The success of recent ambient noise tomographic studies is now understood to arise due to cross-correlation properties documented in the acoustics community since the 1950s. However, despite the fact that Aki's 1957 spatial autocorrelation (SPAC) work yields identical analytical results to certain noise correlation results, the precise relationship between SPAC and time-domain cross-correlation remains not entirely transparent. Here, we present an explicit comparison of the two approaches and clarify that SPAC theory is indeed equivalent to the cross-correlation theory used for recent noise tomography studies. This equivalence allows theoretical work from each field to be applied to the other, and we illustrate a few examples of this.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46220, title ="On establishing the accuracy of noise tomography travel-time measurements in a realistic medium", author = "Tsai, Victor C.", journal = "Geophysical Journal International", volume = "178", number = "3", pages = "1555-1564", month = "September", year = "2009", doi = "10.1111/j.1365-246X.2009.04239.x ", issn = "0956-540X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140611-153303533", note = "© 2009 The Author. Journal compilation © 2009 RAS. \n\nAccepted 2009 May 4. Received 2009 April 16; in original form 2009 January 19. \n\nThe author would like to thank C. A. Dalton, A. M. Dziewonski,\nM. G. Sterenborg, H. Kanamori, G. Ekström, H. Yao, M.\nBetancourt and J. R. Rice for helpful discussion and M. Ritzwoller\nand various anonymous reviewers for constructive criticism. The\nauthor also thanks editors J. Trampert and M. Diament. This research\nwas supported by a National Science Foundation Graduate\nFellowship.", revision_no = "12", abstract = "It has previously been shown that the Green's function between two receivers can be retrieved by cross-correlating time series of noise recorded at the two receivers. This property has been derived assuming that the energy in normal modes is uncorrelated and perfectly equipartitioned, or that the distribution of noise sources is uniform in space and the waves measured satisfy a high frequency approximation. Although a number of authors have successfully extracted travel-time information from seismic surface-wave noise, the reason for this success of noise tomography remains unclear since the assumptions inherent in previous derivations do not hold for dispersive surface waves on the Earth. Here, we present a simple ray-theory derivation that facilitates an understanding of how cross correlations of seismic noise can be used to make direct travel-time measurements, even if the conditions assumed by previous derivations do not hold. Our new framework allows us to verify that cross-correlation measurements of isotropic surface-wave noise give results in accord with ray-theory expectations, but that if noise sources have an anisotropic distribution or if the velocity structure is non-uniform then significant differences can sometimes exist. We quantify the degree to which the sensitivity kernel is different from the geometric ray and find, for example, that the kernel width is period-dependent and that the kernel generally has non-zero sensitivity away from the geometric ray, even within our ray theoretical framework. These differences lead to usually small (but sometimes large) biases in models of seismic-wave speed and we show how our theoretical framework can be used to calculate the appropriate corrections. Even when these corrections are small, calculating the errors within a theoretical framework would alleviate fears traditional seismologists may have regarding the robustness of seismic noise tomography.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46631, title ="Possible mechanisms for glacial earthquakes", author = "Tsai, Victor C. and Rice, James R.", journal = "Journal of Geophysical Research F", volume = "113", number = "F3", pages = "Art. No. F03014", month = "September", year = "2008", doi = "10.1029/2007JF000944", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140702-101254626", note = "© 2008 American Geophysical Union. \n\nReceived 7 November 2007; revised 9 June 2008; accepted 1 July 2008; published 5 August 2008. \n\nWe thank I. Joughin, R. B. Alley, I. M. Howat, M. Nettles, and G. Ekström for helpful discussion, and we thank S. O’Neel, M. Truffer, and S. Tulaczyk for constructive reviews. This work was supported by a National Science Foundation Graduate Research Fellowship to V.C.T. and, in its later stages, by NSF-OPP grant ANT-0739444.", revision_no = "20", abstract = "The large glacial earthquakes reported on by Ekström et al. (2003, 2006) and Tsai and Ekström (2007) have previously been evaluated in terms of their seismic characteristics. In this paper we attempt to take constraints such as known glacial ice properties, outlet glacier size, calving style, and meltwater variability to construct a self-consistent physical model of the glacial earthquake process. Since many glaciological parameters are poorly constrained, we parameterize a number of important processes and estimate a wide range of possible values for some properties. The range of model outputs is thus fairly large, but it is still difficult to match observational constraints under most conditions. We find that only a small class of models is able to satisfy the major observational constraints. These models are characterized by (1) lost basal resistance coupled to viscoelastic deformation with extensive internal crevassing or with low effective elastic modulus and possibly low effective viscosity or (2) by nonequilibrium calving, such as having large icebergs capsize into the glacier front. Although observational constraints cannot definitively rule out any of the proposed classes of mechanisms, the calving model has much stronger support. Fortunately, the various models make different predictions regarding observables that can potentially be measured in the near future.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46344, title ="Ice-front variation and tidewater behavior on Helheim and Kangerdlugssuaq Glaciers, Greenland", author = "Joughin, Ian and Howat, Ian", journal = "Journal of Geophysical Research F", volume = "113", number = "F1", pages = "Art. No. F01004 ", month = "March", year = "2008", doi = "10.1029/2007JF000837 ", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140618-143619582", note = "© 2008 by the American Geophysical Union.\n\nReceived 23 May 2007; accepted 23 October 2007; published 26 January 2008.\n\nThe National Science Foundation (NSF)\nsupported contributions by I. Joughin, T. Moon, and R. Alley, M. Fahnestock,\nand M. Truffer through Arctic System Science Grants: ARC0531270,\nARC0531211, ARC 0531250 and ARC0531075. The National Aerospace\nAdministration supported I. Howat’s contribution (NNG06GE5SG) and an\nNSF Graduate Research Fellowship supported V. Tsai. Comments by\nT. Murray, A. Vieli, and S. O’Neel led to improvements in the final\nmanuscript.", revision_no = "14", abstract = "We used satellite images to examine the calving behavior of Helheim and Kangerdlugssuaq Glaciers, Greenland, from 2001 to 2006, a period in which they retreated and sped up. These data show that many large iceberg-calving episodes coincided with teleseismically detected glacial earthquakes, suggesting that calving-related processes are the source of the seismicity. For each of several events for which we have observations, the ice front calved back to a large, pre-existing rift. These rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high, which agrees well with earlier theoretical predictions. In addition to the recent retreat in a period of higher temperatures, analysis of several images shows that Helheim retreated in the 20th Century during a warmer period and then re-advanced during a subsequent cooler period. This apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water. The cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in Greenland can advance rapidly. Greenland's larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46407, title ="Analysis of glacial earthquakes", author = "Tsai, Victor C. and Ekström, Göran", journal = "Journal of Geophysical Research F", volume = "112", number = "F3", pages = "Art. No. F03S22", month = "September", year = "2007", doi = "10.1029/2006JF000596", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140620-153346884", note = "© 2007 by the American Geophysical.\n\nReceived 12 June 2006; revised 27 October 2006; accepted 22 November 2006; published 14 April 2007.\n\nWe thank R. Anderson, T. Murray,\nS. Anandakrishnan, and an anonymous reviewer for helpful comments.\nThis research was supported by a National Science Foundation Graduate\nFellowship (VCT) and National Science Foundation grants EAR-0207608\nand OPP-0352276. The seismic data were collected and distributed by the\nIncorporated Research Institutions for Seismology and the U.S. Geological\nSurvey.", revision_no = "11", abstract = "In 2003, Ekström et al. reported on the detection of a new class of earthquakes that occur in glaciated regions, with the vast majority being in Greenland. The events have a characteristic radiation pattern and lack the high-frequency content typical of tectonic earthquakes. It was proposed that the events correspond to large and sudden sliding motion of glaciers. Here we present an analysis of all 184 such events detected in Greenland between 1993 and 2005. Fitting the teleseismic long-period surface waves to a landslide model of the source, we obtain improved locations, timing, force amplitudes, and force directions. After relocation, the events cluster into seven regions, all of which correspond to regions of very high ice flow and most of which are named outlet glaciers. These regions are Daugaard Jensen Glacier, Kangerdlugssuaq Glacier, Helheim Glacier, the southeast Greenland glaciers, the northwest Greenland glaciers, Rinks Isbrae, and Jakobshavn Isbrae. Event amplitudes range from 0.1 to 2.0 × 10^(14) kg m. Force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s. Each region has a different temporal distribution of events. All glaciers are more productive in the summer, but have their peak activity in different months. Over the study period, Kangerdlugssuaq has had a constant number of events each year, whereas Jakobshavn had most events in 1998–1999, and the number of events in Helheim and the northwest Greenland glaciers has increased substantially between 1993 and 2005. The size distribution of events in Kangerdlugssuaq is peaked above the detection threshold, suggesting that glacial earthquakes have a characteristic size.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46454, title ="Star patterns on lake ice", author = "Tsai, Victor C. and Wettlaufer, J. S.", journal = "Physical Review E", volume = "75", number = "6", pages = "Art. No. 066105", month = "June", year = "2007", doi = "10.1103/PhysRevE.75.066105", issn = "1539-3755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140623-153920293", note = "© 2007 American Physical Society.\n\nReceived 14 February 2007; published 18 June 2007.\n\nWe thank K. Bradley and J. A. Whitehead for laboratory\nand facilities support and D. H. Rothman for helpful comments.\nThis research, which began at the Geophysical Fluid\nDynamics summer program at the Woods Hole Oceanographic\nInstitution, was partially funded by National Science\nFoundation (NSF) Grant No. OCE0325296, NSF Grant No.\nOPP0440841 (J.S.W.), and Department of Energy Grant No.\nDE-FG02-05ER15741 (J.S.W.). V. C. T. acknowledges financial\nsupport from NSF.", revision_no = "10", abstract = "Star patterns, reminiscent of a wide range of diffusively controlled growth forms from snowflakes to Saffman-Taylor fingers, are ubiquitous features of ice-covered lakes. Despite the commonality and beauty of these “lake stars,” the underlying physical processes that produce them have not been explained in a coherent theoretical framework. Here we describe a simple mathematical model that captures the principal features of lake-star formation; radial fingers of (relatively warm) water-rich regions grow from a central source and evolve through a competition between thermal and porous media flow effects in a saturated snow layer covering the lake. The number of star arms emerges from a stability analysis of this competition and the qualitative features of this meter-scale natural phenomenon are captured in laboratory experiments.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44280, title ="Theoretical constraints on true polar wander", author = "Tsai, Victor C. and Stevenson, David J.", journal = "Journal of Geophysical Research B", volume = "112", number = "B5", pages = "Art. No. B05415", month = "May", year = "2007", doi = "10.1029/2005JB003923", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140312-113944721", note = "© 2007 American Geophysical Union. \n\nManuscript Accepted: 22 Feb 2007. Manuscript Revised: 30 Jan 2007. Manuscript Received: 2 Jul 2005. Article first published online: 23 May 2007. \n\nWe thank G. Acton and an anonymous reviewer for helpful comments. This research was partially supported by a National Science Foundation Graduate Fellowship (VCT).", revision_no = "14", abstract = "For the present geologic epoch, true polar wander (TPW) is relatively small, but simple theoretical considerations suggest that it could have been larger in other epochs. In this work, we use scaling arguments to assess the qualitative behavior of TPW and a simple Maxwell model to analytically describe how changes in mass anomalies translate into TPW. Unlike previous work, we derive simple analytical estimates of TPW based on the characteristic amplitudes and timescales for changes in the moment of inertia. We find estimates for both the amplitude and speed of TPW as a function of Earth properties. The following four main factors influence how large the maximum TPW can be: the (geological) timescale over which the desired TPW occurs τ_(TPW), the viscosity structure of the mantle which yields a weighted average viscosity η, the characteristic amplitude of the nonhydrostatic changes in the moment of inertia δC, and the added moment of inertia due to the equatorial bulge (C − A). For the nominal values η = 3 × 10^(22) Pa s and δC/(C − A) = 0.003, the maximum TPW is 61° over 100 Myr and 8° over 10 Myr. The maximum TPW speed is only sensitive to η, δC, and (C − A), and is 2.4° Myr^(-1) for the nominal values. TPW is shown to act as a low-pass filter; rapid changes in moment of inertia produce smaller and delayed TPW. A consequence of this is that inertial interchange TPW does not have a different character than TPW. TPW can have an important contribution to plate motions over relatively long timescales but not over shorter timescales. Our simple approach allows us to assess whether multiple TPW events are possible but the major uncertainty continues to be the mantle viscosity structure.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46211, title ="Seasonality and increasing frequency of Greenland glacial earthquakes", author = "Ekström, Göran and Nettles, Meredith", journal = "Science", volume = "311", number = "5768", pages = "1756-1758", month = "March", year = "2006", doi = "10.1126/science.1122112 ", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140611-132746586", note = "© 2006 American Association for the Advancement of Science. \n\nReceived 1 November 2005; accepted 9 January 2006. \n\nSupported by National Science Foundation grants EAR-0207608 and OPP-0352276 and a National Science Foundation Graduate Fellowship (V.C.T.). The seismic data were collected and distributed by the Incorporated Research Institutions for Seismology and the U.S. Geological Survey.", revision_no = "11", abstract = "Some glaciers and ice streams periodically lurch forward with sufficient force to generate emissions of elastic waves that are recorded on seismometers worldwide. Such glacial earthquakes on Greenland show a strong seasonality as well as a doubling of their rate of occurrence over the past 5 years. These temporal patterns suggest a link to the hydrological cycle and are indicative of a dynamic glacial response to changing climate conditions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46207, title ="Multiple CMT source analysis of the 2004 Sumatra earthquake", author = "Tsai, Victor C. and Nettles, Meredith", journal = "Geophysical Research Letters", volume = "32", number = "17", pages = "Art. No. L17304", month = "September", year = "2005", doi = "10.1029/2005GL023813", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140611-125859794", note = "© 2005 by the American Geophysical Union. \n\nReceived 14 June 2005; revised 27 July 2005; accepted 10 August 2005; published 9 September 2005. \n\nWe thank H. Kanamori, C. Ammon, M. Antolik, R. Bürgmann, M. Ishii, T. Lay and two anonymous reviewers for helpful comments. The GSN data analyzed were collected and distributed by the Incorporated Research Institutions for Seismology (IRIS) and the USGS. This research was supported by a Harvard University James Mills Peirce Fellowship and a National Science Foundation (NSF) Graduate Research Fellowship (VCT) and by NSF grant EAR-0207608.", revision_no = "14", abstract = "While it is agreed that the great Sumatra earthquake of\nDecember 26, 2004 was among the largest earthquakes of\nthe past century, there has been disagreement on how large\nit was, which part of the fault ruptured, and how the rupture\ntook place. We present a centroid-moment-tensor (CMT)\nanalysis of the earthquake in which multiple point sources\nare used in the inversion to mimic a propagating slip pulse.\nThe final model consists of five point sources, with the\nsouthernmost sources accounting for the majority of the\nmoment release. The presumed fault planes of the southern\nsources strike northwest, while those in the north strike\nnortheast, consistent with the geometry of the subduction\ntrench. Slip on the fault is found to be more oblique in the\nnorth than in the south. The inversion with five sources\nleads to a moment magnitude for the Sumatra earthquake of\nM_W = 9.3, consistent with estimates from long-period\nnormal-mode amplitudes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/46208, title ="The morning glory wave of southern California", author = "Tsai, Victor C. and Kanamori, Hiroo", journal = "Journal of Geophysical Research B", volume = "109", number = "B2", pages = "Art. No. B02307", month = "February", year = "2004", doi = "10.1029/2003JB002596 ", issn = "0148-0227", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140611-131841863", note = "© 2004 by the American Geophysical Union. \n\nReceived 21 May 2003; revised 26 September 2003; accepted 14 November 2003; published 13 February 2004. \n\nThis research was supported by the SURF program at Caltech as well as generous funding from Marcella Bonsall. We thank Sharon Kedar for bringing our attention to the long-period seismic signal observed on 12 October 2001. We would also like to thank Doug Christie, Andy Ingersoll, Don Anderson, and Toshiro Tanimoto for their critique and suggestions. The seismic and barograph data used in this study were downloaded from the IRIS Data Management Center and the Data Center of the Southern California Earthquake Center.", revision_no = "11", abstract = "A pulse-like disturbance traveling across the Los Angeles basin was observed on 12 October 2001 with seismographs of the TriNet network. This wave had a period of about 1000 s and a propagation speed of about 10 m/s, much slower than seismic waves. The seismograph data were compared with barograph data, and a good correlation was found so the wave was determined to be atmospheric in origin. It had amplitude of about 1 mbar, but it was not known what process could produce such a wave. Since the initial finding, we have inspected all the TriNet barograph and seismograph data for a period of two and a half years (from January 2000 to August 2002) and found four more similar events. Each of the events has amplitude between 0.8 and 1.3 mbar, a period between 700 and 1400 s, and a propagation speed between 5 and 25 m/s. We conclude that these waves are internal gravity waves trapped in a stable layer formed by a temperature inversion. Some of these waves have large amplitudes and develop into solitary waves (nonlinear internal gravity waves) similar to the spectacular “morning glory” wave observed in Australia. We call these waves the LA morning glory waves. The LA morning glory wave is probably excited by either stormy weather, winds such as the Santa Ana winds, or large teleseismic events. The morning glory wave could contribute to the recently reported excitation of the background free oscillations of the Earth. Additionally, because of its large amplitude it could have important implications for aviation safety, as was suggested earlier for the morning glory waves in Australia.", }