CaltechAUTHORS: Combined
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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenWed, 07 Aug 2024 13:09:34 -0700The local amplification of surface waves: A new observable to constrain elastic velocities, density, and anelastic attenuation
https://resolver.caltech.edu/CaltechAUTHORS:20120711-153922657
Year: 2012
DOI: 10.1029/2012JB009208
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
method to measure a new surface wave observable: broadband surface wave amplification that provides new and unique constraints on elastic velocities and density within the
crust 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.
Based 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
teleseismic 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
models of the western U.S. that were produced from ambient noise and earthquake data.
Notably, 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.https://resolver.caltech.edu/CaltechAUTHORS:20120711-153922657Joint inversion of Rayleigh wave phase velocity and ellipticity using USArray: Constraining velocity and density structure in the upper crust
https://resolver.caltech.edu/CaltechAUTHORS:20120723-130421973
Year: 2012
DOI: 10.1029/2012GL052196
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.https://resolver.caltech.edu/CaltechAUTHORS:20120723-130421973Interferometry with a dense 3D dataset
https://resolver.caltech.edu/CaltechAUTHORS:20121003-112803929
Year: 2012
In this paper we report on progress using ambient noise
correlation with a dense 3D survey conducted in Long
Beach, California, to estimate subsurface velocity. We
show that both Rayleigh wave and body wave signals can
be clearly observed between 0.2-10 Hz frequency range in
the noise cross-correlations. The observed signals also
compare well with an active source gather. We apply
eikonal tomography to invert for the Rayleigh wave phase
velocity maps at several different frequencies. The phase
velocity maps, which are most sensitive to structure in the
top 600 meters, show clear correlation with known surface
features such as the slow anomaly adjacent to the coast in
the south and a fast anomaly associated with the Newport-
Inglewood fault zone. The results presented in this study
show the potential of using ambient noise interferometry
method to complement active source techniques in studying
high-resolution shallow crustal structure.https://resolver.caltech.edu/CaltechAUTHORS:20121003-112803929Joint inversion of surface wave dispersion and receiver functions: a Bayesian Monte-Carlo approach
https://resolver.caltech.edu/CaltechAUTHORS:20130221-130333341
Year: 2013
DOI: 10.1093/gji/ggs050
A non-linear Bayesian Monte-Carlo method is presented to estimate a Vsv model beneath
stations by jointly interpreting Rayleigh wave dispersion and receiver functions and associated
uncertainties. The method is designed for automated application to large arrays of broad-band
seismometers. As a testbed for the method, 185 stations from the USArray Transportable
Array are used in the IntermountainWest, a region that is geologically diverse and structurally
complex. Ambient noise and earthquake tomography are updated by applying eikonal and
Helmholtz tomography, respectively, to construct Rayleighwave dispersion maps from 8 to 80 s
across the study region with attendant uncertainty estimates.Amethod referred to as 'harmonic
stripping method' is described and applied as a basis for quality control and to generate
backazimuth independent receiver functions for a horizontally layered, isotropic effective
medium with uncertainty estimates for each station. A smooth parametrization between (as
well as above and below) discontinuities at the base of the sediments and crust suffices to fit most
features of both data types jointly across most of the study region. The effect of introducing
receiver functions to surface wave dispersion data is quantified through improvements in
the posterior marginal distribution of model variables. Assimilation of receiver functions
quantitatively improves the accuracy of estimates of Moho depth, improves the determination
of the Vsv contrast across Moho, and improves uppermost mantle structure because of the
ability to relax a priori constraints. The method presented here is robust and can be applied
systematically to construct a 3-D model of the crust and uppermost mantle across the large
networks of seismometers that are developing globally, but also provides a framework for
further refinements in the method.https://resolver.caltech.edu/CaltechAUTHORS:20130221-130333341Extracting seismic core phases with array interferometry
https://resolver.caltech.edu/CaltechAUTHORS:20130627-095929812
Year: 2013
DOI: 10.1002/grl.50237
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.https://resolver.caltech.edu/CaltechAUTHORS:20130627-095929812High-resolution 3D shallow crustal structure in Long Beach, California: Application of ambient noise tomography on a dense seismic array
https://resolver.caltech.edu/CaltechAUTHORS:20130909-085627813
Year: 2013
DOI: 10.1190/GEO2012-0453.1
Ambient noise tomography has proven to be effective in resolving shallow earth structure. We applied ambient noise tomography on a dense seismic array in Long Beach, California. The array was composed of more than 5200 stations with an average spacing close to 100 m. Three weeks of passive ambient noise were crosscorrelated between each station pair, which resulted in more than 13.5 million crosscorrelations within the area. Clear fundamental-mode Rayleigh waves were observed between 0.5 and 4 Hz, which were most sensitive to structure above 1-km depth. For each station pair, we applied frequency-time analysis to determine the phase traveltime dispersion, and, for each frequency, we applied eikonal tomography to determine the Rayleigh wave phase velocity map. The eikonal tomography accounted for ray bending by tracking the wavefront and allowed uncertainties to be estimated through statistical analysis. The compilation of phase velocity maps was then used to invert for 3D shear velocity structure. The inverted model showed clear correlation with the known geologic features such as the shallow south–north velocity dichotomy and a deeper fast anomaly associated with the Newport-Inglewood fault zone. Our results can potentially be used to complement traditional active source studies.https://resolver.caltech.edu/CaltechAUTHORS:20130909-085627813Seismic interferometry with antipodal station pairs
https://resolver.caltech.edu/CaltechAUTHORS:20131108-135759821
Year: 2013
DOI: 10.1002/grl.50907
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 s 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.https://resolver.caltech.edu/CaltechAUTHORS:20131108-1357598213-D crustal structure of the western United States: application of Rayleigh-wave ellipticity extracted from noise cross-correlations
https://resolver.caltech.edu/CaltechAUTHORS:20140613-160117253
Year: 2014
DOI: 10.1093/gji/ggu160
We present a new 3-D seismic model of the western United States crust derived from a joint
inversion of Rayleigh-wave phase velocity and ellipticity measurements using periods from
8 to 100 s. Improved constraints on upper-crustal structure result from use of short-period
Rayleigh-wave ellipticity, or Rayleigh-wave H/V (horizontal to vertical) amplitude ratios,
measurements determined using multicomponent ambient noise cross-correlations. To retain
the amplitude ratio information between vertical and horizontal components, for each station,
we perform daily noise pre-processing (temporal normalization and spectrum whitening) simultaneously
for all three components. For each station pair, amplitude measurements between
cross-correlations of different components (radial–radial, radial–vertical, vertical–radial and
vertical–vertical) are then used to determine the Rayleigh-wave H/V ratios at the two station
locations. We use all EarthScope/USArray Tranportable Array data available between 2007
January and 2011 June to determine the Rayleigh-wave H/V ratios and their uncertainties at all
station locations and construct new Rayleigh-wave H/V ratio maps in the western United States
between periods of 8 and 24 s. Combined with previous longer period earthquake Rayleigh-wave
H/V ratio measurements and Rayleigh-wave phase velocity measurements from both
ambient noise and earthquakes, we invert for a new 3-D crustal and upper-mantle model in the
western United States. Correlation between the inverted model and known geological features
at all depths suggests good resolution in five crustal layers. Use of short-period Rayleigh-wave
H/V ratio measurements based on noise cross-correlation enables resolution of distinct near
surface features such as the Columbia River Basalt flows, which overlie a thick sedimentary
basin.https://resolver.caltech.edu/CaltechAUTHORS:20140613-160117253Site amplification, attenuation, and scattering from noise correlation amplitudes across a dense array in Long Beach, CA
https://resolver.caltech.edu/CaltechAUTHORS:20150423-094707871
Year: 2015
DOI: 10.1002/2014GL062662
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 Hz. 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.https://resolver.caltech.edu/CaltechAUTHORS:20150423-094707871The 11 March 2011 Tohoku tsunami wavefront mapping across offshore Southern California
https://resolver.caltech.edu/CaltechAUTHORS:20150716-124510453
Year: 2015
DOI: 10.1002/2014JB011524
The 11 March 2011 (M_w = 9.0) Tohoku tsunami was recorded by a temporary array of seafloor pressure gauges deployed off the coast of Southern California, demonstrating how dense array data can illustrate and empirically validate predictions of linear tsunami wave propagation characteristics. A noise cross-correlation method was used to first correct for the pressure gauge instrument phase response. Phase and group travel times were then measured for the first arrival in the pressure gauge tsunami waveforms filtered in narrow bands around 30 periods between 200 and 3000 s. For each period, phase velocities were estimated across the pressure gauge array based on the phase travel time gradient using eikonal tomography. Clear correlation was observed between the phase velocity and long-wavelength bathymetry variations where fast and slow velocities occurred for deep and shallow water regions, respectively. In particular, velocity gradients are pronounced at the Patton Escarpment and near island plateaus due to the abrupt bathymetry change. In the deep open ocean area, clear phase velocity dispersion is observed. Comparison with numerically calculated tsunami waveforms validates the approach and provides an independent measure of the finite-frequency effect on phase velocities at long periods.https://resolver.caltech.edu/CaltechAUTHORS:20150716-124510453The Yellowstone magmatic system from the mantle plume to the upper crust
https://resolver.caltech.edu/CaltechAUTHORS:20150504-121706460
Year: 2015
DOI: 10.1126/science.aaa5648
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.https://resolver.caltech.edu/CaltechAUTHORS:20150504-121706460High-resolution probing of inner core structure with seismic interferometry
https://resolver.caltech.edu/CaltechAUTHORS:20160218-105113138
Year: 2015
DOI: 10.1002/2015GL066390
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.https://resolver.caltech.edu/CaltechAUTHORS:20160218-105113138Amplification and Attenuation across USArray using Ambient Noise Wavefront Tracking
https://resolver.caltech.edu/CaltechAUTHORS:20171115-155052574
Year: 2017
DOI: 10.1002/2017JB014804
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.https://resolver.caltech.edu/CaltechAUTHORS:20171115-155052574Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California
https://resolver.caltech.edu/CaltechAUTHORS:20180221-090936349
Year: 2018
DOI: 10.1016/j.epsl.2018.02.009
The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40–80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150–200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a ∼3–4 Ma pulse of basaltic magmatism.https://resolver.caltech.edu/CaltechAUTHORS:20180221-090936349Detection of Building Damage Using Helmholtz Tomography
https://resolver.caltech.edu/CaltechAUTHORS:20180807-145018368
Year: 2018
DOI: 10.1785/0120170322
High‐rise buildings with dense permanent installations of continuously recording accelerometers offer a unique opportunity to observe temporal and spatial variations in the propagation properties of seismic waves. When precise, floor‐by‐floor measurements of frequency‐dependent travel times can be made, accurate models of material properties (e.g., stiffness or rigidity) can be determined using seismic tomographic imaging techniques. By measuring changes in the material properties, damage to the structure can be detected and localized after shaking events such as earthquakes. Here, seismic Helmholtz tomography is applied to simulated waveform data from a high‐rise building, and its feasibility is demonstrated. A 52‐story dual system building—braced‐frame core surrounded by an outrigger steel moment frame—in downtown Los Angeles is used for the computational basis. It is part of the Community Seismic Network and has a three‐component accelerometer installed on every floor. A finite‐element model of the building based on structural drawings is used for the computation of synthetic seismograms for 60 damage scenarios in which the stiffness of the building is perturbed in different locations across both adjacent and distributed floors and to varying degrees. The dynamic analysis loading function is a Gaussian pulse applied to the lowest level fixed boundary condition, producing a broadband response on all floors. After narrowband filtering the synthetic seismograms and measuring the maximum amplitude, the frequency‐dependent travel times and differential travel times are computed. The travel‐time and amplitude measurements are converted to shear‐wave velocity at each floor via the Helmholtz wave equation whose solutions can be used to track perturbations to wavefronts through densely sampled wavefields. These results provide validation of the method's application to recorded data from real buildings to detect and locate structural damage using earthquake, explosion, or ambient seismic noise data in near‐real time.https://resolver.caltech.edu/CaltechAUTHORS:20180807-145018368