CaltechAUTHORS: Article
https://feeds.library.caltech.edu/people/Tsai-V-C/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenWed, 18 Sep 2024 19:26:25 -0700The morning glory wave of southern California
https://resolver.caltech.edu/CaltechAUTHORS:20140611-131841863
Year: 2004
DOI: 10.1029/2003JB002596
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.https://resolver.caltech.edu/CaltechAUTHORS:20140611-131841863Multiple CMT source analysis of the 2004 Sumatra earthquake
https://resolver.caltech.edu/CaltechAUTHORS:20140611-125859794
Year: 2005
DOI: 10.1029/2005GL023813
While it is agreed that the great Sumatra earthquake of
December 26, 2004 was among the largest earthquakes of
the past century, there has been disagreement on how large
it was, which part of the fault ruptured, and how the rupture
took place. We present a centroid-moment-tensor (CMT)
analysis of the earthquake in which multiple point sources
are used in the inversion to mimic a propagating slip pulse.
The final model consists of five point sources, with the
southernmost sources accounting for the majority of the
moment release. The presumed fault planes of the southern
sources strike northwest, while those in the north strike
northeast, consistent with the geometry of the subduction
trench. Slip on the fault is found to be more oblique in the
north than in the south. The inversion with five sources
leads to a moment magnitude for the Sumatra earthquake of
M_W = 9.3, consistent with estimates from long-period
normal-mode amplitudes.https://resolver.caltech.edu/CaltechAUTHORS:20140611-125859794Seasonality and increasing frequency of Greenland glacial earthquakes
https://resolver.caltech.edu/CaltechAUTHORS:20140611-132746586
Year: 2006
DOI: 10.1126/science.1122112
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.https://resolver.caltech.edu/CaltechAUTHORS:20140611-132746586Theoretical constraints on true polar wander
https://resolver.caltech.edu/CaltechAUTHORS:20140312-113944721
Year: 2007
DOI: 10.1029/2005JB003923
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.https://resolver.caltech.edu/CaltechAUTHORS:20140312-113944721Star patterns on lake ice
https://resolver.caltech.edu/CaltechAUTHORS:20140623-153920293
Year: 2007
DOI: 10.1103/PhysRevE.75.066105
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.https://resolver.caltech.edu/CaltechAUTHORS:20140623-153920293Analysis of glacial earthquakes
https://resolver.caltech.edu/CaltechAUTHORS:20140620-153346884
Year: 2007
DOI: 10.1029/2006JF000596
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.https://resolver.caltech.edu/CaltechAUTHORS:20140620-153346884Ice-front variation and tidewater behavior on Helheim and Kangerdlugssuaq Glaciers, Greenland
https://resolver.caltech.edu/CaltechAUTHORS:20140618-143619582
Year: 2008
DOI: 10.1029/2007JF000837
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.https://resolver.caltech.edu/CaltechAUTHORS:20140618-143619582Possible mechanisms for glacial earthquakes
https://resolver.caltech.edu/CaltechAUTHORS:20140702-101254626
Year: 2008
DOI: 10.1029/2007JF000944
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.https://resolver.caltech.edu/CaltechAUTHORS:20140702-101254626On establishing the accuracy of noise tomography travel-time measurements in a realistic medium
https://resolver.caltech.edu/CaltechAUTHORS:20140611-153303533
Year: 2009
DOI: 10.1111/j.1365-246X.2009.04239.x
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.https://resolver.caltech.edu/CaltechAUTHORS:20140611-153303533An explicit relationship between time-domain noise correlation and spatial autocorrelation (SPAC) results
https://resolver.caltech.edu/CaltechAUTHORS:20140612-071051007
Year: 2010
DOI: 10.1111/j.1365-246X.2010.04633.x
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.https://resolver.caltech.edu/CaltechAUTHORS:20140612-071051007A model for turbulent hydraulic fracture and application to crack propagation at glacier beds
https://resolver.caltech.edu/CaltechAUTHORS:20140612-072458326
Year: 2010
DOI: 10.1029/2009JF001474
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.https://resolver.caltech.edu/CaltechAUTHORS:20140612-072458326The relationship between noise correlation and the Green's function in the presence of degeneracy and the absence of equipartition
https://resolver.caltech.edu/CaltechAUTHORS:20140611-150433557
Year: 2010
DOI: 10.1111/j.1365-246X.2010.04693.x
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.https://resolver.caltech.edu/CaltechAUTHORS:20140611-150433557Averaging and sampling for magnetic-observatory hourly data
https://resolver.caltech.edu/CaltechAUTHORS:20140611-145630940
Year: 2010
DOI: 10.5194/angeo-28-2079-2010
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.https://resolver.caltech.edu/CaltechAUTHORS:20140611-145630940A model for seasonal changes in GPS positions and seismic wave speeds due to thermoelastic and hydrologic variations
https://resolver.caltech.edu/CaltechAUTHORS:20140616-103542486
Year: 2011
DOI: 10.1029/2010JB008156
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.https://resolver.caltech.edu/CaltechAUTHORS:20140616-103542486Understanding the amplitudes of noise correlation measurements
https://resolver.caltech.edu/CaltechAUTHORS:20111025-085851425
Year: 2011
DOI: 10.1029/2011JB008483
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.https://resolver.caltech.edu/CaltechAUTHORS:20111025-085851425Constraints on the long-period moment-dip tradeoff for the Tohoku earthquake
https://resolver.caltech.edu/CaltechAUTHORS:20111114-110515606
Year: 2011
DOI: 10.1029/2011GL049129
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.https://resolver.caltech.edu/CaltechAUTHORS:20111114-110515606Are secular correlations between sunspots, geomagnetic activity, and global temperature significant?
https://resolver.caltech.edu/CaltechAUTHORS:20111213-144757573
Year: 2011
DOI: 10.1029/2011GL049380
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.https://resolver.caltech.edu/CaltechAUTHORS:20111213-144757573Quantifying the influence of sea ice on ocean microseism using observations from the Bering Sea, Alaska
https://resolver.caltech.edu/CaltechAUTHORS:20120105-135036867
Year: 2011
DOI: 10.1029/2011GL049791
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.https://resolver.caltech.edu/CaltechAUTHORS:20120105-135036867A physical model for seismic noise generation from sediment transport in rivers
https://resolver.caltech.edu/CaltechAUTHORS:20120228-092625703
Year: 2012
DOI: 10.1029/2011GL050255
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.https://resolver.caltech.edu/CaltechAUTHORS:20120228-092625703Modeling Turbulent Hydraulic Fracture Near a Free Surface
https://resolver.caltech.edu/CaltechAUTHORS:20120523-104717621
Year: 2012
DOI: 10.1115/1.4005879
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.https://resolver.caltech.edu/CaltechAUTHORS:20120523-104717621The 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:20120723-130421973Earthquake in a Maze: Compressional Rupture Branching During the 2012 M_w 8.6 Sumatra Earthquake
https://resolver.caltech.edu/CaltechAUTHORS:20120816-082822417
Year: 2012
DOI: 10.1126/science.1224030
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.https://resolver.caltech.edu/CaltechAUTHORS:20120816-082822417The 2012 Sumatra great earthquake sequence
https://resolver.caltech.edu/CaltechAUTHORS:20121130-081922437
Year: 2012
DOI: 10.1016/j.epsl.2012.07.017
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.https://resolver.caltech.edu/CaltechAUTHORS:20121130-081922437Anomalously steep dips of earthquakes in the 2011 Tohoku-Oki source region and possible explanations
https://resolver.caltech.edu/CaltechAUTHORS:20121214-153612362
Year: 2012
DOI: 10.1016/j.epsl.2012.07.038
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.https://resolver.caltech.edu/CaltechAUTHORS:20121214-153612362Estimating the effect of Earth elasticity and variable water density on tsunami speeds
https://resolver.caltech.edu/CaltechAUTHORS:20130524-103956058
Year: 2013
DOI: 10.1002/grl.50147
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.https://resolver.caltech.edu/CaltechAUTHORS:20130524-103956058Locating a scatterer in the active volcanic area of Southern Peru from ambient noise cross-correlation
https://resolver.caltech.edu/CaltechAUTHORS:20130325-143244581
Year: 2013
DOI: 10.1093/gji/ggs103
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 per cent 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.https://resolver.caltech.edu/CaltechAUTHORS:20130325-143244581Extracting 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:20130627-095929812Using centroid time-delays to characterize source durations and identify earthquakes with unique characteristics
https://resolver.caltech.edu/CaltechAUTHORS:20131017-080058538
Year: 2013
DOI: 10.1016/j.epsl.2013.05.024
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.https://resolver.caltech.edu/CaltechAUTHORS:20131017-080058538Spurious velocity changes caused by temporal variations in ambient noise frequency content
https://resolver.caltech.edu/CaltechAUTHORS:20130919-113717281
Year: 2013
DOI: 10.1093/gji/ggt170
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.https://resolver.caltech.edu/CaltechAUTHORS:20130919-113717281Seismic 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:20131108-135759821Multiple fluvial processes detected by riverside seismic and infrasound monitoring of a controlled flood in the Grand Canyon
https://resolver.caltech.edu/CaltechAUTHORS:20131108-093414888
Year: 2013
DOI: 10.1002/grl.50953
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.https://resolver.caltech.edu/CaltechAUTHORS:20131108-093414888Rupture complexity of the 1994 Bolivia and 2013 Sea of Okhotsk deep earthquakes
https://resolver.caltech.edu/CaltechAUTHORS:20140313-091602304
Year: 2014
DOI: 10.1016/j.epsl.2013.10.028
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 8.3, depth 607 km) and the Great 1994 Bolivia earthquake (M 8.3, 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.https://resolver.caltech.edu/CaltechAUTHORS:20140313-091602304Ambient noise correlation on the Amery Ice Shelf, East Antarctica
https://resolver.caltech.edu/CaltechAUTHORS:20140403-102823691
Year: 2014
DOI: 10.1093/gji/ggt488
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.https://resolver.caltech.edu/CaltechAUTHORS:20140403-1028236913-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:20140613-160117253A physical model for seismic noise generation by turbulent flow in rivers
https://resolver.caltech.edu/CaltechAUTHORS:20150106-082010292
Year: 2014
DOI: 10.1002/2014JF003201
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 Hz 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.https://resolver.caltech.edu/CaltechAUTHORS:20150106-082010292Green's Functions for Surface Waves in a Generic Velocity Structure
https://resolver.caltech.edu/CaltechAUTHORS:20141201-131138693
Year: 2014
DOI: 10.1785/0120140121
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.https://resolver.caltech.edu/CaltechAUTHORS:20141201-131138693Cooling magma model for deep volcanic long-period earthquakes
https://resolver.caltech.edu/CaltechAUTHORS:20150122-080316714
Year: 2014
DOI: 10.1002/2014JB011180
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 m or a cylindrical magma of 74 m radius, thermal strain rates within the intruded magma are higher than tectonic strain rates of ~ 10^(−14) s^(−1) and produce a total strain of 2 × 10^(−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.https://resolver.caltech.edu/CaltechAUTHORS:20150122-080316714Modeling the elastic transmission of tidal stresses to great distances inland in channelized ice streams
https://resolver.caltech.edu/CaltechAUTHORS:20140618-141557404
Year: 2014
DOI: 10.5194/tc-8-2007-2014
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.https://resolver.caltech.edu/CaltechAUTHORS:20140618-141557404An improved model for tidally modulated grounding-line migration
https://resolver.caltech.edu/CaltechAUTHORS:20150312-074943389
Year: 2015
DOI: 10.3189/2015JoG14J152
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.https://resolver.caltech.edu/CaltechAUTHORS:20150312-074943389Marine ice-sheet profiles and stability under Coulomb basal conditions
https://resolver.caltech.edu/CaltechAUTHORS:20150311-105436730
Year: 2015
DOI: 10.3189/2015JoG14J221
The behavior of marine-terminating ice sheets, such as the West Antarctic ice sheet, is of
interest due to the possibility of rapid grounding-line retreat and consequent catastrophic loss of ice.
Critical to modeling this behavior is a choice of basal rheology, where the most popular approach is to
relate the ice-sheet velocity to a power-law function of basal stress. Recent experiments, however,
suggest that near-grounding line tills exhibit Coulomb friction behavior. Here we address how Coulomb
conditions modify ice-sheet profiles and stability criteria. The basal rheology necessarily transitions to
Coulomb friction near the grounding line, due to low effective stresses, leading to changes in ice-sheet
properties within a narrow boundary layer. Ice-sheet profiles 'taper off' towards a flatter upper surface,
compared with the power-law case, and basal stresses vanish at the grounding line, consistent with
observations. In the Coulomb case, the grounding-line ice flux also depends more strongly on flotation
ice thickness, which implies that ice sheets are more sensitive to climate perturbations. Furthermore,
with Coulomb friction, the ice sheet grounds stably in shallower water than with a power-law rheology.
This implies that smaller perturbations are required to push the grounding line into regions of negative
bed slope, where it would become unstable. These results have important implications for ice-sheet
stability in a warming climate.https://resolver.caltech.edu/CaltechAUTHORS:20150311-105436730Seismologically determined bedload flux during the typhoon season
https://resolver.caltech.edu/CaltechAUTHORS:20150310-084646630
Year: 2015
DOI: 10.1038/srep08261
PMCID: PMC4317699
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 Hz) 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.https://resolver.caltech.edu/CaltechAUTHORS:20150310-084646630Site 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 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:20150504-121706460Modeling of subglacial hydrological development following rapid supraglacial lake drainage
https://resolver.caltech.edu/CaltechAUTHORS:20150806-081044372
Year: 2015
DOI: 10.1002/2014JF003333
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.https://resolver.caltech.edu/CaltechAUTHORS:20150806-081044372Time Scale for Rapid Draining of a Surficial Lake Into the Greenland Ice Sheet
https://resolver.caltech.edu/CaltechAUTHORS:20150625-080038328
Year: 2015
DOI: 10.1115/1.4030325
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 hr. 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 °C below melting allow such rapid drainage on a time scale which agrees well with the 2006 observations.https://resolver.caltech.edu/CaltechAUTHORS:20150625-080038328Predicting short-period, wind-wave-generated seismic noise in coastal regions
https://resolver.caltech.edu/CaltechAUTHORS:20150707-153131318
Year: 2015
DOI: 10.1016/j.epsl.2015.06.017
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.https://resolver.caltech.edu/CaltechAUTHORS:20150707-153131318Nonperturbational surface-wave inversion: A Dix-type relation for surface waves
https://resolver.caltech.edu/CaltechAUTHORS:20160211-082242072
Year: 2015
DOI: 10.1190/GEO2014-0612.1
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.https://resolver.caltech.edu/CaltechAUTHORS:20160211-082242072High-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:20160218-105113138Subseasonal changes observed in subglacial channel pressure, size, and sediment transport
https://resolver.caltech.edu/CaltechAUTHORS:20160715-111120593
Year: 2016
DOI: 10.1002/2016GL068337
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.https://resolver.caltech.edu/CaltechAUTHORS:20160715-111120593Offshore Southern California lithospheric velocity structure from noise cross-correlation functions
https://resolver.caltech.edu/CaltechAUTHORS:20160907-162036144
Year: 2016
DOI: 10.1002/2016JB012919
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 km 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.https://resolver.caltech.edu/CaltechAUTHORS:20160907-162036144Evidence for non-self-similarity of microearthquakes recorded at a Taiwan borehole seismometer array
https://resolver.caltech.edu/CaltechAUTHORS:20160818-075652559
Year: 2016
DOI: 10.1093/gji/ggw172
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.https://resolver.caltech.edu/CaltechAUTHORS:20160818-075652559Earthquake ground motion amplification for surface waves
https://resolver.caltech.edu/CaltechAUTHORS:20170217-132237004
Year: 2017
DOI: 10.1002/2016GL071885
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.https://resolver.caltech.edu/CaltechAUTHORS:20170217-132237004Tidal modulation of ice shelf buttressing stresses
https://resolver.caltech.edu/CaltechAUTHORS:20171220-130521553
Year: 2017
DOI: 10.1017/aog.2017.22
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.https://resolver.caltech.edu/CaltechAUTHORS:20171220-130521553Seismic array constraints on reach-scale bedload transport
https://resolver.caltech.edu/CaltechAUTHORS:20170124-083754227
Year: 2017
DOI: 10.1130/G38639.1
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.https://resolver.caltech.edu/CaltechAUTHORS:20170124-083754227Perturbational and nonperturbational inversion of Rayleigh-wave velocities
https://resolver.caltech.edu/CaltechAUTHORS:20170908-092450206
Year: 2017
DOI: 10.1190/GEO2016-0397.1
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.https://resolver.caltech.edu/CaltechAUTHORS:20170908-092450206Explaining Extreme Ground Motion in Osaka Basin during the 2011 Tohoku Earthquake
https://resolver.caltech.edu/CaltechAUTHORS:20170721-073805550
Year: 2017
DOI: 10.1002/2017GL074120
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.https://resolver.caltech.edu/CaltechAUTHORS:20170721-073805550Rayleigh-Wave H/V via Noise Cross Correlation in Southern California
https://resolver.caltech.edu/CaltechAUTHORS:20171102-082548374
Year: 2017
DOI: 10.1785/0120170051
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.https://resolver.caltech.edu/CaltechAUTHORS:20171102-082548374Was the M_w 7.5 1952 Kern County, California, earthquake induced (or triggered)?
https://resolver.caltech.edu/CaltechAUTHORS:20171208-073841565
Year: 2017
DOI: 10.1007/s10950-017-9685-x
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.https://resolver.caltech.edu/CaltechAUTHORS:20171208-073841565Seismologically Observed Spatiotemporal Drainage Activity at Moulins
https://resolver.caltech.edu/CaltechAUTHORS:20180117-155132585
Year: 2017
DOI: 10.1002/2017JB014578
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.https://resolver.caltech.edu/CaltechAUTHORS:20180117-155132585Toward automated directivity estimates in earthquake moment tensor inversion
https://resolver.caltech.edu/CaltechAUTHORS:20171026-083324515
Year: 2017
DOI: 10.1093/gji/ggx354
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.https://resolver.caltech.edu/CaltechAUTHORS:20171026-083324515Amplification 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:20171115-155052574Vital Signs: Seismology of Icy Ocean Worlds
https://resolver.caltech.edu/CaltechAUTHORS:20180122-074827302
Year: 2018
DOI: 10.1089/ast.2016.1612
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.https://resolver.caltech.edu/CaltechAUTHORS:20180122-074827302Expected seismicity and the seismic noise environment of Europa
https://resolver.caltech.edu/CaltechAUTHORS:20171201-112529873
Year: 2018
DOI: 10.1002/2017JE005332
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.https://resolver.caltech.edu/CaltechAUTHORS:20171201-112529873The Seismic Signature of Debris Flows: Flow Mechanics and Early Warning at Montecito, California
https://resolver.caltech.edu/CaltechAUTHORS:20180530-083814651
Year: 2018
DOI: 10.1029/2018GL077683
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.https://resolver.caltech.edu/CaltechAUTHORS:20180530-083814651Observations and modeling of long-period ground-motion amplification across northeast China
https://resolver.caltech.edu/CaltechAUTHORS:20180608-144450318
Year: 2018
DOI: 10.1029/2018GL078212
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.https://resolver.caltech.edu/CaltechAUTHORS:20180608-144450318A simple physics-based improvement to the positive degree day model
https://resolver.caltech.edu/CaltechAUTHORS:20180912-135435614
Year: 2018
DOI: 10.1017/jog.2018.55
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.https://resolver.caltech.edu/CaltechAUTHORS:20180912-135435614A 3D Broadband Seismometer Array Experiment at the Homestake Mine
https://resolver.caltech.edu/CaltechAUTHORS:20180822-151337473
Year: 2018
DOI: 10.1785/0220170228
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 km in each direction, thus spanning a 3D volume of about 3.4 km^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.https://resolver.caltech.edu/CaltechAUTHORS:20180822-151337473A Simple Model for Deglacial Meltwater Pulses
https://resolver.caltech.edu/CaltechAUTHORS:20181022-091459376
Year: 2018
DOI: 10.1029/2018GL080884
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.https://resolver.caltech.edu/CaltechAUTHORS:20181022-091459376Particle transport mechanics and induced seismic noise in steep flume experiments with accelerometer-embedded tracers
https://resolver.caltech.edu/CaltechAUTHORS:20181127-075323708
Year: 2019
DOI: 10.1002/esp.4495
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.https://resolver.caltech.edu/CaltechAUTHORS:20181127-075323708Coherence-based approaches for estimating the composition of the seismic wavefield
https://resolver.caltech.edu/CaltechAUTHORS:20190312-101923328
Year: 2019
DOI: 10.1029/2018jb016608
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.https://resolver.caltech.edu/CaltechAUTHORS:20190312-101923328Direct Observations of Surface‐Wave Eigenfunctions at the Homestake 3D Array
https://resolver.caltech.edu/CaltechAUTHORS:20190625-083425940
Year: 2019
DOI: 10.1785/0120190026
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.https://resolver.caltech.edu/CaltechAUTHORS:20190625-083425940A physical model of the high-frequency seismic signal generated by debris flows
https://resolver.caltech.edu/CaltechAUTHORS:20190604-151340833
Year: 2019
DOI: 10.1002/esp.4677
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.https://resolver.caltech.edu/CaltechAUTHORS:20190604-151340833Validation of a fast semi-analytic method for surface-wave propagation in layered media
https://resolver.caltech.edu/CaltechAUTHORS:20191031-104134697
Year: 2019
DOI: 10.1093/gji/ggz351
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.https://resolver.caltech.edu/CaltechAUTHORS:20191031-104134697Frequency-Dependent P Wave Polarization and Its Subwavelength Near-Surface Depth Sensitivity
https://resolver.caltech.edu/CaltechAUTHORS:20200103-074516932
Year: 2019
DOI: 10.1029/2019gl084892
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.https://resolver.caltech.edu/CaltechAUTHORS:20200103-074516932Geometric and Level Set Tomography using Ensemble Kalman Inversion
https://resolver.caltech.edu/CaltechAUTHORS:20200130-142308238
Year: 2020
DOI: 10.1093/gji/ggz472
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.https://resolver.caltech.edu/CaltechAUTHORS:20200130-142308238Did Oldham Discover the Core After All? Handling Imprecise Historical Data with Hierarchical Bayesian Model Selection Methods
https://resolver.caltech.edu/CaltechAUTHORS:20200506-132625904
Year: 2020
DOI: 10.1785/0220190266
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.https://resolver.caltech.edu/CaltechAUTHORS:20200506-132625904Extension of the Basin Rayleigh-Wave Amplification Theory to Include Basin-Edge Effects
https://resolver.caltech.edu/CaltechAUTHORS:20200506-125019256
Year: 2020
DOI: 10.1785/0120190161
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.https://resolver.caltech.edu/CaltechAUTHORS:20200506-125019256Time-Dependent Stresses From Fluid Extraction and Diffusion With Applications to Induced Seismicity
https://resolver.caltech.edu/CaltechAUTHORS:20200813-074456541
Year: 2020
DOI: 10.1115/1.4047034
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.https://resolver.caltech.edu/CaltechAUTHORS:20200813-074456541Measuring Basal Force Fluctuations of Debris Flows Using Seismic Recordings and Empirical Green's Functions
https://resolver.caltech.edu/CaltechAUTHORS:20200826-103628260
Year: 2020
DOI: 10.1029/2020jf005590
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.https://resolver.caltech.edu/CaltechAUTHORS:20200826-103628260Parsimonious Velocity Inversion Applied to the Los Angeles Basin, CA
https://resolver.caltech.edu/CaltechAUTHORS:20231006-172908203
Year: 2022
DOI: 10.1029/2021jb023103
The proliferation of dense arrays promises to improve our ability to image geological structures at the scales necessary for accurate assessment of seismic hazard. However, combining the resulting local high-resolution tomography with existing regional models presents an ongoing challenge. We developed a framework based on the level-set method that infers where local data provide meaningful constraints beyond those found in regional models - for example the Community Velocity Models (CVMs) of southern California. This technique defines a volume within which updates are made to a reference CVM, with the boundary of the volume being part of the inversion rather than explicitly defined. By penalizing the complexity of the boundary, a minimal update that sufficiently explains the data is achieved. To test this framework, we use data from the Community Seismic Network, a dense permanent urban deployment. We inverted Love wave dispersion and amplification data, from the Mw 6.4 and 7.1 2019 Ridgecrest earthquakes. We invert for an update to CVM-S4.26 using the Tikhonov Ensemble Sampling scheme, a highly efficient derivative-free approximate Bayesian method. We find the data are best explained by a deepening of the Los Angeles Basin with its deepest part south of downtown Los Angeles, along with a steeper northeastern basin wall. This result offers new progress toward the parsimonious incorporation of detailed local basin models within regional reference models utilizing an objective framework and highlights the importance of accurate basin models when accounting for the amplification of surface waves in the high-rise building response band.https://resolver.caltech.edu/CaltechAUTHORS:20231006-172908203Seismic Mapping of Subglacial Hydrology Reveals Previously Undetected Pressurization Event
https://resolver.caltech.edu/CaltechAUTHORS:20220217-687235000
Year: 2022
DOI: 10.1029/2021jf006406
Understanding the dynamic response of glaciers to climate change is vital for assessing water resources and hazards, and subglacial hydrology is a key player in glacier systems. Traditional observations of subglacial hydrology are spatially and temporally limited, but recent seismic deployments on and around glaciers show the potential for comprehensive observation of glacial hydrologic systems. We present results from a high-density seismic deployment spanning the surface of Lemon Creek Glacier, Alaska. Our study coincided with a marginal lake drainage event, which served as a natural experiment for seismic detection of changes in subglacial hydrology. We observed glaciohydraulic tremor across the surface of the glacier that was generated by the subglacial hydrologic system. During the lake drainage, the relative changes in seismic tremor power and water flux are consistent with pressurization of the subglacial system of only the upper part of the glacier. This event was not accompanied by a significant increase in glacier velocity; either some threshold necessary for rapid basal motion was not attained, or, plausibly, the geometry of Lemon Creek Glacier inhibited speedup. This pressurization event would have likely gone undetected without seismic observations, demonstrating the power of cryoseismology in testing assumptions about and mapping the spatial extent of subglacial pressurization.https://resolver.caltech.edu/CaltechAUTHORS:20220217-687235000