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Schulte-Pelkum, V, Monsalve G, Sheehan AF, Shearer P, Wu F, Rajaure S.  2019.  Mantle earthquakes in the Himalayan collision zone. Geology. 47:815-819.   10.1130/g46378.1   AbstractWebsite

Earthquakes are known to occur beneath southern Tibet at depths up to similar to 95 km. Whether these earthquakes occur within the lower crust thickened in the Himalayan collision or in the mantle is a matter of current debate. Here we compare vertical travel paths expressed as delay times between S and P arrivals for local events to delay times of P-to-S conversions from the Moho in receiver functions. The method removes most of the uncertainty introduced in standard analysis from using velocity models for depth location and migration. We show that deep seismicity in southern Tibet is unequivocally located beneath the Moho in the mantle. Deep seismicity in continental lithosphere occurs under normally ductile conditions and has therefore garnered interest in whether its occurrence is due to particularly cold temperatures or whether other factors are causing embrittlement of ductile material. Eclogitization in the subducting Indian crust has been proposed as a cause for the deep seismicity in this area. Our observation of seismicity in the mantle, falling below rather than within the crustal layer with proposed eclogitization, requires revisiting this concept and favors other embrittlement mechanisms that operate within mantle material.

Ross, ZE, Trugman DT, Hauksson E, Shearer PM.  2019.  Searching for hidden earthquakes in Southern California. Science. 364:767-+.   10.1126/science.aaw6888   AbstractWebsite

Earthquakes follow a well-known power-law size relation, with smaller events occurring much more often than larger events. Earthquake catalogs are thus dominated by small earthquakes yet are still missing a much larger number of even smaller events because of signal fidelity issues. To overcome these limitations, we applied a template-matching detection technique to the entire waveform archive of the regional seismic network in Southern California. This effort resulted in a catalog with 1.81 million earthquakes, a 10-fold increase, which provides important insights into the geometry of fault zones at depth, foreshock behavior and nucleation processes, and earthquake-triggering mechanisms. The rich detail resolved in this type of catalog will facilitate the next generation of analyses of earthquakes and faults.

Shearer, PM, Abercrombie RE, Trugman DT, Wang W.  2019.  Comparing EGF methods for estimating corner frequency and stress drop from p wave spectra. Journal of Geophysical Research-Solid Earth. 124:3966-3986.   10.1029/2018jb016957   AbstractWebsite

Empirical Green's functions (EGFs) are widely applied to correct earthquake spectra for attenuation and other path effects in order to estimate corner frequencies and stress drops, but these source parameter estimates often exhibit poor agreement between different studies. We examine this issue by analyzing a compact cluster of over 3,000 aftershocks of the 1992 Landers earthquake. We apply and compare two different analysis and modeling methods: (1) the spectral decomposition and global EGF fitting approach and (2) a more traditional EGF method of modeling spectral ratios. We find that spectral decomposition yields event terms that are consistent with stacks of spectral ratios for individual events, but source parameter estimates nonetheless vary between the methods. The main source of differences comes from the modeling approach used to estimate the EGF. The global EGF-fitting approach suffers from parameter trade-offs among the absolute stress drop, the stress drop scaling with moment, and the high-frequency falloff rate but has the advantage that the relative spectral shapes and stress drops among the different events in the cluster are well resolved even if their absolute levels are not. The spectral ratio approach solves for a different EGF for each target event without imposing any constraint on the corner frequency, f(c), of the smaller events, and so can produce biased results for target event f(c). Placing constraints on the small-event f(c) improves the performance of the spectral ratio method and enables the two methods to yield very similar results.

Wang, W, Shearer PM.  2019.  An improved method to determine coda-Q, earthquake magnitude, and site amplification: Theory and application to Southern California. Journal of Geophysical Research-Solid Earth. 124:578-598.   10.1029/2018jb015961   AbstractWebsite

Seismic coda waves can be used to constrain attenuation, estimate earthquake magnitudes, and determine site amplification factors. We have developed a new multistation and multievent method to determine these three important seismic parameters simultaneously. We analyze 642 representative local (100 km) and shallow (20 km) earthquakes with catalog magnitudes between 1.8 and 5.4 in southern California at multiple frequency bands centered at 1.5, 3, 6, and 12 Hz. We find that the length of the moving average time window can affect the measurement of coda attenuation Q(C), but our tests indicate that the optimal window length is about 15 times the dominant data period. We use linear regression to fit each coda section and use only those portions that agree with the model decay rate with a correlation coefficient larger than 0.9. For a frequency-dependent coda-Q(C) model (Q(C) = Q(0)f(n)) at 1-Hz reference frequency, our results yield estimates for Q(0) and n of 107-288 and 0.42-1.14, respectively. Our coda magnitude estimates are linearly correlated with catalog magnitudes, and our observed lateral variations in coda-Q(C) and our site amplification factors are in general agreement with previous results, although there are notable differences at some locations. This approach provides a unified, accurate, and stable method to measure coda-Q(C), earthquake magnitude, and site amplification using coda waves of locally recorded earthquakes.

Koper, KD, Pankow KL, Pechmann JC, Hale JM, Burlacu R, Yeck WL, Benz HM, Herrmann RB, Trugman DT, Shearer PM.  2018.  Afterslip enhanced aftershock activity during the 2017 earthquake sequence near Sulphur Peak, Idaho. Geophysical Research Letters. 45:5352-5361.   10.1029/2018gl078196   AbstractWebsite

An energetic earthquake sequence occurred during September to October 2017 near Sulphur Peak, Idaho. The normal-faulting M-w 5.3 mainshock of 2 September 2017 was widely felt in Idaho, Utah, and Wyoming. Over 1,000 aftershocks were located within the first 2 months, 29 of which had magnitudes >= 4.0 M-L. High-accuracy locations derived with data from a temporary seismic array show that the sequence occurred in the upper (<10km) crust of the Aspen Range, east of the northern section of the range-bounding, west-dipping East Bear Lake Fault. Moment tensors for 77 of the largest events show normal and strike-slip faulting with a summed aftershock moment that is 1.8-2.4 times larger than the mainshock moment. We propose that the unusually high productivity of the 2017 Sulphur Peak sequence can be explained by aseismic afterslip, which triggered a secondary swarm south of the coseismic rupture zone beginning similar to 1 day after the mainshock. Plain Language Summary During the fall of 2017, an energetic sequence of earthquakes was recorded in southeastern Idaho. The mainshock had a moment magnitude of M-w 5.3, yet thousands of aftershocks were detected. We found that the unusually high productivity of this earthquake sequence can be explained by extra sliding that occurred just after the mainshock. This extra sliding happened too slowly to generate seismic waves, but it was large enough to alter the stress in the crust such that the extra aftershocks were created. Our finding suggests that in this region of Idaho, some of the strain that is built up by tectonic forces is released in slow-slip or creep events. This discovery will ultimately lead to more accurate forecasts of seismic hazard in the region.

Buehler, JS, Mancinelli NJ, Shearer PM.  2018.  S-to-Rayleigh wave scattering from the continental margin observed at USArray. Geophysical Research Letters. 45:4719-4724.   10.1029/2017gl076812   AbstractWebsite

We show examples of teleseismic S waves from western Pacific earthquakes converting to surface waves near the western U.S. coastline. Many of these events originate in the Tonga-Samoa region. We observe these surface wave conversions at USArray stations at relatively long periods (>10s). The amplitudes vary considerably from station to station and appear highly amplified in the Yellowstone region. Two-dimensional spectral element simulations successfully generate scattered Rayleigh waves from incident SV waves and models with surface topography at the coastline and crustal thickness variations across the margin, although simple models cannot explain the large Rayleigh wave amplitudes (greater than the direct S wave amplitude) observed in some regions, suggesting that the wave train is amplified by local structure or 3-D focusing effects. Plain Language Summary We show new observations of body-to-surface wave conversions at the U.S. continental margin observed with USArray. Simulation results show how simple surface topography at the coastline can successfully generate scattered Rayleigh waves from incident SV waves. These converted surface waves may be an important source of signal-generated noise in continental seismology, in particular in studies of seismic phases and parts of the wavefield between direct S and later-arriving surface waves from the source.

Fan, WY, Shearer PM.  2018.  Coherent Seismic Arrivals in the P Wave Coda of the 2012 M(w)7.2 Sumatra Earthquake: Water Reverberations or an Early Aftershock? Journal of Geophysical Research-Solid Earth. 123:3147-3159.   10.1002/2018jb015573   AbstractWebsite

Teleseismic records of the 2012M(w)7.2 Sumatra earthquake contain prominent phases in the P wave train, arriving about 50 to 100s after the direct P arrival. Azimuthal variations in these arrivals, together with back-projection analysis, led Fan and Shearer (, ) to conclude that they originated from early aftershock(s), located approximate to 150 km northeast of the mainshock and landward of the trench. However, recently, Yue et al. (, ) argued that the anomalous arrivals are more likely water reverberations from the mainshock, based mostly on empirical Green's function analysis of a M6 earthquake near the mainshock and a water phase synthetic test. Here we present detailed back-projection and waveform analyses of three M6 earthquakes within 100km of the M(w)7.2 earthquake, including the empirical Green's function event analyzed in Yue et al. (, ). In addition, we examine the waveforms of three M5.5 reverse-faulting earthquakes close to the inferred early aftershock location in Fan and Shearer (, ). These results suggest that the reverberatory character of the anomalous arrivals in the mainshock coda is consistent with water reverberations, but the origin of this energy is more likely an early aftershock rather than delayed and displaced water reverberations from the mainshock.

Trugman, DT, Shearer PM.  2018.  Strong correlation between stress drop and peak ground acceleration for recent m 1-4 earthquakes in the San Francisco Bay Area. Bulletin of the Seismological Society of America. 108:929-945.   10.1785/0120170245   AbstractWebsite

Theoretical and observational studies suggest that between-event variability in the median ground motions of larger (M >= 5) earthquakes is controlled primarily by the dynamic properties of the earthquake source, such as Brune-type stress drop. Analogous results remain equivocal for smaller events due to the lack of comprehensive and overlapping ground-motion and source-parameter datasets in this regime. Here, we investigate the relationship between peak ground acceleration (PGA) and dynamic stress drop for a new dataset of 5297 earthquakes that occurred in the San Francisco Bay area from 2002 through 2016. For each event, we measure PGA on horizontal-component channels of stations within 100 km and estimate stress drop from P-wave spectra recorded on vertical-component channels of the same stations. We then develop a nonparametric ground-motion prediction equation (GMPE) applicable for the moderate (M 1-4) earthquakes in our study region, using a mixed-effects generalization of the Random Forest algorithm. We use the Random Forest GMPE to model the joint influence of magnitude, distance, and near-site effects on observed PGA. We observe a strong correlation between dynamic stress drop and the residual PGA of each event, with the events with higher-than-expected PGA associated with higher values of stress drop. The strength of this correlation increases as a function of magnitude but remains significant even for smaller magnitude events with corner frequencies that approach the observable bandwidth of the acceleration records. Mainshock events are characterized by systematically higher stress drop and PGA than aftershocks of equivalent magnitude. Coherent local variations in the distribution of dynamic stress drop provide observational constraints to support the future development of nonergodic GMPEs that account for variations in median stress drop at different source locations. Electronic Supplement: Figures showing the relation between M-w and M-L, comparison of the ground-motion measurements from this study with the cross-listed records in the Next Generation Attenuation ground-motion database, the validation curve used to select the optimal tree depth for the Random Forest ground-motion prediction equation (GMPE) used in this study, the between-event ground-motion residual is plotted versus: (a) stress drop, (b) magnitude-adjusted stress drop, (c) depth, and (d) depth-adjusted stress drop, a table containing the ground-motion and stressdrop measurements associated with this study, and an example Python notebook.

Fan, WY, Bassett D, Jiang JL, Shearer PM, Ji C.  2017.  Rupture evolution of the 2006 Java tsunami earthquake and the possible role of splay faults. Tectonophysics. 721:143-150.   10.1016/j.tecto.2017.10.003   AbstractWebsite

The 2006 Mw 7.8 Java earthquake was a tsunami earthquake, exhibiting frequency-dependent seismic radiation along strike. High-frequency global back-projection results suggest two distinct rupture stages. The first stage lasted similar to 65 s with a rupture speed of similar to 1.2 km/s, while the second stage lasted from similar to 65 to 150 s with a rupture speed of similar to 2.7 km/s. High-frequency radiators resolved with back-projection during the second stage spatially correlate with splay fault traces mapped from residual free-air gravity anomalies. These splay faults also colocate with a major tsunami source associated with the earthquake inferred from tsunami first-crest back-propagation simulation. These correlations suggest that the splay faults may have been reactivated during the Java earthquake, as has been proposed for other tsunamigenic earthquakes, such as the 1944 Mw 8.1 Tonankai earthquake in the Nankai Trough.

Fan, WY, Shearer PM.  2017.  Investigation of Backprojection Uncertainties With M6 Earthquakes. Journal of Geophysical Research-Solid Earth. 122:7966-7986.   10.1002/2017jb014495   AbstractWebsite

We investigate possible biasing effects of inaccurate timing corrections on teleseismic P wave backprojection imaging of large earthquake ruptures. These errors occur because empirically estimated time shifts based on aligning P wave first arrivals are exact only at the hypocenter and provide approximate corrections for other parts of the rupture. Using the Japan subduction zone as a test region, we analyze 46 M6-M7 earthquakes over a 10year period, including many aftershocks of the 2011 M9 Tohoku earthquake, performing waveform cross correlation of their initial P wave arrivals to obtain hypocenter timing corrections to global seismic stations. We then compare backprojection images for each earthquake using its own timing corrections with those obtained using the time corrections from other earthquakes. This provides a measure of how well subevents can be resolved with backprojection of a large rupture as a function of distance from the hypocenter. Our results show that backprojection is generally very robust and that the median subevent location error is about 25km across the entire study region (approximate to 700km). The backprojection coherence loss and location errors do not noticeably converge to zero even when the event pairs are very close (<20km). This indicates that most of the timing differences are due to 3-D structure close to each of the hypocenter regions, which limits the effectiveness of attempts to refine backprojection images using aftershock calibration, at least in this region.

Trugman, DT, Dougherty SL, Cochran ES, Shearer PM.  2017.  Source spectral properties of small to moderate earthquakes in southern Kansas. Journal of Geophysical Research: Solid Earth. :n/a-n/a.   10.1002/2017JB014649   Abstract

The source spectral properties of injection-induced earthquakes give insight into their nucleation, rupture processes, and influence on ground motion. Here we apply a spectral decomposition approach to analyze P wave spectra and estimate Brune-type stress drop for more than 2,000 ML1.5–5.2 earthquakes occurring in southern Kansas from 2014 to 2016. We find that these earthquakes are characterized by low stress drop values (median ∼0.4 MPa) compared to natural seismicity in California. We observe a significant increase in stress drop as a function of depth, but the shallow depth distribution of these events is not by itself sufficient to explain their lower stress drop. Stress drop increases with magnitude from M1.5 to M3.5, but this scaling trend may weaken above M4 and also depends on the assumed source model. Although we observe a nonstationary, sequence-specific temporal evolution in stress drop, we find no clear systematic relation with the activity of nearby injection wells.

Wang, W, Shearer PM.  2017.  Using direct and coda wave envelopes to resolve the scattering and intrinsic attenuation structure of Southern California. Journal of Geophysical Research-Solid Earth. 122:7236-7251.   10.1002/2016jb013810   AbstractWebsite

Characterizing scattering and absorbing properties and the power spectrum of crustal heterogeneity is a fundamental problem for informing strong ground motion estimates at high frequencies, where scattering and attenuation effects are critical. We perform a comprehensive study of local earthquake coda waves in Southern California to constrain scattering and intrinsic attenuation structure. We analyze data from 1195 spatially distributed earthquakes from 1981 to 2013 at source depths of 10 to 15 km and epicentral distances from 0 to 250 km with magnitudes larger than 1.8. We stack envelope functions from 28,127 vertical component and 27,521 transverse component seismograms, filtered from 2 to 4 Hz. We model these observations using a particle-based Monte Carlo algorithm that includes intrinsic attenuation as well as both P and S wave scattering and both single and multiple scattering events. We find that spatially averaged coda wave behavior for Southern California can be explained only with models containing an increase in scattering strength and intrinsic attenuation within the uppermost crust, i.e., they are poorly fit with half-space models of constant scattering strength. A reasonable fit to our data is obtained with a two-layer model, composed of a shallow crustal layer with strong wide-angle scattering and high P and S intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation (top 5.5 km: (alpha)Q(I) = 250, (beta)Q(I) = 125, heterogeneity correlation length a = 50 m, fractional velocity heterogeneity epsilon = 0.4; lower crust: (alpha)Q(I) = 900, (beta)Q(I) = 400, a = 2 km, epsilon = 0.05). Plain Language Summary In summary, we have built a one-dimensional depth-dependent intrinsic and scattering attenuation model of Southern California to resolve the characteristics of the high-frequency (2-4 Hz) wavefield for events from 1981 to 2013. The envelope function stacking method provides the spatially averaged coda energy, including both the P and S wave coda decay and their relative amplitude information. We model the data stacks using energy-conserving and multiscattering regional Monte Carlo simulations. The synthetic results show the Southern California region can be reasonably fit with a two-layered model composed of a shallow crustal layer with strong wide-angle scattering and high intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation.

Wei, SS, Shearer PM.  2017.  A sporadic low-velocity layer atop the 410 km discontinuity beneath the Pacific Ocean. Journal of Geophysical Research-Solid Earth. 122:5144-5159.   10.1002/2017jb014100   AbstractWebsite

Waveforms of SS precursors recorded by global stations are analyzed to investigate lateral heterogeneities of upper mantle discontinuities on a global scale. A sporadic low-velocity layer immediately above the 410 km discontinuity (LVL-410) is observed worldwide, including East Asia, western North America, eastern South America, the Pacific Ocean, and possibly the Indian Ocean. Our best data coverage is for the Pacific Ocean, where the LVL-410 covers 33-50% of the resolved region. Lateral variations of our LVL-410 observations show no geographical correlation with 410 km discontinuity topography or tomographic models of seismic velocity, suggesting that the LVL-410 is not caused by regional thermal anomalies. We interpret the LVL-410 as partial melting due to dehydration of ascending mantle across the 410 km discontinuity, which is predicted by the transition zone water filter hypothesis. Given the low vertical resolution of SS precursors, it is possible that the regions without a clear LVL-410 detection also have a thin layer. Therefore, the strong lateral heterogeneity of the LVL-410 in our observations suggests partial melting with varying intensities across the Pacific and further provides indirect evidence of a hydrous mantle transition zone with laterally varying water content.

Buehler, JS, Shearer PM.  2017.  Uppermost mantle seismic velocity structure beneath USArray. Journal of Geophysical Research-Solid Earth. 122:436-448.   10.1002/2016jb013265   AbstractWebsite

We apply Pn tomography beneath the entire USArray footprint to image uppermost mantle velocity structure and anisotropy, as well as crustal thickness constraints, beneath the United States. The sparse source distribution in the eastern United States and the resulting longer raypaths provide new challenges and justify the inclusion of additional parameters that account for the velocity gradient in the mantle lid. At large scale, Pn velocities are higher in the eastern United States compared to the west, but we observe patches of lower velocities around the New Madrid seismic zone and below the eastern Appalachians. For much of the mantle lid below the central and eastern United States we find a moderate positive velocity gradient. In the western United States, we observe a moderate gradient in the region of the Juan de Fuca subduction zone, but no significant gradient to the south and east of this region. In terms of anisotropy, we find that the Pn fast axes generally do not agree with SKS splitting orientations, suggesting significant vertical changes in anisotropy in the upper mantle. In particular the circular pattern of the fast polarization direction of SKS in the western United States is much less pronounced in the Pn results, and in the eastern US the dominant Pn fast direction is approximately north-south, whereas the SKS fast polarizations are oriented roughly parallel to the absolute plate motion direction.

Trugman, DT, Shearer PM.  2017.  Application of an improved spectral decomposition method to examine earthquake source scaling in Southern California. Journal of Geophysical Research: Solid Earth. Abstract
Matoza, RS, Green DN, Le Pichon A, Shearer PM, Fee D, Mialle P, Ceranna L.  2017.  Automated detection and cataloging of global explosive volcanism using the International Monitoring System infrasound network. Journal of Geophysical Research: Solid Earth. Abstract
Buehler, JS, Shearer PM.  2016.  Characterizing earthquake location uncertainty in North America using source-receiver reciprocity and USArray. Bulletin of the Seismological Society of America. 106:2395-2401.   10.1785/0120150173   AbstractWebsite

The Comprehensive Nuclear-Test-Ban Treaty community often uses calibration events with well-determined origins to improve absolute locations of nearby seismic events by accounting for the biasing effects of unknown velocity structure, but the number of these ground-truth events is limited. To provide additional constraints, source-receiver reciprocity allows us to use seismic stations as calibration events with known locations. The dense and uniform spacing of the USArray transportable array stations makes them ideal to measure the spatial coherence of mislocation vectors across North America and hence to assess how close calibration events (or stations) need to be to target events to improve locations for a given region. We use a gridsearch approach for the station"relocations," using both teleseismic earthquakes and simulated regional events. Our results show that the mislocation vectors are spatially coherent for scales up to 500 km in many regions, but that in some places, such as regions that can be associated with strong velocity anomalies in the upper mantle, mislocation vectors exhibit large changes over short distances.

Fan, WY, Shearer PM.  2016.  Local near instantaneously dynamically triggered aftershocks of large earthquakes. Science. 353:1133-1136.   10.1126/science.aag0013   AbstractWebsite

Aftershocks are often triggered by static- and/or dynamic-stress changes caused by mainshocks. The relative importance of the two triggering mechanisms is controversial at near-to-intermediate distances. We detected and located 48 previously unidentified large early aftershocks triggered by earthquakes with magnitudes between >= 7 and 8 within a few fault lengths (approximately 300 kilometers), during times that high-amplitude surface waves arrive from the mainshock (less than 200 seconds). The observations indicate that near-to-intermediate-field dynamic triggering commonly exists and fundamentally promotes aftershock occurrence. The mainshocks and their nearby early aftershocks are located at major subduction zones and continental boundaries, and mainshocks with all types of faulting-mechanisms (normal, reverse, and strike-slip) can trigger early aftershocks.

Denolle, MA, Shearer PM.  2016.  New perspectives on self-similarity for shallow thrust earthquakes. Journal of Geophysical Research-Solid Earth. 121:6533-6565.   10.1002/2016jb013105   AbstractWebsite

Scaling of dynamic rupture processes from small to large earthquakes is critical to seismic hazard assessment. Large subduction earthquakes are typically remote, and we mostly rely on teleseismic body waves to extract information on their slip rate functions. We estimate the P wave source spectra of 942 thrust earthquakes of magnitude M-w 5.5 and above by carefully removing wave propagation effects (geometrical spreading, attenuation, and free surface effects). The conventional spectral model of a single-corner frequency and high-frequency falloff rate does not explain our data, and we instead introduce a double-corner-frequency model, modified from the Haskell propagating source model, with an intermediate falloff of f(-1). The first corner frequency f(1) relates closely to the source duration T-1, its scaling follows M0T13for M-w<7.5, and changes to M0T12 for larger earthquakes. An elliptical rupture geometry better explains the observed scaling than circular crack models. The second time scale T-2 varies more weakly with moment, M0T25, varies weakly with depth, and can be interpreted either as expressions of starting and stopping phases, as a pulse-like rupture, or a dynamic weakening process. Estimated stress drops and scaled energy (ratio of radiated energy over seismic moment) are both invariant with seismic moment. However, the observed earthquakes are not self-similar because their source geometry and spectral shapes vary with earthquake size. We find and map global variations of these source parameters.

Fan, WY, Shearer PM, Ji C, Bassett D.  2016.  Multiple branching rupture of the 2009 Tonga-Samoa earthquake. Journal of Geophysical Research-Solid Earth. 121:5809-5827.   10.1002/2016jb012945   AbstractWebsite

Several source models have been proposed to explain the enigmatic 2009 Tonga-Samoa earthquake. The long-period data require a composite source model and can be fit with a normal-faulting subevent followed by one or more reverse-faulting subevents. The short-period data, in contrast, indicate a more compact rupture pattern around the epicenter. The lack of a unified source model reflects the complexity of the event. We analyze the spatiotemporal evolution of this earthquake with P wave back-projection from globally distributed stations in different frequency bands (low frequency: 0.05-0.2Hz, high frequency: 0.2-2Hz) and a multiple moment tensor inversion. The rupture propagation revealed by back-projection exhibits frequency-dependent behavior, with two branches of high-frequency-enriched bilateral rupture around the epicenter and a high-frequency-deficient rupture branch at the subduction interface. A composite source model with one M(w)8.0 normal-faulting earthquake east of the trench axis (seaward) followed by one M(w)8.1 reverse-faulting earthquake along the subduction interface west of the trench axis (landward) can explain the very long period data (200 approximate to 500s). Combined with high-resolution swath bathymetry data, the back-projection images show that the azimuth of rupture branches east of the trench axis were controlled by the geometry of bending-related faults on the Pacific plate and that the rupture branch west of the trench axis may correlate with the along-strike fore-arc segmentation. The rupture along the subduction interface was triggered by the seaward rupture and a partially subducted normal fault may have played a key role in facilitating the triggering. The apparent normal-reverse faulting interactions pose a higher seismic risk to this region than their individual strands at the northernmost corner of the Tonga subduction zone.

Mancinelli, N, Shearer P.  2016.  Scattered energy from a rough core-mantle boundary modeled by a Monte Carlo seismic particle method: Application to PKKP precursors. Geophysical Research Letters. 43:7963-7972.   10.1002/2016gl070286   AbstractWebsite

We stack a large global data set of 1Hz PKKP waveforms to constrain globally averaged properties of PKKP precursors. We find that the precursor observations are better explained by scattering from core-mantle boundary (CMB) topography than by scattering from the near surface, lower mantle, outer core, or inner core. However, as previously noted, simple models of CMB topography and standard 1-D seismic velocity models fail to model the range dependence of the relative amplitude between PKKPbc and its precursors. We find that this systematic mismatch is due, at least in part, to the assumed velocity gradient in the lowermost 250km of the outer core. Our globally averaged PKKP precursor observations are consistent with random CMB topography with RMS variations of approximate to 390m and a horizontal correlation length of approximate to 7km.

Mancinelli, N, Shearer P, Liu QY.  2016.  Constraints on the heterogeneity spectrum of Earth's upper mantle. Journal of Geophysical Research-Solid Earth. 121:3703-3721.   10.1002/2015jb012641   AbstractWebsite

We constrain the heterogeneity spectrum of Earth's upper mantle at scales from a few kilometers to tens of thousands of kilometers using observations from high-frequency scattering, long-period scattering, and tomography. Tomography and high-frequency scattering constraints are drawn from previous studies, but constraints on mantle heterogeneity at intermediate scales (5-500 km) are lacking. To address this, we stack similar to 15,000 long-period P coda envelopes to characterize the globally averaged scattered wavefield at periods from 5 to 60 s and at ranges from 50 to 98 degrees. To fit these observations, we consider models of random mantle heterogeneity and compute the corresponding global wavefield using both a ray theoretical "seismic particle" approach and full spectral element simulations. Von Karman random media distributed throughout the uppermost 600 km of the mantle with a = 2000 km, epsilon = 10%, and kappa = 0.05 provide a good fit to the time, range, and frequency dependence of the stacks, although there is a trade-off between epsilon and the thickness of the assumed scattering layer. This random media model also fits previously published 1 Hz stacks of P coda and agrees with constraints on long-wavelength structure from tomography. Finally, we explore geodynamically plausible scenarios that might be responsible for the RMS and falloff rate of the proposed spectrum, including a self-similar mixture of basalt and harzburgite.

Zhang, Q, Shearer PM.  2016.  A new method to identify earthquake swarms applied to seismicity near the San Jacinto Fault, California. Geophysical Journal International. 205:995-1005.   10.1093/gji/ggw073   AbstractWebsite

Understanding earthquake clustering in space and time is important but also challenging because of complexities in earthquake patterns and the large and diverse nature of earthquake catalogues. Swarms are of particular interest because they likely result from physical changes in the crust, such as slow slip or fluid flow. Both swarms and clusters resulting from aftershock sequences can span a wide range of spatial and temporal scales. Here we test and implement a new method to identify seismicity clusters of varying sizes and discriminate them from randomly occurring background seismicity. Our method searches for the closest neighbouring earthquakes in space and time and compares the number of neighbours to the background events in larger space/time windows. Applying our method to California's San Jacinto Fault Zone (SJFZ), we find a total of 89 swarm-like groups. These groups range in size from 0.14 to 7.23 km and last from 15 min to 22 d. The most striking spatial pattern is the larger fraction of swarms at the northern and southern ends of the SJFZ than its central segment, which may be related to more normal-faulting events at the two ends. In order to explore possible driving mechanisms, we study the spatial migration of events in swarms containing at least 20 events by fitting with both linear and diffusion migration models. Our results suggest that SJFZ swarms are better explained by fluid flow because their estimated linear migration velocities are far smaller than those of typical creep events while large values of best-fitting hydraulic diffusivity are found.

Mai, PM, Shearer P, Ampuero JP, Lay T.  2016.  Standards for documenting finite-fault earthquake rupture models. Seismological Research Letters. 87:712-U292.   10.1785/0220150204   AbstractWebsite

In this article, we propose standards for documenting and disseminating finite-fault earthquake rupture models, and related data and metadata. A comprehensive documentation of the rupture models, a detailed description of the data processing steps, and facilitating the access to the actual data that went into the earthquake source inversion are required to promote follow-up research and to ensure interoperability, transparency, and reproducibility of the published slip-inversion solutions. We suggest a formatting scheme that describes the kinematic rupture process in an unambiguous way to support subsequent research. We also provide guidelines on how to document the data, metadata, and data processing. The proposed standards and formats represent a first step to establishing best practices for comprehensively documenting input and output of finitefault earthquake source studies.