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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.

Fan, WY, Shearer PM.  2016.  Fault interactions and triggering during the 10 January 2012 M-w 7.2 Sumatra earthquake. Geophysical Research Letters. 43:1934-1942.   10.1002/2016gl067785   AbstractWebsite

The 10 January 2012 M-w 7.2 Sumatra earthquake in the Wharton basin occurred 3months before the great M-w 8.6 and M-w 8.2 earthquakes in the same region, which had complex ruptures and are the largest strike-slip earthquakes ever recorded. Teleseismic P wave back projection of the M-w 7.2 earthquake images a unilateral rupture lasting approximate to 40s without observable frequency dependency (low frequency, 0.05-0.3Hz, high frequency, 0.3-1Hz). In addition to radiation bursts during the M-w 7.2 main shock, coherent energy releases from 50 to 75s and from 100 to 125s are observed about 143km northeast of the main shock rupture and landward of the trench. Analysis of globally recorded P waves, in both 0.02-0.05Hz velocity records and 1-5Hz stacked envelope functions, confirms the presence of coherent sources during the time windows. The observed energy bursts are likely to be large early aftershocks occurring on or near the subduction interface. Both dynamic and static triggering could have induced these early aftershocks, as they initiated after the surface wave passed by, and the Coulomb stress perturbations from the M-w 7.2 main shock promote earthquakes in the observed locations. The earthquake sequence is a clear example of a seaward-intraplate strike-slip earthquake triggering landward-intraplate earthquakes in the same region, in contrast to previously reported normal-reverse or reverse-normal interactions at subduction zones.

Mancinelli, N, Shearer P, Thomas C.  2016.  On the frequency dependence and spatial coherence of PKP precursor amplitudes. Journal of Geophysical Research-Solid Earth. 121:1873-1889.   10.1002/2015jb012768   AbstractWebsite

Studies now agree that small-scale (approximate to 10km) weak (approximate to 0.1%) velocity perturbations throughout the lowermost mantle generate the globally averaged amplitudes of 1Hz precursors to the core phase, . The possible frequency dependence and spatial coherence of this scattered phase, however, has been given less attention. Using a large global data set of approximate to 150,000 PKP precursor recordings, we characterize the frequency dependence of PKP precursors at central frequencies ranging from 0.5 to 4Hz. At greater frequencies, we observe more scattered energy (relative to the reference phase PKPdf), particularly at shorter ranges. We model this observation by invoking heterogeneity at length scales from 2 to 30km. Amplitudes at 0.5Hz, in particular, suggest the presence of more heterogeneity at scales >8km than present in previously published models. Using a regional bootstrap approach, we identify large (>20 degrees), spatially coherent regions of anomalously strong scattering beneath the West Pacific, Central/North America, andto a lesser extentEast Africa. Finally, as proof of concept, we use array processing techniques to locate the origin of scattered energy observed in Southern California by the Anza and Southern California Seismic Networks. The energy appears to come primarily from out-of-plane scattering on the receiver side. We suggest that such improvised arrays can increase global coverage and may reveal whether a majority of precursor energy comes from localized heterogeneity in the lowermost mantle.

Melgar, D, Fan WY, Riquelme S, Geng JH, Liang CR, Fuentes M, Vargas G, Allen RM, Shearer PM, Fielding EJ.  2016.  Slip segmentation and slow rupture to the trench during the 2015, M(w)8.3 Illapel, Chile earthquake. Geophysical Research Letters. 43:961-966.   10.1002/2015gl067369   AbstractWebsite

The 2015 M(w)8.3 Illapel, Chile earthquake is the latest megathrust event on the central segment of that subduction zone. It generated strong ground motions and a large (up to 11m runup) tsunami which prompted the evacuation of more than 1 million people in the first hours following the event. Observations during recent earthquakes suggest that these phenomena can be associated with rupture on different parts of the megathrust. The deep portion generates strong shaking while slow, large slip on the shallow fault is responsible for the tsunami. It is unclear whether all megathrusts can have shallow slip during coseismic rupture and what physical properties regulate this. Here we show that the Illapel event ruptured both deep and shallow segments with substantial slip. We resolve a kinematic slip model using regional geophysical observations and analyze it jointly with teleseismic backprojection. We find that the shallow and deep portions of the megathrust are segmented and have fundamentally different behavior. We forward calculate local tsunami propagation from the resolved slip and find good agreement with field measurements, independently validating the slip model. These results show that the central portion of the Chilean subduction zone has accumulated a significant shallow slip deficit and indicates that, given enough time, shallow slip might be possible everywhere along the subduction zone.

Trugman, DT, Shearer PM, Borsa AA, Fialko Y.  2016.  A comparison of long-term changes in seismicity at The Geysers, Salton Sea, and Coso geothermal fields. Journal of Geophysical Research-Solid Earth. 121:225-247.   10.1002/2015jb012510   AbstractWebsite

Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake-earthquake triggering and (2) the smoothly varying background rate controlled by other time-dependent stresses, including anthropogenic forcing. We apply our methodology to compare long-term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short-term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg-Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field-wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.

Lin, GQ, Amelung F, Shearer PM, Okubo PG.  2015.  Location and size of the shallow magma reservoir beneath Klauea caldera, constraints from near-source V-p/V-s ratios. Geophysical Research Letters. 42:8349-8357.   10.1002/2015gl065802   AbstractWebsite

We present high-resolution compressional wave to shear wave velocity ratios (V-p/V-s) beneath Klauea's summit caldera by applying an in situ estimation method using waveform cross-correlation data for three similar earthquake clusters. We observe high V-p/V-s ratios (1.832 and 1.852) for two event clusters surrounded by the low background V-p/V-s value of 1.412 at similar to 2.1km depth below the surface. These high and low V-p/V-s ratios can be explained by melt- and CO2-filled cracks, respectively, based on a theoretical crack model. The event cluster with the highest V-p/V-s ratio consists of long-period events that followed the 1997 East Rift Zone eruption, indicating their association with fluid and magma movement. The depths of the two clusters with high V-p/V-s ratios are consistent with the magma reservoir location inferred from geodetic observations. Their locations east and north of Halemaumau crater suggest a horizontal extent of a few kilometers for the reservoir.

Denolle, MA, Fan WY, Shearer PM.  2015.  Dynamics of the 2015 M7.8 Nepal earthquake. Geophysical Research Letters. 42:7467-7475.   10.1002/2015gl065336   AbstractWebsite

The 2015 M7.8 Nepal earthquake ruptured part of the Main Himalayan Thrust beneath Kathmandu. To study the dynamics of this event, we compute P wave spectra of the main shock and of two large aftershocks to estimate stress drop and radiated energy. We find that surface reflections (depth phases) of these shallow earthquakes produce interference that severely biases spectral measurements unless corrections are applied. Measures of earthquake dynamics for the main shock are within the range of estimates from global and regional earthquakes. We explore the azimuthal and temporal variations of radiated energy and highlight unique aspects of the M7.8 rupture. The beginning of the earthquake likely experienced a dynamic weakening mechanism immediately followed by an abrupt change in fault geometry. Correlation of backprojection results with frequency-dependent variations in the radiated energy rate and with the suggested geometry of the Main Himalayan Thrust yields new constraints on dynamic ruptures through geometrical barriers.

Wang, W, Shearer PM.  2015.  No clear evidence for localized tidal periodicities in earthquakes in the central Japan region. Journal of Geophysical Research-Solid Earth. 120:6317-6328.   10.1002/2015jb011937   AbstractWebsite

We search for possible localized tidal triggering in earthquake occurrence near Japan by testing for tidal periodicities in seismicity within a variety of space/time bins. We examine 74,610 earthquakes of M3 in the Japan Meteorological Agency catalog from January 2000 to April 2013. Because we use many earthquakes for which accurate focal mechanisms are not available, we do not compute tidal stresses on individual fault planes but instead assume that the mechanisms are likely to be similar enough among nearby events that tidal triggering will promote earthquake occurrence at specific tidal phases. After dividing the data into cells at a range of spatial (0.2 degrees, 0.5 degrees, and 1.0 degrees) and temporal dimensions (100, 200, and 400days), we apply Schuster's test for nonrandom event occurrence with respect to both the semidiurnal and semimonthly tidal phases. Because the resulting p values will be biased by temporal clustering caused by aftershocks, we apply a declustering method that retains only one event per tidal cycle per phase increment. Our results show a wide range of p values for the localized earthquake bins, but the number of bins with very small p values (e.g., p < 0.05) is no more than might be expected due to random chance, and there is no correlation of low p value bins with the time of the 2010 M 9.0 Tohoku-Oki earthquake.

Zhan, ZW, Shearer PM.  2015.  Possible seasonality in large deep-focus earthquakes. Geophysical Research Letters. 42:7366-7373.   10.1002/2015gl065088   AbstractWebsite

Large deep-focus earthquakes (magnitude>7.0, depth>500km) have exhibited strong seasonality in their occurrence times since the beginning of global earthquake catalogs. Of 60 such events from 1900 to the present, 42 have occurred in the middle half of each year. The seasonality appears strongest in the northwest Pacific subduction zones and weakest in the Tonga region. Taken at face value, the surplus of northern hemisphere summer events is statistically significant, but due to the ex post facto hypothesis testing, the absence of seasonality in smaller deep earthquakes, and the lack of a known physical triggering mechanism, we cannot rule out that the observed seasonality is just random chance. However, we can make a testable prediction of seasonality in future large deep-focus earthquakes, which, given likely earthquake occurrence rates, should be verified or falsified within a few decades. If confirmed, deep earthquake seasonality would challenge our current understanding of deep earthquakes.