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Matoza, RS, Shearer PM, Lin GQ, Wolfe CJ, Okubo PG.  2013.  Systematic relocation of seismicity on Hawaii Island from 1992 to 2009 using waveform cross correlation and cluster analysis. Journal of Geophysical Research-Solid Earth. 118:2275-2288.   10.1002/jgrb.50189   AbstractWebsite

The analysis and interpretation of seismicity from mantle depths to the surface play a key role in understanding how Hawaiian volcanoes work. We present results from a comprehensive and systematic re-analysis of waveforms from 130,902 seismic events recorded by the U.S. Geological Survey Hawaiian Volcano Observatory permanent seismic network from January 1992 to March 2009. We compute high-precision relative relocations for 101,390 events (77% of all events considered) using waveform cross correlation and cluster analysis, resulting in a multiyear systematically processed catalog of seismicity for all of Hawaii Island. The 17 years of relocated seismicity exhibit a dramatic sharpening of earthquake clustering along faults, streaks, and magmatic features, permitting a more detailed understanding of fault geometries and volcanic and tectonic processes. Our relocation results are generally consistent with previous studies that have focused on more specific regions of Hawaii. The relocated catalog includes crustal seismicity at Kilauea and its rift zones, seismicity delineating crustal detachment faults separating volcanic pile and old oceanic crust on the flanks of Kilauea and Mauna Loa, events along inferred magma conduits, and events along inferred mantle fault zones. The relocated catalog is available for download in the supporting information.

Warren, LM, Shearer PM.  2006.  Systematic determination of earthquake rupture directivity and fault planes from analysis of long-period P-wave spectra. Geophysical Journal International. 164:46-62.   10.1111/j.1365-246X.2005.02769   AbstractWebsite

If an earthquake has a primarily unilateral rupture, the pulse width observed on seismograms will vary depending on the angle between the rupture direction and the takeoff vector to the station. We have developed a method to estimate the amount of pulse broadening from the spectrum and apply it to a long-period database of large, globally distributed earthquakes that occurred between 1988 and 2000. We select vertical-component P-waves at epicentral distances of 20 degrees-98 degrees. We compute the spectrum from a 64-s-long window around each P-wave arrival. Each spectrum is the product of source, receiver and propagation response functions as well as local source- and receiver-side effects. Since there are multiple receivers for each source and multiple sources for each receiver, we can estimate and remove the source- and receiver-side terms by stacking the appropriate P log spectra. For earthquakes deeper than similar to 200 km, source effects dominate the residual spectra. We use our pulse-width estimates to determine the best rupture direction and to identify which nodal plane of the Harvard centroid moment tensor (CMT) solution is most consistent with this rupture direction for 66 events. In about 30 per cent of the cases, one of the two nodal planes produces a much better fit to the data and can be identified as the true fault plane. When results from previous studies are available for comparison, our rupture directions are usually consistent with their results, particularly for earthquakes with simple rupture histories.

Vidale, JE, Shearer PM.  2006.  A survey of 71 earthquake bursts across southern California: Exploring the role of pore fluid pressure fluctuations and aseismic slip as drivers. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb004034   AbstractWebsite

[ 1] We investigate the cause of seismicity bursts by examining a waveform-relocated catalog for southern California between 1984 and 2002 and systematically identifying 71 isolated sequences of 40 or more earthquakes occurring within a 2-km-radius volume and a 4-week interval. Fifty-seven of the 71 bursts are difficult to interpret as primarily a main shock and its Omori-law-abiding foreshocks and aftershocks because they exhibit a more complicated evolution in space, time, and magnitude; we identify 18 of these sequences as particularly swarm-like. Evidence against a simple cascade of elastic stress triggering includes the presence of an interval of steady seismicity rate, the tendency of the largest event to strike later in the sequence, the large spatial extent of some of the swarms compared to their cumulative moment, and the weak correlation between the number of events in each burst and the magnitude of the largest event in each burst. Shallow sequences and normal faulting mechanism sequences are most likely to be swarm-like. The tendencies of the hypocenters in the swarm-like sequences to occur on vertical planes and expand over time suggest pore fluid pressure fluctuations as the most likely mechanism driving the swarm-like seismicity bursts. However, episodic aseismic slip could also be at least partly responsible and might provide a more compelling explanation for the steady rate of seismicity during swarms, whereas fluid pressure perturbations might be expected to diminish more rapidly with time. Both aftershock-like and swarm-like seismicity bursts are distributed across the entire study region, indicating that they are a general feature of tectonic faulting, rather than limited to a few geological conditions such as volcanic or geothermal areas.

Shearer, PM, Orcutt JA.  1987.  Surface and Near-Surface Effects on Seismic-Waves - Theory and Borehole Seismometer Results. Bulletin of the Seismological Society of America. 77:1168-1196.Website
Zhan, ZW, Helmberger DV, Kanamori H, Shearer PM.  2014.  Supershear rupture in a M-w 6.7 aftershock of the 2013 Sea of Okhotsk earthquake. Science. 345:204-207.   10.1126/science.1252717   AbstractWebsite

Earthquake rupture speeds exceeding the shear-wave velocity have been reported for several shallow strike-slip events. Whether supershear rupture also can occur in deep earthquakes is unclear, because of their enigmatic faulting mechanism. Using empirical Green's functions in both regional and teleseismic waveforms, we observed supershear rupture during the 2013 moment magnitude (M-w) 6.7 deep earthquake beneath the Sea of Okhotsk, an aftershock of the large deep earthquake (M-w 8.3). The M-w 6.7 event ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers per second, suggesting efficient seismic energy generation. Comparing it to the highly dissipative 1994 M-w 8.3 Bolivia earthquake, the two events represent end members of deep earthquakes in terms of energy partitioning and imply that there is more than one rupture mechanism for deep earthquakes.

Masters, TG, Shearer PM.  1990.  Summary of Seismological Constraints on the Structure of the Earth Core. Journal of Geophysical Research-Solid Earth and Planets. 95:21691-21695.   10.1029/JB095iB13p21691   Website
Yao, HJ, Shearer PM, Gerstoft P.  2012.  Subevent location and rupture imaging using iterative backprojection for the 2011 Tohoku Mw 9.0 earthquake. Geophysical Journal International. 190:1152-1168.   10.1111/j.1365-246X.2012.05541.x   AbstractWebsite

Knowledge of the rupture speed and spatialtemporal distribution of energy radiation of earthquakes is important for earthquake physics. Backprojection of teleseismic waves is commonly used to image the rupture process of large events. The conventional backprojection method typically performs temporal and spatial averaging to obtain reliable rupture features. We present an iterative backprojection method with subevent signal stripping to determine the distribution of subevents (large energy bursts) during the earthquake rupture. We also relocate the subevents initially determined by iterative backprojection using the traveltime shifts from subevent waveform cross-correlation, which provides more accurate subevent locations and source times. A bootstrap approach is used to assess the reliability of the identified subevents. We apply this method to the Mw 9.0 Tohoku earthquake in Japan using array data in the United States. We identify 16 reliable subevents in the frequency band 0.21 Hz, which mostly occurred around or west of the hypocentre in the downdip region. Analysis of Tohoku aftershocks shows that depth phases can often produce artefacts in the backprojection image, but the position and timing of our main shock subevents are inconsistent with depth-phase artefacts. Our results suggest a complicated rupture with a component of bilateral rupture along strike. The dominant energy radiation (between 0.2 and 1 Hz) is confined to a region close to the hypocentre during the first 90 s. A number of subevents occurred around the hypocentre in the first 90 s, suggesting a low initial rupture speed and repeated rupture or slip near the hypocentre. The rupture reached the coastal region about 106 km northwest of the hypocentre at 43 s and the region about 110 km north of the hypocentre at 105 s with a northward rupture speed similar to 2.0 km s-1 at 60110 s. After 110 s, a series of subevents occurred about 120220 km southwest of the hypocentre, consistent with a 3 km s-1 along-strike rupture speed. The abundant high-frequency radiation in the downdip region close to the coast suggests intermittent rupture probably in the brittleductile transition zone. The lack of high-frequency radiation in the updip region suggests the rupture near the trench was more continuous, probably due to more homogeneous frictional properties of the shallow slab interface. The lack of early aftershocks in the updip region indicates that most of the accumulated slip in the updip region during the interseismic period was probably released during the main shock.

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.

Yang, ZH, Sheehan A, Shearer P.  2011.  Stress-induced upper crustal anisotropy in southern California. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007655   AbstractWebsite

We use an automated method to analyze shear wave splitting from local earthquakes recorded by the Southern California Seismic Network between 2000 and 2005. The observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression sigma(Hmax), suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern California is regional stress. However, at other stations, fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock. Splitting delay times range widely within 0.2 s. These upper crustal anisotropy observations, together with previous studies of SKS shear wave splitting, surface waves, and receiver functions, suggest different mechanisms of anisotropy at different depths under southern California. Anisotropy in the upper crust appears to be in response to the current horizontal maximum compression sigma(Hmax), which differs from the cause of anisotropy in the lower crust and mantle. We also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays.

Goebel, THW, Hauksson E, Shearer PM, Ampuero JP.  2015.  Stress-drop heterogeneity within tectonically complex regions: a case study of San Gorgonio Pass, southern California. Geophysical Journal International. 202:514-528.   10.1093/gji/ggv160   AbstractWebsite

In general, seismic slip along faults reduces the average shear stress within earthquake source regions, but stress drops of specific earthquakes are observed to vary widely in size. To advance our understanding of variations in stress drop, we analysed source parameters of small-magnitude events in the greater San Gorgonio area, southern California. In San Gorgonio, the regional tectonics are controlled by a restraining bend of the San Andreas fault system, which results in distributed crustal deformation, and heterogeneous slip along numerous strike-slip and thrust faults. Stress drops were estimated by fitting a Brune-type spectral model to source spectra obtained by iteratively stacking the observed amplitude spectra. The estimates have large scatter among individual events but the median of event populations shows systematic, statistically significant variations. We identified several crustal and faulting parameters that may contribute to local variations in stress drop including the style of faulting, changes in average tectonic slip rates, mineralogical composition of the host rocks, as well as the hypocentral depths of seismic events. We observed anomalously high stress drops (>20 MPa) in a small region between the traces of the San Gorgonio and Mission Creek segments of the San Andreas fault. Furthermore, the estimated stress drops are higher below depths of similar to 10 km and along the San Gorgonio fault segment, but are lower both to the north and south away from San Gorgonio Pass, showing an approximate negative correlation with geologic slip rates. Documenting controlling parameters of stress-drop heterogeneity is important to advance regional hazard assessment and our understanding of earthquake rupture processes.

Uchide, T, Shearer PM, Imanishi K.  2014.  Stress drop variations among small earthquakes before the 2011 Tohoku-oki, Japan, earthquake and implications for the main shock. Journal of Geophysical Research-Solid Earth. 119:7164-7174.   10.1002/2014jb010943   AbstractWebsite

It is important to assess the likely rupture characteristics of future megathrust earthquakes. One approach is to study the spatiotemporal variation of geophysical properties in active subduction zones. We explore this idea by examining stress drops of 1563 small earthquakes (M-w 3.0-4.5) shallower than 80km in the Tohoku-oki region before the 2011 Tohoku-oki earthquake. Although individual stress drop estimates exhibit considerable scatter, we find a strong increase in stress drop with depth between 30km and 60km, whereas stress drops for shallower and deeper events, respectively, are nearly constant. We also identify lateral variations in stress drop along strike. Higher-than-average stress drops are found in East Aomori-oki and Miyagi-oki, whereas Sanriku-oki is a moderate stress drop area. The high stress drop zone in Miyagi-oki is located just south of the large slip area of the 2011 Tohoku-oki earthquake, and possibly acted as a barrier to further rupture propagation during the event. In addition, the frequency dependence of the seismic radiation observed during the main shock, with proportionally higher frequencies coming from the deeper parts of the fault, mimics the depth dependence we see in small earthquakes in the same region. These results imply that smaller pre-main shock earthquakes can provide insight into the fault properties and consequent rupture processes of future megathrust earthquakes.

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.

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.

Allmann, BR, Shearer PA, Hauksson E.  2008.  Spectral discrimination between quarry blasts and earthquakes in southern California. Bulletin of the Seismological Society of America. 98:2073-2079.   10.1785/0120070215   AbstractWebsite

We compare P-wave spectra of quarry blasts and earthquakes recorded by the southern California seismic network (SCSN) between 2000 and 2005, with the goal of developing methods to discriminate between these events. We process the spectra using an iterative robust least-squares method to isolate source, receiver, and propagation path contributions. This corrects for first-order attenuation structure, as well as near-receiver site effects and any errors in the instrument response functions. Using the earthquake spectra and a simple omega(-2) source model, we compute an empirical Green's function (EGF) to remove the trade-off between the source terms and other parameters in our model. A constant stress-drop model gives a good fit to the observed average earthquake spectra over a wide range of moment magnitude, but provides a mediocre fit to the average quarry blast spectra, which have a generally steeper fall-off at high frequencies than omega(-2). We also compare P- and S-wave amplitudes and find modestly smaller average S amplitudes for the explosions compared to the earthquakes. For southern California, the root-mean-square (rms) misfit of P-wave spectra to an omega(-2) source model is a more reliable explosion discriminant than the S-to-P amplitude ratio and works for about 90% of the events.

Uchide, T, Yao HJ, Shearer PM.  2013.  Spatio-temporal distribution of fault slip and high-frequency radiation of the 2010 El Mayor-Cucapah, Mexico earthquake. Journal of Geophysical Research-Solid Earth. 118:1546-1555.   10.1002/jgrb.50144   AbstractWebsite

Earthquake slip history and moment release are best resolved using long period seismic waves, but details in the rupture process, such as sharp changes in rupture velocity or direction, can be imaged more clearly using higher frequency waves. Here, we investigate the slip and the high-frequency radiation histories of the 2010 El Mayor-Cucapah, Baja California, Mexico earthquake (Mw 7.2). The slip distribution inferred from inversion of strong motion data between 0.02 and 0.25Hz indicates northwest propagating rupture, followed by bilateral rupture for 40s. The sources of high-frequency radiation between 0.3 and 2Hz inferred from back-projection analysis using teleseismic data are adjacent to, but not within, the high-slip patches from the finite slip model in time and space. This implies relatively smooth rupture during the times and regions of maximum moment release. As theoretical models have predicted, high-frequency radiation seems mostly associated with changes in rupture velocity or slip magnitude. Strong high-frequency radiation is also found where the rupture propagated to a branch fault 50-km northwest of the hypocenter. Complementary constraints on both fault slip and high-frequency radiation provide increased understanding of earthquake rupture mechanics and may help improve strong motion evaluation at high frequencies.

Chen, X, Shearer PM, Abercrombie RE.  2012.  Spatial migration of earthquakes within seismic clusters in Southern California: Evidence for fluid diffusion. Journal of Geophysical Research-Solid Earth. 117   10.1029/2011jb008973   AbstractWebsite

Seismicity within many earthquake swarms is observed to migrate slowly with time, which may reflect event triggering due to slow fault slip or fluid flow. We search for this behavior in Southern California by applying a weighted least squares method to quantify event migration within 69 previously observed seismicity bursts. We obtain best-fitting migration directions and velocities, and compute a statistical migration significance s(m) for each burst using a bootstrap resampling method. We define 37 bursts with s(m) >= 0.8 as the migration group, and 32 bursts with s(m) < 0.8 as the non-migration group. To explore differences between the two groups, for each burst we compute effective stress drop (Delta sigma(quasi), the ratio between total moment and radius), the skew of the moment release time series (mu), the timing of the largest event (t(max)), and the distance separation between the first half and second half of the sequence (d(s)). As expected, the migration group features larger d(s) and lower Delta sigma(quasi), consistent with higher migration significance. It also features lower mu and higher t(max), similar to observations from swarms in the Salton Trough, while the non-migration group is more similar to main shock-aftershock sequences. To explore possible fluid involvement, we model the migration behavior with the fluid diffusion equation, and identify 18 bursts with diffusion coefficients ranging from 0.01 to 0.8 m(2)/s, with the majority below 0.16 m(2)/s. The obtained diffusion coefficients and migration behavior are similar to the Reservoir-induced seismicity beneath the Acu reservoir in Brazil. The majority of normal faulting events are associated with these 18 bursts, while the non-migration group has the most reverse faulting events, indicating a possible link between sequence type and focal mechanism.

Allmann, BP, Shearer PM.  2007.  Spatial and temporal stress drop variations in small earthquakes near Parkfield, California. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004395   AbstractWebsite

[1] We estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the Parkfield segment of the San Andreas Fault in central California. We use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed P wave source spectra with a spatially varying empirical Green's function (EGF). Our Brune-type stress drop estimates vary from 0.1 to over 100 MPa with a median value of 6.75 MPa, which is nearly constant with moment, implying self-similarity over the M-L = 0.5 to 3.0 range of our data. The corner frequency decreases for earthquakes at shallower depths, consistent with slower rupture velocities and reduced shear wave velocities in local velocity models. The estimated median stress drops show significant lateral variations: we find lower stress drops in the Middle Mountain asperity and along the creeping fault section, and higher stress drops in the hypocentral region of the 2004 M6.0 Parkfield earthquake. The main shock did not alter the overall pattern of high and low stress drop regions. However, a statistical test reveals areas with significant changes in computed stress drops after the main shock, which we compare to estimated absolute shear stress changes from a main shock slip model. By calculating Delta t* from the spectral EGF ratio, we also identify areas with increased attenuation after the main shock, and we are able to distinguish source effects and near-source attenuation effects in the spectral analysis. These results are confirmed independently from spectral ratios of repeating microearthquake clusters.

Shearer, PM.  2012.  Space-time clustering of seismicity in California and the distance dependence of earthquake triggering. Journal of Geophysical Research-Solid Earth. 117 Abstract

Using two recent high-resolution earthquake catalogs, I examine clustering in California seismicity by plotting the average rate of earthquakes as a function of both space and time from target events of M 2 to 5. Comparisons between pre- and post-target event activity can be used to resolve earthquake-to-earthquake triggering associated with target events of different magnitudes. The results are more complicated than predicted by computer simulations of earthquake triggering that begin with background events occurring at random times. In particular, at least some of the temporal clustering of seismicity at short scales (0.1 to 5 km) does not appear to be caused by local earthquake triggering, but instead reflects an underlying physical process that temporarily increases the seismicity rate, such as is often hypothesized to drive earthquake swarms. Earthquake triggering for M < 4.5 earthquakes is only resolvable in average seismicity rates at times less than about one day and to distances of less than about 10 km, and its linear density decreases as r(-1.5) to r(-2.5), significantly steeper than some previous studies have found.

Shearer, P, Hauksson E, Lin GQ.  2005.  Southern California hypocenter relocation with waveform cross-correlation, part 2: Results using source-specific station terms and cluster analysis. Bulletin of the Seismological Society of America. 95:904-915.   10.1785/01200401168   AbstractWebsite

We obtain precise relative relocations for more than 340,000 southern California earthquakes between 1984 and 2002 by applying the source-specific station-term (SSST) method to existing P- and S-phase picks and a differential location method to about 208,000 events within similar-event clusters identified with waveform cross-correlation. The entire catalog is first relocated by using existing phase picks, a reference ID velocity model, and the SSST method of Richards-Dinger and Shearer (2000). We also perform separate relocations of Imperial Valley events by using a velocity model more suited to this region. Next, we apply cluster analysis to the waveform cross-correlation output to identify similar-event clusters. We relocate earthquakes within each similar-event cluster by using the differential times alone, keeping the cluster centroid fixed to its initial SSST location. We estimate standard errors for the relative locations from the internal consistency of differential locations between individual event pairs; these errors are often as small as tens of meters. In many cases the relocated events within each similar-event cluster align in planar features suggestive of faults. We observe a surprising number of such faults at small scales that strike nearly perpendicular to the main seismicity trends. In general, the fine-scale details of the seismicity reveal a great deal of structural complexity in southern California fault systems.

Hauksson, E, Shearer P.  2005.  Southern California hypocenter relocation with waveform cross-correlation, Part 1: Results using the double-difference method. Bulletin of the Seismological Society of America. 95:896-903.   10.1785/0120040167   AbstractWebsite

We present the results of relocating 327,000 southern California earthquakes that occurred between 1984 and 2002. We apply time-domain waveform cross-correlation for P and S waves between each event and 100 neighboring events identified from the catalog based on a 3D velocity model. To simplify the computation, we first divide southern California into five polygons, such that there are similar to 100,000 events or less in each region. The polygon boundaries are chosen to lie in regions of sparse seismicity. We calculate and save differential times from the peaks in the cross-correlation functions and use a spline interpolation method to achieve a nominal timing precision of 0.001 sec. These differential times, together with existing P- and S-phase picks, are input to the double-difference program of Waldhauser and Ellsworth (2000, 2002) to calculate refined hypocenters. We divide the southern California region into grid cells and successively relocate hypocenters within each grid cell. The overall resulting pattern of seismicity is more focused than the previously determined pattern from 1D or 3D models. The new improved locations are more clustered, in many cases by a factor of two or three, and often show clear linear alignments. In particular, the depth distribution is improved and less affected by layer boundaries in velocity models or other similar artifacts.

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.

Matoza, RS, Chouet BA, Dawson PB, Shearer PM, Haney MM, Waite GP, Moran SC, Mikesell TD.  2015.  Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion. Journal of Geophysical Research-Solid Earth. 120:6351-6364.   10.1002/2015jb012279   AbstractWebsite

Long-period (LP, 0.5-5Hz) seismicity, observed at volcanoes worldwide, is a recognized signature of unrest and eruption. Cyclic LP drumbeating was the characteristic seismicity accompanying the sustained dome-building phase of the 2004-2008 eruption of Mount St. Helens (MSH), WA. However, together with the LP drumbeating was a near-continuous, randomly occurring series of tiny LP seismic events (LP subevents), which may hold important additional information on the mechanism of seismogenesis at restless volcanoes. We employ template matching, phase-weighted stacking, and full-waveform inversion to image the source mechanism of one multiplet of these LP subevents at MSH in July 2005. The signal-to-noise ratios of the individual events are too low to produce reliable waveform inversion results, but the events are repetitive and can be stacked. We apply network-based template matching to 8days of continuous velocity waveform data from 29 June to 7 July 2005 using a master event to detect 822 network triggers. We stack waveforms for 359 high-quality triggers at each station and component, using a combination of linear and phase-weighted stacking to produce clean stacks for use in waveform inversion. The derived source mechanism points to the volumetric oscillation (approximate to 10m(3)) of a subhorizontal crack located at shallow depth (approximate to 30m) in an area to the south of Crater Glacier in the southern portion of the breached MSH crater. A possible excitation mechanism is the sudden condensation of metastable steam from a shallow pressurized hydrothermal system as it encounters cool meteoric water in the outer parts of the edifice, perhaps supplied from snow melt.

Buehler, JS, Shearer PM.  2013.  Sn propagation in the Western United States from common midpoint stacks of USArray data. Geophysical Research Letters. 40:6106-6111.   10.1002/2013gl057680   AbstractWebsite

The regional seismic phase Sn propagates horizontally in the uppermost mantle and is sensitive to lateral variations in mantle lid thickness, temperature, and melt. Sn is therefore often used as an indicator for physical properties of the lithosphere. It has previously been noticed that Sn is not observed at many stations in the Western United States and Sn seems especially highly attenuated for paths across the Basin and Range. Here we apply stacking methods to USArray data to identify highly attenuating regions in the uppermost mantle and increase the spatial resolution of the Sn propagation image. We find evidence for Sn propagation at short ranges in the central Great Basin and the northeastern part of the Colorado Plateau, both regions where lithospheric stability and thickness is debated, and observe strong Sn attenuation around the perimeter of the central Great Basin.

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.