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Shearer, PM, Toy KM, Orcutt JA.  1988.  Axi-Symmetric Earth Models and Inner-Core Anisotropy. Nature. 333:228-232.   10.1038/333228a0   Website
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
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Hauksson, E, Shearer PM.  2006.  Attenuation models (Q(P) and Q(S)) in three dimensions of the southern California crust: Inferred fluid saturation at seismogenic depths. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003947   AbstractWebsite

[ 1] We analyze high dynamic range waveform spectra to determine t* values for both P and S waves from earthquakes in southern California. We invert the t* values for three-dimensional (3-D) frequency-independent Q(P) and Q(S) regional models of the crust. The models have 15 km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth, and extend from the U.S.-Mexico border to the Coast Ranges in the south and Sierra Nevada in the north. In general, Q(P) and Q(S) increase rapidly with depth, consistent with crustal densities and velocities. The 3-D Q(P) and Q(S) models image the major tectonic structures and to a much lesser extent the thermal structure of the southern California crust. The near-surface low Q(P) and Q(S) zones coincide with major sedimentary basins such as the San Bernardino, Chino, San Gabriel Valley, Los Angeles, Ventura, and Santa Maria basins and the Salton Trough. In contrast, at shallow depths beneath the Peninsular Ranges, southern Mojave Desert, and southern Sierras, we image high Q(P) and Q(S) zones, which correspond to the dense and high-velocity rocks of the mountain ranges. Several clear transition zones of rapidly varying Q(P) and Q(S) coincide with major late Quaternary faults and connect regions of high and low Q(P) and Q(S). At midcrustal depths, the Q(P) and Q(S) exhibit modest variation in slightly higher and lower Q(P) or Q(S) zones, which is consistent with reported crustal reflectivity. In general, for the southern California crust, Q(S)/Q(P) is greater than 1.0, suggesting partially fluid-saturated crust. A few limited regions of Q(S)/Q(P) less than 1.0 correspond to areas mostly outside the major sedimentary basins, including areas around the San Jacinto fault, suggesting a larger reduction in the shear modulus compared to the bulk modulus or almost complete fluid saturation.

Lin, GQ, Shearer PM, Hauksson E.  2007.  Applying a three-dimensional velocity model, waveform cross correlation, and cluster analysis to locate southern California seismicity from 1981 to 2005. Journal of Geophysical Research-Solid Earth. 112   10.1029/2007jb004986   AbstractWebsite

[1] We compute high-precision earthquake locations using southern California pick and waveform data from 1981 to 2005. Our latest results are significantly improved compared to our previous catalog by the following: (1) We locate events with respect to a new crustal P and S velocity model using three-dimensional ray tracing, (2) we examine six more years of waveform data and compute cross-correlation results for many more pairs than our last analysis, and (3) we compute locations within similar event clusters using a new method that applies a robust fitting method to obtain the best locations satisfying all the differential time constraints from the waveform cross correlation. These results build on the relocated catalogs of Hauksson and Shearer (2005) and Shearer et al. (2005) and provide additional insight regarding the fine-scale fault structure in southern California and the relationship between the San Andreas Fault (SAF) and nearby seismicity. In particular, we present results for two regions in which the seismicity near the southern SAF seems to align on dipping faults.

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
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Buehler, JS, Shearer PM.  2014.  Anisotropy and Vp/Vs in the uppermost mantle beneath the western United States fromjoint analysis of Pn and Sn phases. Journal of Geophysical Research-Solid Earth. 119:1200-1219.   10.1002/2013jb010559   AbstractWebsite

Pn and Sn phases are valuable for resolving velocity structure in the mantle lid, as they propagate horizontally right below the Moho. Relatively few Sn tomography attempts have been made compared to Pn, because Sn is often highly attenuated or buried in P wave coda. USArray has greatly increased data coverage for regional phases, and both Pn and Sn are routinely picked by network analysts. Here we jointly invert Pn and Sn arrival time residuals with a modified time-term analysis and a regularized tomography method and present new maps of crustal thickness, uppermost mantle P velocity perturbations, Vp/Vs ratios, and azimuthal anisotropy strength and orientation beneath the western United States. The results indicate partially molten mantle below the Snake River Plain and the Colorado Plateau. The seismic structure of the top approximate to 40 km of the mantle below the Colorado Plateau differs from that seen at greater depths in other studies, such as surface wave or teleseismic body wave tomography, whereas the Snake River Plain anomaly just below the Moho is comparable to structures seen at about approximate to 200 km depth. Pn fast axes provide complementary information to SKS shear wave splitting observations, and our analysis indicates that in several regions in the western United States the orientation of azimuthal anisotropy changes with depth in the upper mantle. However, we have so far been unable to resolve shear wave splitting directly in Sn waveforms, which seem to be dominated by Sn-SV energy.

Shearer, PM, Hardebeck JL, Astiz L, Richards-Dinger KB.  2003.  Analysis of similar event clusters in aftershocks of the 1994 Northridge, California, earthquake. Journal of Geophysical Research-Solid Earth. 108   10.1029/2001jb000685   AbstractWebsite

[1] We perform waveform cross-correlation on over 14,000 aftershocks of the 1994 Northridge M-W 6.7 earthquake in southern California as recorded by short-period stations of the Southern California Seismic Network (SCSN). Approximately 10-30% of the events belong to similar event clusters, depending upon the similarity criteria that are applied. We relocate events within 218 of these clusters to a relative location accuracy of about 30 m using the differential times obtained from the cross-correlation. These relocated event clusters often show planar features suggestive of faults at depth, and we apply principal parameter analysis to characterize the shape of each cluster and to compute best fitting planes. In several cases, these planes are parallel to the main shock fault plane; however, more generally, the seismicity planes exhibit a wide range of orientations, suggesting complexity in the aftershock faulting. Composite focal mechanisms can be obtained for each cluster by combining the P polarity data from individual events. A comparison of polarity measurement differences within similar event clusters provides constraints on the error rate in the individual focal mechanisms. For some clusters, we are able to resolve the primary versus auxiliary fault plane ambiguity by comparing the computed focal mechanisms with the best fitting seismicity planes. Individual event focal mechanisms are in general agreement with the composite focal mechanisms for the similar event clusters. Events occurring along the main shock rupture plane are mainly thrust, whereas events in the hanging wall are predominately strike-slip.

Hedlin, MAH, Shearer PM.  2000.  An analysis of large-scale variations in small-scale mantle heterogeneity using Global Seismographic Network recordings of precursors to PKP. Journal of Geophysical Research-Solid Earth. 105:13655-13673.   10.1029/2000jb900019   AbstractWebsite

High-frequency precursors to the core phase PKP are caused by scattering off heterogeneities in the lowermost mantle and D " regions, and they provide a unique window into the small-scale structure of the deep Earth. We study lower mantle scattering by analyzing 412 high-quality PKP precursor records at ranges between 120 degrees and 137.5 degrees as obtained from the global seismic networks during the last 10 years. To examine regional variations in scattering strength, we compare individual records with the globally averaged PKP precursor stack of Hedlin et al. [1997]. We identify strong differences in apparent scattering strength among specific source-receiver paths. Inversion of these data for scattering source regions is complicated by ambiguity between source- and recever-side scattering and the sparse and uneven data coverage. Synthetic tests, however, suggest that inversions with applied smoothness constraints can resolve large-scale differences in scattering strength over significant parts of the lower mantle. We use a conjugate gradient method based on an approximation to Rayleigh-Born scattering theory to image differences in the average strength of scattering within the lowermost 1000 km of the mantle. Our results indicate particularly strong scattering beneath central Africa, parts of North America, and just north of India, whereas weaker scattering is seen beneath South and Central America, eastern Europe, and Indonesia. Some regions of strong scattering correlate roughly with large-scale anomalies revealed by seismic tomography including the African plume and the Tethys trench. These correlations are tentative rather than definitive because bootstrap resampling tests show that many details in our model are not reliably resolved and the network data alone do not permit complete resolution of the source-receiver ambiguity in all areas. Further progress in this area will require integration of available network recordings with data collected by regional networks the phase velocity of the precursors as well as their and arrays and consideration of temporal variations.

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.

Grant, LB, Shearer PM.  2004.  Activity of the offshore Newport-Inglewood Rose Canyon fault zone, coastal southern California, from relocated microseismicity. Bulletin of the Seismological Society of America. 94:747-752.   10.1785/0120030149   AbstractWebsite

An offshore zone of faulting approximately 10 km from the southern California coast connects the seismically active strike-slip Newport-Inglewood fault zone in the Los Angeles metropolitan region with the active Rose Canyon fault zone in the San Diego area. Relatively little seismicity has been recorded along the offshore Newport-Inglewood Rose Canyon fault zone, although it has long been suspected of being seismogenic. Active low-angle thrust faults and Quaternary folds have been imaged by seismic reflection profiling along the offshore fault zone, raising the question of whether a through-going, active strike-slip fault zone exists. We applied a waveform cross-correlation algorithm to identify clusters of microseis-micity consisting of similar events. Analysis of two clusters along the offshore fault zone shows that they are associated with nearly vertical, north-northwest-striking faults, consistent with an offshore extension of the Newport-Inglewood and Rose Canyon strike-slip fault zones. P-wave polarities from a 1981 event cluster are consistent with a right-lateral strike-slip focal mechanism solution.