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Hedlin, MAH, Shearer PM.  2002.  Probing mid-mantle heterogeneity using PKP coda waves. Physics of the Earth and Planetary Interiors. 130:195-208.   10.1016/s0031-9201(02)00007-9   AbstractWebsite

We investigate the utility of PKP coda waves for studying weak scattering from small-scale heterogeneity in the mid-mantle. Coda waves are potentially a useful probe of heterogeneity in the mid-mantle because they are not preferentially scattered near the CMB, as PKP precursors are, but are sensitive to scattering at all depths. PKP coda waves have not been used for this purpose historically because of interference with other late-arriving energy due to near-surface resonance and scattering. Any study of deep mantle scattering using coda waves requires the removal of near-surface effects from the data. We have analyzed 3624 recordings of PKP precursors and coda made by stations in the Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network (GSN). To study the range and time dependence of the scattered waves, we binned and stacked envelopes of the recordings. We have considered precursors that arrive within a 20 s window before PKP and coda waves in a 60 s window after PKP. The PKP scattered waves increase in amplitude rapidly with range as predicted by scattering theory. At ranges below similar to125degrees, we predict and observe essentially no scattered energy preceding PKP. Coda amplitudes at these ranges are independent of range and provide an estimate of energy due to near-surface effects that we can expect at all ranges. We use the average coda amplitude at ranges from 120 to 125degrees to correct coda amplitudes at other ranges. PKP coda waves show a strong dependence on time and range and are clearly influenced by scattering in the lower mantle. PKP coda waves, however, do not provide a tighter constraint on the vertical distribution of mantle heterogeneity than is provided by precursors. This is due, in part, to relatively large scatter in coda amplitudes as revealed by a resampling analysis. Modeling using Rayleigh-Born scattering theory and an exponential autocorrelation function shows that PKP coda amplitudes are not highly sensitive to the vertical distribution of heterogeneity in the mantle. To illustrate this we consider single-scattering in two extreme models of mantle heterogeneity. One allows heterogeneity just at the CMB; the other includes heterogeneity throughout the mantle. The amplitudes of precursors are tightly constrained by our stack and support our earlier conclusion that small-scale heterogeneity is uniformly distributed throughout the lower mantle. The best-fit model includes 8 km scale length heterogeneity with an rms velocity contrast throughout the mantle of 1%. (C) 2002 Elsevier Science B.V. All rights reserved.

Astiz, L, Shearer PM, Agnew DC.  2000.  Precise relocations and stress change calculations for the upland earthquake sequence in southern California. Journal of Geophysical Research-Solid Earth. 105:2937-2953.   10.1029/1999jb900336   AbstractWebsite

We relocate earthquakes that occurred near the 1988 (M-L = 4.7) and the 1990 (M-L = 5.5) Upland, California, earthquakes to map the fault geometry of the poorly defined San Jose fault and to test the static:Stress triggering hypothesis for this sequence. We adopt the L1 norm, waveform cross-correlation method of Shearer [1997] to obtain precise relocations for 1573 events: between 1981 and 1997 in the Upland area. To limit computation time, we only perform waveform cross correlation on 60 of the nearest neighbors of leach relocated event. Our final relocations show two linear features. The first is imaged,by the locations of the initial month of aftershocks of the 1988 Upland earthquake, which delineate a fault with a,dip angle of similar to 45 degrees between 7 and 9 km depth, consistent with the mainshock focal mechanism. The second linear feature is a plane, dipping at about 74 degrees from 2 to 9 km depth, which is illuminated by both the 1988:and 1990 Upland sequences, in agreement with the inferred location of the San Jose fault at depth. However, below 9 km the event locations become more diffuse, giving rise to two different interpretations of the fate of the San Jose fault at depth. One possibility is that the fault shallows at depth, consistent with our relocations: but not with the focal mechanism of a M-L = 4.7 deep aftershock. Alternatively, the. fault may be offset at depth by the more shallow dipping fault strand broken during the 1988 earthquake, Using these inferred fault geometries, we compute stress changes resulting from slip during the mainshocks to test whether the relocated aftershocks are consistent with the:hypothesis that more aftershocks occur where the change in static Coulomb failure stress is positive (on faults optimally oriented for failure). This requires an extension of previous models of changes in the failure stress to three dimensions and arbitrary fault orientation. We find that patterns of change in Coulomb failure stress differ little between the different fault geometries: all are nearly symmetric about the fault and so do not match the aftershock distribution, in which most of the off-fault events occur to one side of the fault plane.

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.

Buehler, JS, Shearer PM.  2010.  Pn tomography of the western United States using USArray. Journal of Geophysical Research-Solid Earth. 115   10.1029/2009jb006874   AbstractWebsite

USArray has now provided several years of high-quality seismic data and improved ray coverage for much of the western United States, which will enable increased resolution for studies of the lithospheric and deeper structure of the North American continent. Here we analyze Pn arrival times from the transportable stations of USArray to resolve crustal thickness and uppermost mantle structure. We use 123,008 Pn picks from April 2004 to October 2009 as measured by the Array Network Facility at epicentral distances from 180 to 1450 km. These picks are derived from 778 stations at similar to 70 km spacing and 7903 earthquakes and quarry blasts. Applying the classic time-term method, we use a regularized least squares inversion to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle just below the Moho. We also consider upper mantle anisotropy and describe the velocity perturbations with a cos 2 phi azimuthal variation. Our crustal thickness map generally agrees with receiver function results from other researchers but differs in some details. We obtain an average upper mantle velocity of 7.93 km/s, with higher velocities beneath eastern Washington and northern Idaho, and lower velocities near the California-Mexico border, the Sierra Nevada, the northern coastal California region, and the greater Yellowstone area. We observe large anisotropic anomalies in southern California as well as in the Snake River Plain area. These results should complement other seismic studies (e.g., body and surface wave tomography and shear wave splitting) to provide information about composition, temperature, and tectonic processes in the western United States.

Shearer, PM, Toy KM.  1991.  PKP(BC) Versus PKP(DF) Differential Travel-Times and Aspherical Structure in the Earths Inner Core. Journal of Geophysical Research-Solid Earth and Planets. 96:2233-2247.   10.1029/90jb02370   AbstractWebsite

PKP(BC) versus PKP(DF) differential travel times show coherent patterns of residuals indicating aspherical structure within the Earth's inner core. Using a waveform correlation method, we measure 318 differential PKP(BC) versus PKP(DF) travel times from 5 years of Global Digital Seismograph Network (GDSN) short-period data at ranges between 145-degrees and 155-degrees. In addition, we obtain 19,470 probable BC versus DF travel times at these ranges by searching through 23 years of the International Seismological Centre (ISC) catalog. Plots of residuals versus turning point location and residuals versus ray direction show clear patterns which indicate that the differential travel times are highly correlated along similar ray paths. This pattern is robust with respect to range and source depth. These anomalies are almost certainly due to aspherical inner-core structure since PKP(BC) and PKP(DF) have nearly coincident ray paths in the mantle and outer core. The GDSN and ISC residual patterns are in general agreement, but the ISC residuals underpredict the GDSN residuals by about a factor of 2. The ISC data have better spatial coverage, particularly for N-S ray paths within the inner core. These N-S ray paths show positive residuals of about 0.5 s, indicating faster inner-core P velocities in this direction, a result consistent with previous studies of absolute PKP(DF) travel times but derived from an independent data set. Due to the sparse ray coverage of our data, either heterogeneity or anisotropy within the inner core with about 1% velocity variations could explain these residual patterns. However, inversions of the ISC data for heterogeneity require many more free parameters to achieve the same variance reduction as a simple two-parameter anisotropy model.

Shearer, PM.  2002.  Parallel fault strands at 9-km depth resolved on the Imperial Fault, Southern California. Geophysical Research Letters. 29   10.1029/2002gl015302   AbstractWebsite

[1] Precision relocation of hundreds of small earthquakes occurring along the Imperial Fault in Southern California during the last two decades reveals parallel steaks of seismicity at 9-km depth. These strands are spaced about 0.5 km apart within a 2 km wide zone of earthquakes near the brittle-ductile transition between the shallow locked part of the fault and a creeping zone at depth. These results suggest that the lower crustal shear zone below the Imperial Fault, site of major earthquakes in 1940 and 1979, must be at least two kilometers wide.