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I
Warren, LM, Shearer PM.  2000.  Investigating the frequency dependence of mantle Q by stacking P and PP spectra. Journal of Geophysical Research-Solid Earth. 105:25391-25402.   10.1029/2000jb900283   AbstractWebsite

Using seismograms from globally distributed, shallow earthquakes between 1988 and 1998, we compute spectra for P arrivals from epicentral distances of 40 degrees to 80 degrees and PP arrivals from 80 degrees to 160 degrees. Selecting records with estimated signal-to-noise ratios greater than 2, we find 17,836 P and 14,721 PP spectra. We correct each spectrum for the known instrument response and for an omega (-2) source model that accounts for varying event sizes. Next, we stack the logarithms of the P and PP spectra in bins of similar source-receiver range. The stacked log spectra, denoted as log(D'(P)) and log(D'(PP)), appear stable between about 0.16 and 0.86 Hz, with noise and/or bias affecting the results at higher frequencies. Assuming that source spectral differences are randomly distributed, then for shallow events, when the PP range is twice the P range, the average residual source spectrum may be estimated as 2 log(D'(P))- log(D'(PP)), and the average P wave attenuation spectrum may be Estimated as log(D'(PP)) - log(D'(P)). The residual source spectral estimates exhibit a smooth additional falloff as omega (-0.15+/-0.05) between 0.16 and 0.86 Hz, indicating that omega (-2.15+/-0.05) is an appropriate average source model for shallow events. The attenuation spectra show little distance dependence over this band and have a P wave (t) over bar* value of similar to0.5 s. We use (t) over bar* measurements from individual P and PP spectra to invert for a frequency-independent Q model and find that the upper mantle is nearly 5 times as attenuating as the lower mantle. Frequency dependence in Q, is difficult to resolve directly in these data but, as previous researchers have noted, is required to reconcile these values with long-period Q estimates. Using Q model QL6 [Durek and Ekstrom, 1996] as a long-period constraint, we experiment with fitting our stacked log spectra with an absorption band model. We find that the upper corner frequency f(2) in the absorption band must be depth-dependent to account for the lack of a strong distance dependence in our observed (t) over bar* values. In particular, our results indicate that f(2) is higher in the top 220 km of the mantle than at greater depths; the lower layer is about twice as attenuating at 1 Ha than at 0.1 Hz, whereas the upper mantle attenuation is relatively constant across this band.

L
Castro, RR, Shearer PM, Astiz L, Suter M, Jacques-Ayala C, Vernon F.  2010.  The Long-Lasting Aftershock Series of the 3 May 1887 M-w 7.5 Sonora Earthquake in the Mexican Basin and Range Province. Bulletin of the Seismological Society of America. 100:1153-1164.   10.1785/0120090180   AbstractWebsite

We study local and regional body-wave arrival times from several seismic networks to better define the active regional fault pattern in the epicentral region of the 3 May 1887 M-w 7.5 Sonora, Mexico (southern Basin and Range Province) earthquake. We determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network RESNES (Red Sismica del Noreste de Sonora) and stations of the Network of Autonomously Recording Seismographs (NARS)-Baja array. For events between April and December 2007, we also incorporated arrival times from USArray stations located within 150 km of the United States-Mexico border. We first obtained preliminary earthquake locations with the Hypoinverse program (Klein, 2002) and then relocated these initial hypocenter coordinates with the source-specific station term (SSST) method (Lin and Shearer, 2005). Most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long-lasting series of aftershocks. The region of aftershock activity extends, along the same fault zone, 40-50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 May 1913 (I-max VIII, M-I 5.0-0.4) and 18 December 1923 (I-max IX, M-I 5.7-0.4) Granados-Huasabas, Sonora, earthquakes, which themselves are likely to be aftershocks of the 1887 event. The long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887.

M
Lawrence, JF, Shearer PM, Masters G.  2006.  Mapping attenuation beneath North America using waveform cross-correlation and cluster analysis. Geophysical Research Letters. 33   10.1029/2006gl025813   AbstractWebsite

We measure seismic attenuation beneath North America using waveform cross-correlation and cluster analysis, and obtain images of the laterally varying anelastic structure of the upper mantle. Cluster analysis improves attenuation measurements by systematically comparing only highly similar waveforms, which reduces bias from scattering, directional differences in source functions, and source-side structure. While lacking station coverage in many areas, the P- and S-wave results are correlated (R-2 >= 0.5) in both travel time and attenuation. Much weaker correlations are observed between travel-time and attenuation measurements. Similarities and differences between attenuation and travel times may be used to infer the source of the observed anomalies. The observed anelastic structure has a long-wavelength pattern crudely similar to that of seismic velocity, which likely indicates higher temperatures beneath western North America than in the east. Shorter-wavelength structure suggests complex variations requiring alternate explanations such as variable water content.

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

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

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.

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.