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Fan, WY, Shearer PM.  2018.  Coherent Seismic Arrivals in the P Wave Coda of the 2012 M(w)7.2 Sumatra Earthquake: Water Reverberations or an Early Aftershock? Journal of Geophysical Research-Solid Earth. 123:3147-3159.   10.1002/2018jb015573   AbstractWebsite

Teleseismic records of the 2012M(w)7.2 Sumatra earthquake contain prominent phases in the P wave train, arriving about 50 to 100s after the direct P arrival. Azimuthal variations in these arrivals, together with back-projection analysis, led Fan and Shearer (, ) to conclude that they originated from early aftershock(s), located approximate to 150 km northeast of the mainshock and landward of the trench. However, recently, Yue et al. (, ) argued that the anomalous arrivals are more likely water reverberations from the mainshock, based mostly on empirical Green's function analysis of a M6 earthquake near the mainshock and a water phase synthetic test. Here we present detailed back-projection and waveform analyses of three M6 earthquakes within 100km of the M(w)7.2 earthquake, including the empirical Green's function event analyzed in Yue et al. (, ). In addition, we examine the waveforms of three M5.5 reverse-faulting earthquakes close to the inferred early aftershock location in Fan and Shearer (, ). These results suggest that the reverberatory character of the anomalous arrivals in the mainshock coda is consistent with water reverberations, but the origin of this energy is more likely an early aftershock rather than delayed and displaced water reverberations from the mainshock.

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.

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.

Zhang, J, Gerstoft P, Shearer PM, Yao HJ, Vidale JE, Houston H, Ghosh A.  2011.  Cascadia tremor spectra: Low corner frequencies and earthquake-like high-frequency falloff. Geochemistry Geophysics Geosystems. 12   10.1029/2011gc003759   AbstractWebsite

The discovery of non-volcanic tremor (NVT) has opened a new window to observe major Earth plate boundaries. However, the spectral characteristics of NVT have not been well studied due to poor signal-to-noise ratio (SNR) on individual seismograms. We estimate the spectral content of Cascadia tremor between 2.5 and 20 Hz by suppressing noise using array analysis, and compute empirical path corrections using nearby small earthquakes. We demonstrate that the displacement spectra of the Cascadia tremor have corner frequencies around 3-8 Hz and fall off at f(-2) to f(-3) at higher frequencies. Our results have the following implications. (1) The high-frequency falloff of tremor agrees with the observations of regular earthquakes, suggesting that tremor can be analyzed using standard spectral models. Prior analyses that have shown a tremor spectral falloff proportional to f(-1) may reflect only the spectral behavior over a limited frequency band. (2) Tremor may be no different from a swarm of microearthquakes with abnormally small stress drops on the order of kPa, likely due to the presence of fluids. Alternatively the low corner frequencies of tremor may reflect abnormally slow ruptures. (3) Fitting a standard Brune (1970) spectral model implies a moment release rate of Cascadia tremor of 3.8 x 10(10) N.m/s assuming the tremor signals are P waves (or 1.4 x 10(10) N.m/s assuming S-waves). This implies that a typical 20-day long tremor episode releases moment equivalent to Mw 5.1 (P-wave) or Mw 4.9 (S-wave), although these may be underestimates if the spectra deviate substantially from the Brune model at very low frequencies.

Allmann, BP, Shearer PM.  2007.  A high-frequency secondary event during the 2004 Parkfield earthquake. Science. 318:1279-1283.   10.1126/science.1146537   AbstractWebsite

By using seismic records of the 2004 magnitude 6.0 Parkfield earthquake, we identified a burst of high-frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation. We imaged this event in three dimensions by using a waveform back-projection method, as well as by timing distinct arrivals visible on many of the seismograms. The high-frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions, indicating that slip may have grown abruptly at this point. The time history obtained from full-waveform back projection suggests a rupture velocity of 2.5 kilometers per second. Energy estimates for the subevent, together with long-period slip inversions, indicate a lower average stress drop for the northern part of the Parkfield earthquake compared with that for the region near its hypocenter, which is in agreement with stress-drop estimates obtained from small-magnitude aftershocks.

Aster, RC, Shearer PM.  1992.  Initial Shear-Wave Particle Motions and Stress Constraints at the Anza Seismic Network. Geophysical Journal International. 108:740-748.   10.1111/j.1365-246X.1992.tb03465.x   AbstractWebsite

We use focal plane solutions to constrain principal stress directions in the vicinity of six Anza Network stations which show evidence for shallow shear wave anisotropy in the vicinity of the Anza seismic gap region of the San Jacinto fault. Faulting near all stations is consistent with approximately N-S maximum compressive stress. Five of these stations show nearly N-S initial particle motion alignment, consistent with the anisotropy-stress relationship expected for stress-aligned microcracks. However, one station (KNW) has a well-defined preferred initial shear wave polarization direction of N40-degrees-W. Although this polarization direction differs dramatically from the local maximum compressive stress direction, it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals, particularly biotite. Thus, anisotropy observed at this station most likely reflects a fixed, palaeostrain alignment of anisotropic minerals and/or microcracks and does not require a dependence on the current stress field.