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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.  2015.  Detailed rupture imaging of the 25 April 2015 Nepal earthquake using teleseismic P waves. Geophysical Research Letters. 42:5744-5752.   10.1002/2015gl064587   AbstractWebsite

We analyze the rupture process of the 25 April 2015 Nepal earthquake with globally recorded teleseismic P waves. The rupture propagated east-southeast from the hypocenter for about 160km with a duration of similar to 55s. Backprojection of both high-frequency (HF, 0.2 to 3Hz) and low-frequency (LF, 0.05 to 0.2Hz) P waves suggest a multistage rupture process. From the low-frequency images, we resolve an initial slow downdip (northward) rupture near the nucleation area for the first 20s (Stage 1), followed by two faster updip ruptures (20 to 40s for Stage 2 and 40 to 55s for Stage 3), which released most of the radiated energy northeast of Kathmandu. The centroid rupture power from LF backprojection agrees well with the Global Centroid Moment Tensor solution. The spatial resolution of the backprojection images is validated by applying similar analysis to nearby aftershocks. The overall rupture pattern agrees well with the aftershock distribution. A multiple-asperity model could explain the observed multistage rupture and aftershock distribution.