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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, Bassett D, Jiang JL, Shearer PM, Ji C.  2017.  Rupture evolution of the 2006 Java tsunami earthquake and the possible role of splay faults. Tectonophysics. 721:143-150.   10.1016/j.tecto.2017.10.003   AbstractWebsite

The 2006 Mw 7.8 Java earthquake was a tsunami earthquake, exhibiting frequency-dependent seismic radiation along strike. High-frequency global back-projection results suggest two distinct rupture stages. The first stage lasted similar to 65 s with a rupture speed of similar to 1.2 km/s, while the second stage lasted from similar to 65 to 150 s with a rupture speed of similar to 2.7 km/s. High-frequency radiators resolved with back-projection during the second stage spatially correlate with splay fault traces mapped from residual free-air gravity anomalies. These splay faults also colocate with a major tsunami source associated with the earthquake inferred from tsunami first-crest back-propagation simulation. These correlations suggest that the splay faults may have been reactivated during the Java earthquake, as has been proposed for other tsunamigenic earthquakes, such as the 1944 Mw 8.1 Tonankai earthquake in the Nankai Trough.

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.  Fault interactions and triggering during the 10 January 2012 M-w 7.2 Sumatra earthquake. Geophysical Research Letters. 43:1934-1942.   10.1002/2016gl067785   AbstractWebsite

The 10 January 2012 M-w 7.2 Sumatra earthquake in the Wharton basin occurred 3months before the great M-w 8.6 and M-w 8.2 earthquakes in the same region, which had complex ruptures and are the largest strike-slip earthquakes ever recorded. Teleseismic P wave back projection of the M-w 7.2 earthquake images a unilateral rupture lasting approximate to 40s without observable frequency dependency (low frequency, 0.05-0.3Hz, high frequency, 0.3-1Hz). In addition to radiation bursts during the M-w 7.2 main shock, coherent energy releases from 50 to 75s and from 100 to 125s are observed about 143km northeast of the main shock rupture and landward of the trench. Analysis of globally recorded P waves, in both 0.02-0.05Hz velocity records and 1-5Hz stacked envelope functions, confirms the presence of coherent sources during the time windows. The observed energy bursts are likely to be large early aftershocks occurring on or near the subduction interface. Both dynamic and static triggering could have induced these early aftershocks, as they initiated after the surface wave passed by, and the Coulomb stress perturbations from the M-w 7.2 main shock promote earthquakes in the observed locations. The earthquake sequence is a clear example of a seaward-intraplate strike-slip earthquake triggering landward-intraplate earthquakes in the same region, in contrast to previously reported normal-reverse or reverse-normal interactions at subduction zones.

Wang, W, Shearer PM.  2015.  No clear evidence for localized tidal periodicities in earthquakes in the central Japan region. Journal of Geophysical Research-Solid Earth. 120:6317-6328.   10.1002/2015jb011937   AbstractWebsite

We search for possible localized tidal triggering in earthquake occurrence near Japan by testing for tidal periodicities in seismicity within a variety of space/time bins. We examine 74,610 earthquakes of M3 in the Japan Meteorological Agency catalog from January 2000 to April 2013. Because we use many earthquakes for which accurate focal mechanisms are not available, we do not compute tidal stresses on individual fault planes but instead assume that the mechanisms are likely to be similar enough among nearby events that tidal triggering will promote earthquake occurrence at specific tidal phases. After dividing the data into cells at a range of spatial (0.2 degrees, 0.5 degrees, and 1.0 degrees) and temporal dimensions (100, 200, and 400days), we apply Schuster's test for nonrandom event occurrence with respect to both the semidiurnal and semimonthly tidal phases. Because the resulting p values will be biased by temporal clustering caused by aftershocks, we apply a declustering method that retains only one event per tidal cycle per phase increment. Our results show a wide range of p values for the localized earthquake bins, but the number of bins with very small p values (e.g., p < 0.05) is no more than might be expected due to random chance, and there is no correlation of low p value bins with the time of the 2010 M 9.0 Tohoku-Oki earthquake.

Yao, HJ, Shearer PM, Gerstoft P.  2013.  Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures. Proceedings of the National Academy of Sciences of the United States of America. 110:4512-4517.   10.1073/pnas.1212790110   AbstractWebsite

Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above similar to 15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient high-frequency radiation, and few early aftershocks. The slab interface at intermediate depths (similar to 15-35 km) is the main unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediate- to high-frequency radiation. The deeper portion of the slab interface (similar to 35-45 km) is seismically unstable, however with small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.