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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.

Kane, DL, Shearer PM, Goertz-Allmann BP, Vernon FL.  2013.  Rupture directivity of small earthquakes at Parkfield. Journal of Geophysical Research-Solid Earth. 118:212-221.   10.1029/2012jb009675   AbstractWebsite

Theoretical modeling of strike-slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction. We test this model with 450 small earthquakes (2 < M < 5) from Parkfield, California, to look for evidence of consistent rupture directivity along the San Andreas Fault. We analyze azimuthal variations in earthquake source spectra after applying an iterative correction for wave propagation effects. Our approach avoids directly modeling source spectra because these models generally assume symmetric rupture; instead, we look for azimuthal variations in the amplitudes of the source spectra over specified frequency bands. Our overall results show similar proportions of events exhibiting characteristics of rupture directivity toward either the southeast or northwest. However, the proportion of events with southeast rupture directivity increases as we limit the data set to larger magnitudes, with 70% of the 46 events M > 3 exhibiting southeast rupture characteristics. Some spatial and temporal variability in rupture directivity is also apparent. We observe a higher proportion of northwest directivity ruptures following the 2004 M 6 Parkfield earthquake, which ruptured toward the northwest. Our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors. Citation: Kane, D. L., P. M. Shearer, B. P. Goertz-Allmann, and F. L. Vernon (2013), Rupture directivity of small earthquakes at Parkfield, J. Geophys. Res. Solid Earth, 118, 212-221, doi: 10.1029/2012JB009675.

Walker, KT, Ishii M, Shearer PM.  2005.  Rupture details of the 28 March 2005 Sumatra Mw 8.6 earthquake imaged with teleseismic P waves. Geophysical Research Letters. 32   10.1029/2005gl024395   AbstractWebsite

We image the rupture of the 28 March 2005 Sumatra Mw 8.6 earthquake by back-projecting teleseismic P waves recorded by the Global Seismic Network and the Japanese Hi-net to their source. The back-projected energy suggests that the rupture started slowly, had a total duration of about 120 s, and propagated at 2.9 to 3.3 km/s from the hypocenter in two different directions: first toward the north for similar to 100 km and then, after a similar to 40 s delay, toward the southeast for similar to 200 km. Our images are consistent with a rupture area of similar to 40,000 km 2, the locations of the first day of aftershocks, and the Harvard CMT Mw of 8.6, which implies an average slip of similar to 6 m. The earthquake is similar in its location, size, and geometry to a Mw similar to 8.5 event in 1861. Our estimated average slip is consistent with a partially coupled subduction interface, GPS forearc velocities, and the similar to 59 mm/yr convergence rate if the 2005 earthquake released elastic strain that accumulated over many hundreds of years rather than just since the last 1861 event.

Zhang, J, Gerstoft P, Shearer PM.  2010.  Resolving P-wave travel-time anomalies using seismic array observations of oceanic storms. Earth and Planetary Science Letters. 292:419-427.   10.1016/j.epsl.2010.02.014   AbstractWebsite

Array analysis of seismic noise has the potential to be very useful in improving body-wave tomography of Earth structure, just as noise cross-correlation methods have recently proven successful in surface-wave tomography. Beamforming of seismic noise recorded in southern California reveals P-wave arrivals from distant storms in open oceans. In this case, the noise can be processed using cross-correlation among different station pairs and optimal P-wave relative arrival times can be estimated using the same approach traditionally used to analyze earthquake arrival times. Using three storms in the Gulf of Mexico, the Western Pacific (near Japan), and the South Pacific (near Fiji) respectively, we demonstrate that travel-time anomalies can be obtained from P waves generated by a distant storm, and that they are similar to those obtained from using an earthquake close to the storm. Our results suggest using oceanic storms as additional seismic sources for resolving P-wave travel-time anomalies. (C) 2010 Elsevier B.V. All rights reserved.

Rychert, CA, Shearer PM.  2010.  Resolving crustal thickness using SS waveform stacks. Geophysical Journal International. 180:1128-1137.   10.1111/j.1365-246X.2009.04497.x   AbstractWebsite

We image lithospheric interfaces using variations in the character of SS waveform stacks, a method we term SS Lithospheric Interface Profiling (SSLIP). The variations are caused by reflected phases, that is, underside reflections (SS precursors) and topside multiples (SS reverberations), created at velocity discontinuities near the midpoint of the SS ray path. Stacks from continental versus oceanic bounce point regions produce distinctly different SS waveforms, consistent with the large continent/ocean difference in crustal thickness. To investigate the potential of SS waveform stacks to constrain Moho depths under continents, we develop a method to fit continental bounce point stacks with a reference SS waveform convolved with a crustal operator. The SSLIP inferred Moho depths agree with the CRUST 2.0 model in Asia for those regions where the SS bounce point density is the highest. The SSLIP depths are correlated (correlation coefficient 0.82) with the CRUST 2.0 values averaged over sample bins of 10 degrees radius. The SSLIP method has broad lateral resolution in comparison to most other methods for resolving crustal thickness, but has the potential to sample regions where station coverage may be sparse.

Mancinelli, NJ, Shearer PM.  2013.  Reconciling discrepancies among estimates of small-scale mantle heterogeneity from PKP precursors. Geophysical Journal International. 195:1721-1729.   10.1093/gji/ggt319   AbstractWebsite

We stack amplitudes of over 10 000 high-frequency (similar to 1 Hz) PKP precursor waveforms, amassed from broad-band global seismic data with source-receiver distances between 120 degrees and 145 degrees recorded from 1990 to 2012. We forward model the stacked precursor envelope with an energy-conserving, multiple-scattering algorithm to find that an rms velocity perturbation of similar to 0.1 per cent fits the data reasonably well, in agreement with Margerin & Nolet. Similar results can be obtained using single-scattering (Born) theory, given the relatively weak scattering produced by our preferred model. The ramp-like increase in PKP precursor amplitudes with time is best fit with whole mantle scattering rather than models where scattering is restricted to the core-mantle boundary. Correctly modelling the relative amplitude of PKP precursor amplitudes compared to PKPdf requires taking into account the pulse broadening and coda of PKPdf, which can be done either empirically or by including a strongly scattering lithospheric layer in the multiple-scattering code. Several mantle scattering models proposed to explain other scattered seismic phases predict PKP precursor amplitudes much larger than those observed.

Chapman, CH, Shearer PM.  1989.  Ray Tracing in Azimuthally Anisotropic Media: 2. Quasi-Shear Wave Coupling. Geophysical Journal-Oxford. 96:65-83.   10.1111/j.1365-246X.1989.tb05251.x   Website
Shearer, PM, Chapman CH.  1988.  Ray Tracing in Anisotropic Media with a Linear Gradient. Geophysical Journal-Oxford. 94:575-580.   10.1111/j.1365-246X.1988.tb02277.x   Website