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Denolle, MA, Fan WY, Shearer PM.  2015.  Dynamics of the 2015 M7.8 Nepal earthquake. Geophysical Research Letters. 42:7467-7475.   10.1002/2015gl065336   AbstractWebsite

The 2015 M7.8 Nepal earthquake ruptured part of the Main Himalayan Thrust beneath Kathmandu. To study the dynamics of this event, we compute P wave spectra of the main shock and of two large aftershocks to estimate stress drop and radiated energy. We find that surface reflections (depth phases) of these shallow earthquakes produce interference that severely biases spectral measurements unless corrections are applied. Measures of earthquake dynamics for the main shock are within the range of estimates from global and regional earthquakes. We explore the azimuthal and temporal variations of radiated energy and highlight unique aspects of the M7.8 rupture. The beginning of the earthquake likely experienced a dynamic weakening mechanism immediately followed by an abrupt change in fault geometry. Correlation of backprojection results with frequency-dependent variations in the radiated energy rate and with the suggested geometry of the Main Himalayan Thrust yields new constraints on dynamic ruptures through geometrical barriers.

Earle, PS, Shearer PM.  2001.  Distribution of fine-scale mantle heterogeneity from observations of P-diff coda. Bulletin of the Seismological Society of America. 91:1875-1881.   10.1785/0120000285   AbstractWebsite

We present stacked record sections of Global Seismic Network data that image the average amplitude and polarization of the high-frequency P-diff coda and investigate their implications on the depth extent of fine-scale (similar to10 km) mantle heterogeneity. The extended 1-Hz coda lasts for at least 150 sec and is observed to a distance of 130degrees. The coda's polarization angle is about the same as the main P-diff, arrival (4.4 sec/deg) and is nearly constant with time. Previous studies show that multiple scattering from heterogeneity restricted to the lowermost mantle generates an extended P-diff coda with a constant polarization. Here we present an alternative model that satisfies our P-diff observations. The model consists of single scattering from weak (similar to1%) fine-scale (similar to2 km) structures distributed throughout the mantle. Although this model is nonunique, it demonstrates that P-diff coda observations do not preclude the existence of scattering contributions from the entire mantle.

Shearer, PM, Oppenheimer DH.  1982.  A Dipping Moho and Crustal Low-Velocity Zone from PN Arrivals at the Geyers-Clear Lake, California. Bulletin of the Seismological Society of America. 72:1551-1566.Website
Houser, C, Masters G, Flanagan M, Shearer P.  2008.  Determination and analysis of long-wavelength transition zone structure using SS precursors. Geophysical Journal International. 174:178-194.   10.1111/j.1365-246X.2008.03719.x   AbstractWebsite

Global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry, dynamics and mineralogy. Numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite. Underside reflections of the 410 and 660 km discontinuities arrive as precursors to SS. With the recent development of a semi-automated method of determining SS arrivals, we have more than tripled the Flanagan and Shearer (1998a) data set of handpicked SS waveforms. We are able to increase resolution by stacking waveforms in 5 degrees rather than 10 degrees radius bins as well as increasing data coverage significantly in the southern hemisphere. The resulting SS-S410S and SS-S660S times are heavily influenced by upper-mantle velocity structure. We perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods. The 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not. In addition, the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography, rather anticorrelated, as expected due to the opposite signs of the Clapeyron slopes of their respective phase changes. These results suggest that, whereas the topography of 660 km discontinuity could be dominated by thermal effects, the topography of the 410 km discontinuity is likely dominated by compositional effects. In addition, unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity, our 410 and 660 km topography have similar amplitudes.

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.

Shearer, P, Masters G.  1990.  The Density and Shear Velocity Contrast at the Inner Core Boundary. Geophysical Journal International. 102:491-498.   10.1111/j.1365-246X.1990.tb04481.x   Website
Fialko, Y, Sandwell D, Agnew D, Simons M, Shearer P, Minster B.  2002.  Deformation on nearby faults induced by the 1999 Hector Mine earthquake. Science. 297:1858-1862.   10.1126/science.1074671   AbstractWebsite

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.