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Schulte-Pelkum, V, Monsalve G, Sheehan AF, Shearer P, Wu F, Rajaure S.  2019.  Mantle earthquakes in the Himalayan collision zone. Geology. 47:815-819.   10.1130/g46378.1   AbstractWebsite

Earthquakes are known to occur beneath southern Tibet at depths up to similar to 95 km. Whether these earthquakes occur within the lower crust thickened in the Himalayan collision or in the mantle is a matter of current debate. Here we compare vertical travel paths expressed as delay times between S and P arrivals for local events to delay times of P-to-S conversions from the Moho in receiver functions. The method removes most of the uncertainty introduced in standard analysis from using velocity models for depth location and migration. We show that deep seismicity in southern Tibet is unequivocally located beneath the Moho in the mantle. Deep seismicity in continental lithosphere occurs under normally ductile conditions and has therefore garnered interest in whether its occurrence is due to particularly cold temperatures or whether other factors are causing embrittlement of ductile material. Eclogitization in the subducting Indian crust has been proposed as a cause for the deep seismicity in this area. Our observation of seismicity in the mantle, falling below rather than within the crustal layer with proposed eclogitization, requires revisiting this concept and favors other embrittlement mechanisms that operate within mantle material.

Uchide, T, Yao HJ, Shearer PM.  2013.  Spatio-temporal distribution of fault slip and high-frequency radiation of the 2010 El Mayor-Cucapah, Mexico earthquake. Journal of Geophysical Research-Solid Earth. 118:1546-1555.   10.1002/jgrb.50144   AbstractWebsite

Earthquake slip history and moment release are best resolved using long period seismic waves, but details in the rupture process, such as sharp changes in rupture velocity or direction, can be imaged more clearly using higher frequency waves. Here, we investigate the slip and the high-frequency radiation histories of the 2010 El Mayor-Cucapah, Baja California, Mexico earthquake (Mw 7.2). The slip distribution inferred from inversion of strong motion data between 0.02 and 0.25Hz indicates northwest propagating rupture, followed by bilateral rupture for 40s. The sources of high-frequency radiation between 0.3 and 2Hz inferred from back-projection analysis using teleseismic data are adjacent to, but not within, the high-slip patches from the finite slip model in time and space. This implies relatively smooth rupture during the times and regions of maximum moment release. As theoretical models have predicted, high-frequency radiation seems mostly associated with changes in rupture velocity or slip magnitude. Strong high-frequency radiation is also found where the rupture propagated to a branch fault 50-km northwest of the hypocenter. Complementary constraints on both fault slip and high-frequency radiation provide increased understanding of earthquake rupture mechanics and may help improve strong motion evaluation at high frequencies.

Prieto, GA, Parker RL, Vernon FL, Shearer PM, Thomson DJ.  2006.  Uncertainties in earthquake source spectrum estimation using empirical Green functions. Earthquakes; radiated energy and the physics of faulting. 170( Abercrombie RE, McGarr A, Kanamori H, Di Toro G, Eds.).:69-74., Washington: American Geophysical Union   10.1029/170gm08   Abstract

We analyze the problem of reliably estimating uncertainties of the earthquake source spectrum and related source parameters using Empirical Green Functions (EGF). We take advantage of the large dataset available from 10 seismic stations at hypocentral distances (10 km < d <50 km) to average spectral ratios of the 2001 M5.1 Anza earthquake and 160 nearby aftershocks. We estimate the uncertainty of the average source spectrum of the M5.1 target earthquake by performing propagation of errors, which, due to the large number of EGFs used, is significantly smaller than that obtained using a single EGF. Our approach provides estimates of both the earthquake source spectrum and its uncertainties, plus confidence intervals on related source parameters such as radiated seismic energy or apparent stress, allowing the assessment of statistical significance. This is of paramount importance when comparing different sized earthquakes and analyzing source scaling of the earthquake rupture process. Our best estimate of radiated energy for the target earthquake is 1.24×1011 Joules with 95% confidence intervals (0.73×1011, 2.28×1011). The estimated apparent stress of 0.33 (0.19, 0.59) MPa is relatively low compared to previous estimates from smaller earthquakes (1MPa) in the same region.