DTIC ADA519480: Geophysical Imaging Of Asia And Siberia: Tomography For Seismic Velocity, Upper Mantle Gradient, Lg Attenuation, And Joint Inversion Of Surface Wave Dispersion, Receiver Functions And Satellite Gravity Data - Info and Reading Options

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Our geophysical modeling efforts have focused on a variety of special studies. We continue to develop models for Eastern Russia using the Michigan State Siberia database, for which we have expanded over previous Pn velocity tomography by also inverting Lg amplitudes for a regional attenuation study, and a preliminary tomographic image of Pg travel times in Eurasia, including the Siberia region. Both Lg attenuation and Pg velocity tomographic studies illuminate features consistent with known aspects of Siberian and Russian Far East tectonics and are consistent with features derived in our earlier Pn study. Lg attenuation studies will help to improve our understanding of phase amplitudes for calibration across eastern Russia, and an improved Pg model will allow us to better calibrate travel times in the area when secondary phase arrivals are needed for event location. We are exploring the effects of upper mantle velocity gradients on the two-dimensional (2-D) tomographic image for P-waves that travel through the upper mantle for regional-distance source-receiver pairs (2.5 to 18 degrees). By adding an upper mantle gradient term to the inversion for 2-D varying Pn velocity and crustal time terms, we are able to map a 2-D varying upper mantle gradient, based on travel times from high-quality event locations. Lateral gradients in the upper mantle across central and eastern Eurasia vary from -0.001 s(-1) to 0.003 s(-1). High gradients appear to be associated with regions of tectonic convergence, both continental and oceanic, as well as stable cratonic regions. Variance reduction is 63% with respect to Pn tomography without gradients. An improved mapping not only of Pn velocities but also of the upper mantle gradients that influence the travel times for this phase is important for ongoing efforts to improve event location throughout central and eastern Asia.

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