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A 3D Tomographic Model of South Central Asia Based on Pn Travel Times from GT Events
SANDIA NATIONAL LABS LIVERMORE CA
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Increasingly, nuclear explosion monitoring is focusing on detection, location, and identification of small events recorded at regional distances. Because Earth structure is highly variable on regional scales, locating events accurately at these distances requires the use of region-specific models to provide accurate travel times. Improved results have been achieved with composites of 1D models and with approximate 3D models with simplified upper mantle structures, but both approaches introduce nonphysical boundaries that are problematic for operational monitoring use. Ultimately, what is needed is a true, seamless 3D model of the Earth. Towards that goal, we have developed a 3D tomographic model of the P velocity of the crust and upper mantle for the region of southcentral Asia centered around the Tibetan Plateau. Our model is derived from almost 140,000 Pn picks for more than 5300 events recorded at 563 stations from a Ground Truth GT dataset assembled by Los Alamos National Laboratory LANL. Our starting model is the a priori model of East Asia developed by LANL, which is based on various global and regional studies. The topmost layers come from the Laske and Masters global sedimentary model from 1997. As our dataset lacks the resolution to improve this sedimentary portion of our model, we fix the velocity and depth of these layers, as well as the depths of the major mantle discontinuities Moho, 410 km, 660 km. We invert for P velocities from the crust down through the upper mantle, along with source and receiver terms to account for the effects of event mislocation and fine-scale structure near the receiver not accounted for in the crustal model. Forward calculations are made using our own implementation of the Um and Thurber ray pseudo-bending approach Ballard, 2008, these Proceedings with full enforcement of Snells Law in 3D at the major discontinuities.
APPROVED FOR PUBLIC RELEASE