Pn and Sn tomography across Eurasia to improve seismic event locations

This paper has three motivations: first, to map P-n and S-n velocities beneath most of Eurasia to reveal information on a length scale relevant to regional tectonics, second, to test recently constructed 3-D mantle models and, third, to develop and test a method to produce P-n and S-n travel time correction surfaces which are the 3-D analogue of travel time curves for a 1-D model. Our third motive is inspired by the need to improve regional location capabilities in monitoring nuclear treaties such as the nuclear Comprehensive Test Ban Treaty (CTBT). To a groomed version of the ISC/NEIC data, we apply the tomographic method of Barmin et al. [Pure Appl. Geophys. (2001)], augmented to include station and event corrections and an epicentral distance correction. The P-n and S-n maps are estimated on a 1degrees x 1degrees grid throughout Eurasia. We define the phases P-n and S-n as arriving between epicentral distances of 3degrees and 15degrees. After selection, the resulting data set consists of about 1,250,000 P,, and 420,000 S-n travel times distributed inhomogeneously across Eurasia. The rms misfit to the entire Eurasian data set from the P-n and S-n model increases nearly linearly with distance and averages about 1.6 s for P-n and 3.2 s for S-n, but is better for events that occurred on several nuclear test sites and for selected high-quality data subsets. The P-n and S-n maps compare favorably with recent 3-D models of P and S in the uppermost mantle and with recently compiled teleseismic station corrections across the region. The most intriguing features on the maps are the low-velocity anomalies that characterize most tectonically deformed regions such as the anomaly across central and southern Asia and the Middle East that extends along a tortuous path from Turkey in the west to Lake Baikal in the east. These anomalies are related to the closing of the Neo-Tethys Ocean and the collision of India with Asia. The uppermost mantle beneath the Pacific Rim back-arc is also very slow, presumably due to upwelling that results from back-arc spreading, as is the Red Sea rift, the Tyrrhenian Sea and other regions undergoing active extension. (C) 2002 Elsevier Science B.V. All rights reserved.