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Department of Physics New Mexico State University, Las Cruces, New Mexico 88003
Abstract
A comprehensive study of the gross characteristics of high-frequency seismic-wave propagation in China and its surrounding regions was accomplished to learn more about how regional geology affects high-frequency phases. Amplitudes of the regional seismic waves Sn and Lg relative to P coda were used to map lateral variations of shear-wave attenuation in the crust and upper mantle. Over 7000 digital seismograms from 13 broadband stations of the Chinese Digital Seismic Network and Global Seismic Network in the frequency range 0.5 to 5.0 Hz were visually examined. Propagation efficiencies of Sn and Lg were qualitatively analyzed by ranking their amplitudes relative to the P-wave coda. Efficient Sn propagation is observed in the Tien Shan, Tarim Platform, southern Tibet, Yangzi Paraplatform, and Sino-Korean Platform. We have confirmed results of previous studies by finding a lack of Sn transmission in north central Tibet, the Ryukyu and Japan Arcs, Burma, and the Baikal Rift. An important observation, which has not been previously reported, is that Sn does not propagate across Mongolia. The elimination of Sn and the occurrence of recent volcanism in north central Tibet and Mongolia suggest that there is partial melt in the uppermost mantle beneath those regions. High-frequency Lg waves propagate efficiently for most of China, Indochina, and Mongolia. Lg signals are attenuated within central Tibet as well as along its southern boundary. Crustal structure variations along with an anomalously low-Q crust are primarily responsible for weakened Lg amplitudes in Tibet. Propagation paths across oceanic crust in the Sea of Japan and the Andaman Sea show blockage of the Lg phase as well. This investigation has relevance to Comprehensive Test Ban Treaty monitoring by showing where certain regional phases will or will not propagate efficiently.
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