Uppermost mantle shear wave attenuation in China
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] We have used attenuation tomography of the regional seismic phase Sn to characterize the uppermost mantle shear wave Q (Qs) over a large part of China. The Sn phase is often a difficult phase to identify for continental paths since it usually has a relatively small amplitude compared with the later arriving Lg phase. It is often a high frequency phase and thus it is often blocked for paths that cross tectonically active regions. We have used seismic data from the Chinese National and Provincial Networks combined with temporary stations that were deployed across China over the last five years. These stations combine to give us an unprecedented amount of earthquake data in this region. We have been to be able to successfully identify Sn phases and use them to measure Sn Q using a reverse two station method (RTM). This method has advantages over other methods since it explicitly eliminates the influence of earthquake source, instrument response, and site responses. Sn waveforms from 150 earthquakes recorded by 892 stations were manually picked in order to obtain the ratio of Sn amplitude from each two-station pair. Those ratios describe Sn attenuation along each inter-station path. The Q value is typically assumed to be the intrinsic Q, whose lateral variation suggests temperature structure of the uppermost mantle. We have observed large variations in our Sn Q model; for example, we observe a low Q anomaly along the Shanxi rift suggesting hot uppermost mantle and high Q anomaly beneath the Ordos plateau and Sichuan basin, consistent with the cold/thick lithosphere. Also, we may have observed strong Sn scattering along the east coast of China, which results in low Q with a strong frequency dependency.
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