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Structure of the Upper Crust in Japan from S-Wave Attenuation Tomography

Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China

Youshun Sun and M. Nafi Toksöz

Earth Resources Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02319

Charlotte A. Rowe

Los Alamos National Laboratory, Los Alamos, New Mexico 87545

Xing Gao, Junmeng Zhao, Hongbing Liu, and Jiankun He

Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China

F. Dale Morgan

Earth Resources Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02319

Seismic attenuation (Q-value) can be estimated by extracting the amplitude-frequency information contained in seismic waveforms. We apply the attenuation tomography method of Pei et al. (2006) using M_L amplitude data to estimate attenuation within the upper crust in Japan. More than 60,000 Sg-wave maximum amplitude readings from 5559 events, recorded by 971 stations, were selected from the dense High-Sensitivity Seismography Network (Hi-net) under the condition that epicentral distance is less than 2º and event depth is less than 10 km. The lateral S-wave Q variations of the upper crust at 1 Hz in Japan were obtained. The results indicate that low Q-values exist in the central Japanese islands, with almost the same distribution as volcanoes, while high Q-values exist mainly between the front of volcanoes and the Japanese east coast. In addition, a low Q was found between the eastern coast and the subducted trench. Most large crustal earthquakes occur in or around zones of low Q or the boundaries between areas of low and high Q, which will be very helpful in estimating the risk of large earthquakes.