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

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 2 degrees 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.