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 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.