Attenuation structure beneath the volcanic front in northeastern Japan from broad-band seismograms

Anelastic structure in the asthenosphere beneath the volcanic front in nort heastern Japan are is estimated by using the spectral amplitude ratio data of P and S waves from about 100 events which occurred in the subducting Pac ific slab below Japan. These earthquakes occurred within a 90 km radius cen tered about the station Sawauchi (SWU), with focal depths ranging from 60 t o 200 km. Waveforms were recorded by the Carnegie broad-band three-componen t seismograph and were corrected for instrument responses, crustal reverber ations, corner frequencies, and superimposed noise. Ray paths and travel ri mes of P and S waves are calculated using a three-dimensional velocity mode l [Zhao, D., Hasegawa, A., Horiuchi, S., 1992. J. Geophys. Res. 97, 19909-1 9928]. We find a low-Q region (Q(s) similar to 70) extending down to 55 km depth from the lower crust beneath the volcanic front. Using Q-temperature laboratory results [Sato, H., Sacks, I.S., Murase, T., Muncill, G., Fukushi ma, H., 1989. J. Geophys. Res. 94, 10647-10661], this implies a temperature of about 130 degreesC higher than the eastern forearc region and about 30 degreesC higher than the western backarc region, in good agreement with the tomographic results of Zhao et al. [Zhao, D., Hasegawa, A., Horiuchi, S., 1992. J. Geophys. Res. 97, 19909-19928]. This suggests that low velocities in the crust and uppermost mantle beneath SWU may be explained by a subsoli dus temperature increase without partial melting. (C) 2000 Elsevier Science B.V. All rights reserved.