Three-dimensional attenuation structure beneath the northeastern Japan arcestimated from spectra of small earthquakes

A fine-scale three-dimensional attenuation structure beneath the northeaste rn Japan are has been obtained using a joint inversion for source parameter s, site response and Q values. Data used for this study are P wave spectra of microearthquakes occurring in the subducting Pacific plate. The subducti ng plate is found to have higher Q values than the overlying mantle. There is an along-are variation in the distribution of low-Q zones in the crust a nd the mantle wedge within the study area. For the southern part of the stu dy area, low-Q zones are distributed in the crust beneath the volcanic fron t. They extend into the mantle wedge and become deep toward the west, that is, toward the backarc region. On the other hand, the northern part of the study area has low-Q zones at two depth ranges; at shallower depths just be neath the volcanic front and at the deeper part of the mantle wedge beneath the backarc region. The region without active volcanoes along the volcanic front has relatively high mantle Q values. Seismic activity in the lower p lane of the double-planed deep seismic zone is high beneath this relatively high-Q region of the mantle wedge. Spatial distribution of low-Q zones fou nd in the present study is nearly consistent with that of the low-velocity zones estimated from studies of travel times tomography. Partial melting zo nes in the mantle wedge, whose existence has been suggested from the study of seismic velocity structure and laboratory experiments, are located in th e low Q zones obtained in the present study. These results suggest that spa tial distribution of Q values is closely related to heterogeneous distribut ion of temperature in the mantle wedge. (C) 2000 Elsevier Science B.V. All rights reserved.