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

A fine-scale three-dimensional attenuation structure beneath the northeastern Japan are has been obtained using a joint inversion for source parameters, site response and Q values. Data used for this study are P wave spectra of microearthquakes occurring in the subducting Pacific plate. The subducting 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 and the mantle wedge within the study area. For the southern part of the study area, low-Q zones are distributed in the crust beneath the volcanic front. 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 beneath 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 plane 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 found in the present study is nearly consistent with that of the low-velocity zones estimated from studies of travel times tomography. Partial melting zones in the mantle wedge, whose existence has been suggested from the study of seismic velocity structure and laboratory experiments, are located in the low Q zones obtained in the present study. These results suggest that spatial distribution of Q values is closely related to heterogeneous distribution of temperature in the mantle wedge. (C) 2000 Elsevier Science B.V. All rights reserved.