FINE-STRUCTURE OF DEEP WADATI-BENIOFF ZONE IN THE IZU-BONIN REGION ESTIMATED FROM S-TO-P CONVERTED PHASE

Mechanism of deep earthquakes is one of the most important and still u nsolved problems in the field of solid geophysics. Determining precise location of the deep earthquakes can provide us with clues to approac h this problem. In this study, we address the problem by estimating th e separation distances between hypocenters of the deep-earthquakes and slab-asthenosphere interface. We use later arrivals immediately after initial P arrivals from the deep earthquake events in the Izu-Bonin r egion. We interpret these later arrivals as an sP phase which is conve rted S-to-P at the upper slab interface. Our results prove that the hy pocenters underlie the subducting upper slab interface. Using time del ays between P and sP phases, we find that the interface is located 20 km above the earthquake hypocenters. Moreover, our results also indica te that the almost all deep earthquake hypocenters are distributed wit hin a thin zone about 7 km wide rather than being scattered throughout the slab. This observation fits a hypothesis that deep earthquakes ar e caused by transformational faulting of metastable olivine model. (C) 1998 Published by Elsevier Science B.V.