Fine structure of deep Wadati–Benioff zone in the Izu–Bonin region estimated from S-to-P converted phase

Abstract

Mechanism of deep earthquakes is one of the most important and still unsolved problems in the field of solid geophysics. Determining precise location of the deep earthquakes can provide us with clues to approach this problem. In this study, we address the problem by estimating the 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 region. We interpret these later arrivals as an sP phase which is converted S-to-P at the upper slab interface. Our results prove that the hypocenters underlie the subducting upper slab interface. Using time delays between P and sP phases, we find that the interface is located 20 km above the earthquake hypocenters. Moreover, our results also indicate that the almost all deep earthquake hypocenters are distributed within a thin zone about 7 km wide rather than being scattered throughout the slab. This observation fits a hypothesis that deep earthquakes are caused by transformational faulting of metastable olivine model.