プレート境界における地震活動と沈み込む海洋性地殻の地震波速度構造との関係について : 特集「地震発生の物理からみた地震発生帯堀削」

Ipresent a seismic velocity structure model to explain strong seismic reflections from the aseismic part of the plate boundary recently reported from an airgun-OBS survey in the northern Japan trench subduction zone. Although no P wave velocity anomaly models could explain the reflection amplitudes and the first arrival travel times consistently, it is possible to simulate significant amplitudes observed in relative small offset ranges using a model with a low S wave velocity in the subducting oceanic crust. The upper oceanic crust with the low S wave velocity anomaly may reduce the shear strength of the plate boundary thrust, as fault damaged zones with a considerably low S wave velocity make faults weak. The result of this study indicates the importance of clarifying the S wave structure in the vicinity of the plate interface to understand the frictional propeties of subduction megathrust seismogenic zone. Not only conventional seismic surveys but also seismic observations in deep boreholes are required to do this