Using seismic arrival times from dense land and ocean floor networks, this study constructs a detailed seismic tomography model of the megathrust zone beneath the Tohoku forearc to understand the 2011 Tohoku-oki earthquake (Mw 9.0). Results show coseismic slip initiated at a boundary between a down-dip high-velocity and an up-dip low-velocity anomaly. The shallow low-velocity anomaly near the Japan trench, possibly reflecting low-rigidity materials, explains the large slip and weak high-frequency radiation observed.

Yuanyuan Hua, Dapeng Zhao, Genti Toyokuni, Yixian Xu