Seismic evidence of fluid migration in northeastern Japan after the 2011 Tohoku-Oki earthquake

We use ambient-noise-based seismic monitoring to detect an anomalous velocity decrease (?0.01%) widely distributed in Honshu that arose about 1 year after the 2011 Mw 9.0 Tohoku-Oki earthquake. The anomaly is located along central quaternary volcanic axis, and it suggests changes are related processes. After correction for possible external environmental forcing-related changes, remains, which implies associated with some internal physical process. show a general strong positive correlation between intensity of ground motion derived from daily cumulative moment. However, lack during itself reveals this reduction not directly caused by earthquake shaking. Tiltmeter low-pass observations temporal variations correlated changes. These strengthen hypothesis actual deformation. A previously reported fault strength (?10%) same period as further supports property change upper crust. also note simultaneous increase activity low-frequency events area, pore pressure propose observed early 2012 due induced upward fluid migration, at time triggered fluid-driven swarm seismicity events. recall depth-dependent derive average diffusion m2/s over around 11 months