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 seismic velocity decrease (similar to 0.01%) widely distributed in Honshu that arose about 1 year after the 2011 M-w 9.0 Tohoku-Oki earthquake. The anomaly is located along the central quaternary volcanic axis, and it suggests that the changes are related to volcanic processes. After correction for possible external environmental forcing-related velocity changes, the anomaly in the seismic velocity remains, which implies that it is associated with some internal physical process. We show a general strong positive correlation between the seismic velocity changes and the intensity of ground motion derived from the daily cumulative seismic moment. However, the lack of correlation during the anomaly itself reveals that this reduction is not directly caused by earthquake shaking. Tiltmeter low-pass observations show temporal variations that are correlated with the velocity changes. These observations strengthen the hypothesis of actual physical deformation. A previously reported decrease in fault strength (similar to 10%) for the same period as the velocity anomaly further supports a physical property change in the upper crust. We also note a simultaneous increase in activity of low-frequency events in the volcanic area, which suggests an increase in pore pressure in the upper crust. We propose that the observed anomalous seismic velocity decrease in early 2012 is due to an increase in pore pressure induced by an upward fluid migration, which at the same time triggered the increase in fluid-driven swarm seismicity and low-frequency events. We recall the depth-dependent seismic velocity changes in Honshu and derive an average diffusion of 1 m(2)/s over around 11 months after the Tohoku-Oki earthquake. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Using ambient-noise-based seismic monitoring, we detected an anomalous seismic velocity decrease in Honshu about one year after the 2011 Tohoku-Oki earthquake, suggesting a possible relation to volcanic processes and internal physical changes. Tiltmeter observations revealed temporal variations correlated with velocity changes, supporting the hypothesis of actual physical deformation in the crust. The observed decrease in fault strength and concurrent increase in low-frequency events in the volcanic area further indicate changes in physical properties and an increase in pore pressure in the upper crust.