Abstract

After the 2011 Tohoku earthquake, the dominant type of focal mechanism in the offshore forearc in Northeast Japan changed dramatically from reverse faulting to normal faulting. The aim of this study was to clarify the formation mechanism of the stress field in the offshore forearc and the changes in the mechanism caused by the 2011 Tohoku earthquake. To this end, we modeled the absolute stress field using the finite-element method considering the long-term steady plate subduction, the gravitational forces, and the gigantic earthquake cycles. The model results indicate that the shallow part of the offshore forearc exhibits a normal-fault stress state both before and after the 2011 Tohoku earthquake. This normal-fault stress field originated from the gravitational forces and the shallower bending stress due to the steady plate subduction. The deeper bending stress and frictional stress at the plate interface caused the reverse-fault stress state in the offshore forearc near the plate interface. Our results suggest that the shift in the dominant type of focal mechanism is caused primarily by the following mechanism: Reverse faulting and normal faulting are facilitated and inhibited, respectively, by interplate coupling; however, they were inhibited and facilitated, respectively, by the effects of the 2011 Tohoku earthquake. Our model does not require a reversal of the stress field across the entire offshore forearc, although such a reversal may have occurred locally. Additionally, the results suggest that the plate interface is quite weak. The magnitude of frictional stress is 5–15 MPa for a distance of 40–160 km from the trench after the 2011 Tohoku earthquake.