Weak Japan forearc crust revealed by coseismic stress change and aftershock mechanisms from M9 Tohoku-Oki megathrust earthquake

The 11 March 2011 Tohoku-Oki megathrust earthquake (Mw 9.0) off the east coast of Honshu, Japan resulted in fault displacement up to 30 m and excited a large tsunami that caused huge catastrophe on the east coast of Japan. Off-shore focal mechanisms before the Tohoku-Oki earthquake near the hypocenter show mainly thrust earthquakes but a large number of normal faulting earthquakes occurred in this area after Tohoku-Oki earthquake. Does this imply the stress change induced by the Mw 9.0 Tohoku-Oki megathrust earthquake is large enough to significantly rotate the principal stress orientations? In other words, does this imply that the absolute deviatoric crustal stress in the Japan forearc is of similar magnitude to the coseismic stress change? We use focal mechanisms solutions from 1997 to 11 May 2011 and the computed coseismic stress change produced by the Tohoku-Oki mainshock to invert for the 3D distribution of absolute deviatoric stress in Japan forearc. The differential stress increases from ~50 MPa near the surface to ~170 MPa at 50 km depth, significantly lower than values of 50-700 MPa expected from Byerlee friction. Assuming the strength of the crust is limited by friction, τ=μ(σ-pw ), these results imply low strength for the crust with either elevated pore pressure, pw, or low coefficient of friction, μ≈0.17. The coseismic stress changes were large relative to the background stress with maximum shear stress change ranging from ~20-30 MPa near the surface to ~50-60 MPa near the subduction interface. These stress changes result in rotations of principal stress directions of 30 to 90 degrees.