Localized extensional tectonics in an overall reverse-faulting regime, Northeast Japan

Umeda, Koji

A particularly notable event was the shallow, normal-faulting earthquake swarms with a T-axis oriented in the E-W or NW-SE directions that occurred immediately after the 2011 Tohoku-Oki earthquake near the Pacific coast in the southeast Tohoku district. The stress tensor inversion represents the pre-Tohoku-Oki earthquake stress field in this area as a normal-faulting stress regime with the minimum principal horizontal stress oriented in a roughly NW-SE direction. Additionally, the stress regime varies with depth from normal faulting at shallow depths ( 15 km) to thrust faulting at greater depths. A plausible explanation for these drastic changes in the stress regime is upward flexure of the upper crust due to partly anelastic deformation in the weakened lower crust. Spatial variation of the late Pleistocene uplift rates near the Pacific coast in the southeast Tohoku district can be interpreted as the interaction between regional upwraping and localized extensional tectonics.