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

圧縮場における局所的な引張テクトニクス

梅田 浩司

Umeda, Koji

2011年東北沖地震の直後から福島県の浜通りでは、群発地震が発生し、現在もその活動が継続している。この地域では東北沖地震の前から引張場であり、沈み込み帯の前弧域としては特異な地殻応力であることが知られていた。本報では地震テクトニクス、物理探査、地形・地質調査等の最近の研究成果のレビューを行うとともに、局所的に引張場が生じるメカニズムについて検討した。

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.