Normal Faulting Earthquake Sequence Induced in Pacific Coastal Area of Abukuma Belt due to Reactivation of Ancient Riedel Shears

Normal faulting earthquake sequence was induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw9.0) in Hamadori region of Fukushima prefecture and northern part of Ibaraki prefecture. T-axes derived from focal mechanisms of events occurred in Hamadori region of Fukushima prefecture and northern part of Ibaraki prefecture are perpendicular to each other. It should be noted that these normal faulting earthquakes are distributed in the Pacific coastal area of Abukuma belt. Based on the investigation about past activity of Tanakura Tectonic Line, Hatagawa Tectonic Line and Futaba fault, we found that Pacific coastal area of Abukuma belt should have been deformed as sinistral and dextral shear zones. According to Riedel shear experiment as performed by previous study, Riedel shears should have been developed by the sinistral and dextral shear deformation of Pacific coastal area of Abukuma belt. We concluded that normal faulting earthquakes occurred in Hamadori region of Fukushima prefecture can be regarded as reactivation of Riedel shears formed by dextral shear deformation of Pacific coastal area of Abukuma belt. In contrast, those events occurred in northern part of Ibaraki prefecture can be regarded as reactivation of Riedel shears formed by sinistral shear deformation of Pacific coastal area of Abukuma belt. Yunodake fault and western trace of Itozawa fault were ruptured by the occurrence of 2011 Fukushima Hamadori earthquake (Mj7.0). These normal faults can be regarded as ancient Riedel shears formed by sinistral shear deformation of Pacific coastal area of Abukuma belt.