The 2011 Tohoku-Oki offshore subduction earthquake (M w 9.0) triggered many normal-type earthquakes inland in northeastern Japan. Among these were two very similar normal-faulting earthquakes in 2011 (M w 5.8) and 2016 (M w 5.9), which created surface ruptures along the newly named Mochiyama fault within the southern Abukuma Mountains, northeastern Japan, where no active faults had been previously mapped by interpretation of aerial photographs. We conducted field surveys in this area immediately after both earthquakes, and we performed trench excavations and observations of fault fracture zones after the 2016 event. These activities were complemented by an interferometric synthetic aperture radar analysis that mapped the areas of deformation and locations of surface discontinuities for both events. The combined results document the coseismic behavior of the Mochiyama fault during both events. Subtle tectonic geomorphic features associated with the fault were evident in a lidar digital elevation model of the area, and layered structures of gouge were documented in the field. These lines of evidence indicate repeated activity at shallow crustal levels and the possibility of Quaternary activity. In addition, our trench excavations revealed at least one faulting event before 2011. Our comparison of paleoseismic records on this and two other normal faults in the Abukuma Mountains suggests that great earthquakes in the Japan Trench supercycle of 500-700 years do not consistently trigger ruptures on these faults, and the case of 2011, in which the Tohoku-Oki megathrust earthquake triggered all three faults, is a rare occurrence.

The geological structure and tectonic evolution of the Paleozoic-Mesozoic accretionary complexes of the Ultra-Tamba and Tamba terranes in the Maizuru and Obama districts in SW Japan were clarified through geological surveys and investigations of fault rocks. A detailed geological map was created by precise field surveys and compilation of past investigation results. Based on the new geological map, we discuss the geotectonic history of the study area. The Ultra-Tamba terrane, consisting of a late Permian-possibly Triassic accretionary complex, is subdivided into the Kozuki, Oi, and Hikami formations. The Kozuki Formation is newly identified in the study area and occurs sporadically along the boundary between the Yakuno Ophiolitic Complex of the Maizuru terrane and the Oi Formation of the Ultra-Tamba terrane. The Tamba terrane comprises mainly an Early-Late Jurassic accretionary complex and is subdivided into the Shuzan, Kumogahata, Haiya, Tsurugaoka, and Yuragawa complexes, and the Furuya Formation. The complexes are separated by thrust faults, each comprising a single nappe, and display an imbricate nappe structure as a whole. Layer-parallel, brittle-ductile deformation structures are predominant in the Ultra-Tamba terrane and the Shuzan Complex of the Tamba terrane. These complexes were folded along E-W trending vertical axial planes during the Early Cretaceous. The Kanbayashigawa Fault is characterized by a NE-SW trending cataclastic shear zone with a sinistral shear sense that formed under a paleo-stress field cutoff at these fold structures. The southwestern part of this fault has been reactivated with a dextral shear sense under the current stress field. This slip-sense inversion resulted from the conversion of the stress-field and/or the rotation of regional blocks, and the opening of the Sea of Japan played an important role in the reactivation and slip-sense inversion of old fault structures in SW Japan.