Developing process of active fault system in the west-central Japan

The developing process of an active fault system is useful for estimating the crustal deformation. Especially, a spatial and temporal evolution of Quaternary faulting can provide geological constraints on estimating a potential area of future faulting. In this study, a spatial and temporal evolution of faulting in the Atera fault zone, which limits the southwestern border of the Northern Japan Alps, is revealed based on geomorphological and geological information. In addition, we discuss the developing process of an active fault system in the west-central Japan (WCJ). The WCJ is characterized by many active faults with a strike-slip or a reverse dip-slip. In particular, the Atera fault zone is one of the major strike-slip faults with high slip rates in Japan. We carried out an investigation of paleo-topography, a measurement of tectonic relief and a chronology of sediment to clarify total displacements and slip rates of the fault zone. On the basis of a started age of a present faulting pattern calculated by the total displacement and the slip rate, the Atera fault zone during the last few million years can be divided into three segments: the initiation segment of a present faulting pattern, the slowed segment of fault activity, and the extending segment of faulting. The initiation segment before the Early Pleistocene had been situated not only in the southeastern half of present Atera fault zone but also in the southeastern prolongation. Although a southeastern domain of the initiation segment became inactive as the slowed segment in the Middle Pleistocene, micro-earthquakes still occur frequently in this area. On the other hand, the northwestern prolongation of the initiation segment has become active as the extending segment since the Middle Pleistocene. In brief, the spatial and temporal evolution of faulting suggests northwestward shift of activated fault domain for the last few million years. We calculated a started age of present faulting pattern of other active faults in the WCJ by the same method as the Atera fault zone. As a result, the start age of active faults in the northern part tends to be younger than those in the southern part. This tendency suggests that the distribution of active faults in the WCJ extended to the north during the last few million years. The future perspective of this study will discuss the developing process of an active fault system with a volcanism, considering a relationship to the plate motion. In particular, we need geomorphological and geological information on total-displacements and slip-rates accurately in many active faults. In addition, a sedimentation process and a cooling history of the plutonic rock around active faults are expected to be useful indicators for estimating the developing process of an active fault.