Abstract

The incoming oceanic plate bends while approaching the subduction zone in the trench, and normal faults, which can cause earthquakes, develop because of the plate bending. Near the Japan Trench, historical outer-rise earthquakes (M8-class) that generated huge tsunamis had previously occurred after megathrust earthquakes. Following the Tohoku-Oki earthquake, there have been several outer-rise earthquakes in this area, including a few M7-class earthquakes. However, it is still unclear which faults are most likely to cause a major outer-rise earthquake. It is important to understand the geometry of the normal faults developing in the outer-rise as well as the development process and activity of the normal faults in terms of evaluating the risk of earthquake occurrence under current stress field. In this study, we use a method that evaluates the activity of normal faults with slip tendency calculated by stress field and fault geometry. The near-fault stress field was calculated from the earthquake focal mechanism, and the fault geometry was extracted from the seafloor bathymetry map and the 2D seismic sections after a time-to-depth conversion of time-migrated profiles. We calculated the slip tendency of normal faults developing in the outer-rise of the Japan Trench as one typical example. We found the slip tendency in the west-dipping faults to be significantly higher than that in the east-dipping faults. We also noticed older abyssal-hill faults show low activity. An area of low slip tendency faults is recognized around 38.8°–39°N near the Japan Trench, where the magnetic anomalies do not continue and fracture zone faults are developing. Based on the fault activity analysis, this study shows that the earthquake occurrence in the outer-rise of the Japan Trench is spatially heterogeneous, but more likely to occur on the west-dipping faults.