Shallow structure of the Fujikawa-kako fault system revealed by 3D FWI

The results of this study are summarized as follows. (1) The Omiya and the Iriyamase faults that represent the frontal faults of the Fujikawa-kako fault system, form a single w-dipping reverse at depth. The dip of the fault becomes gentler towards the surface, and nearly horizontal beneath the eastern cliff of the Hoshiyama Hills, although it does not breach the surface. As a result, the cliff does not correspond to the fault scarp, but to the flexure of the hanging wall associated with the subsurface faulting. (2) The hinge of the flexure follows the eastern edge of the cliffs forming a horseshoe shape. This suggests that the eastward displacement of the hanging wall is estimated to be larger at the tip of the horseshoe and smaller at its sides, which is common for hanging walls of thrusts. (3) It is necessary to reexamine MEXT’s (2010) evaluation of the Fijikawa-kako fault system activity assuming the Omiya and the Iriyamase faults to be high-angle normal faults.