[S-SS13] Active faults and paleoseismology

[SSS13-08] Active geologic structures at the border area of the Kyoto basin and Osaka Plain based on subsurface geologic data

*Hiroyuki Tsutsumi¹ (1.Department of Environmental Systems Science, Faculty of Science and Engineering, Doshisha University)

Keywords:Active faults, Kyoto basin, Osaka plain, subsurface geologic data

The border area of the Kyoto basin and Osaka Plain is bounded and transected by a complex system of the east- and north-trending active faults. The 2018 Northern Osaka Prefecture earthquake (M_j 6.1) exhibited a complex source fault mechanism and reminded serious seismic hazards to the densely populated region between the Osaka and Kyoto metropolitan areas. However, active geologic structures have yet to be mapped sufficiently due to intense artificial modification of the landform and erosion/sedimentation by the Yodo River and its tributaries. To better understand the active faults' location and late Quaternary activity, we analyzed published seismic reflection profiles and borehole data and conducted tectonic geomorphic analyses. The Kyoto basin and Osaka Plain are divided by the north-trending Enmyoji and Otokoyama faults that form a continuous west-dipping reverse fault with evidence of late Quaternary movement. The Arima-Takatsuki Tectonic Line fault zone extends about 9 km further east of the geomorphologically mapped surface trace. We identified that the Hirakata flexure extends northeast beneath the Yodo River as a southeast-dipping reverse fault along the northwest margin of the Hirakata Hills. The area bounded by the Arima-Takatsuki Tectonic Line fault zone and the northeast extension of the Hirakata flexure is a northeast-trending graben called the Yodogawa graben. A series of north-trending geologic cross-sections based on borehole data revealed the precise location of the Ujigawa fault, which is several hundred meters south of the trace mapped primarily by seismic reflection profiling. The Ujigawa fault is about 10 km long, with north-up displacement extending from Yawata City to Uji City. The fault does not extend west across the Otokoyama fault but truncates the north-trending Momoyama fault. The complex faulting may reflect that the area is located at the boundary between the area to the north where the east-vergent reverse faults are dominant and the area to the south where the west-vergent reverse faults are dominant.