[S-SS13] Active faults and paleoseismology

[SSS13-P05] Geometry and slip distribution of the southern part of the eastern marginal faults of the Nara basin based on shallow subsurface data

*Shozo Kawashima¹, Hiroyuki Tsutsumi² (1.Division of Science of Environment and Mathematical Modeling, Graduate School of Science and Engineering, Doshisha University, 2.Department of Environmental Systems Science, Faculty of Science and Engineering, Doshisha University)

Keywords:the eastern marginal faults of the Nara basin, borehole database, shallow subsurface structure, reverse fault, Obitoke fault

The eastern marginal faults of the Nara basin consists of east-dipping reverse faults running along the topographic boundary between the eastern hills and mountains and western basins from the south-eastern rim of the Kyoto basin to the southeastern rim of the Nara basin. The slip rate of this fault system is calculated as 0.6–0.7 mm/yr by adding the ones of the Obitoke fault and the Tenri flexure. However, the slip rate of the Obitoke fault is not measured sufficiently. The Obitoke fault and the adjacent faults mainly offset lower terrace and the down-thrown side along the fault scarp is buried under the alluvial plain, therefore the spatial distribution of these faults is unclear. The author mapped the top of the terrace gravel to examine the spatial distribution of these faults and measure the fault displacement along them by analyzing a dense boring database.
Based on north-south geologic cross sections of the study area, alluvium is thickly deposited above subsided terrace gravels especially from Kodono, Nara City to Ichinomoto, Tenri City. Geologic cross sections across the faults enable us to measure more correct vertical displacement and its distribution suggests that the offset of the Obitoke fault is partially dispersed on the Ichinomoto flexure. The maximum vertical slip rate is calculated as 0.3–0.5 mm/yr for the Obitoke fault and 0.5–0.7 mm/yr for the fault zone. This study greatly improves our understanding of the subsurface structures and the slip rates of the eastern marginal fault of the Nara basin.