[S-SS10] Active faults and paleoseismology

[SSS10-07] Paleoseismic activity and slip rate of the Obitoke and Tenri faults in the Nara basin

★Invited Papers

*Hiroyuki Tsutsumi¹, Nobuhiko Sugito², Haruo Kimura³, Shozo Kawashima¹, Toranosuke Morishima¹, Kaoru Taniguchi⁴, Masashi Omata⁴ (1.Department of Environmental Systems Science, Faculty of Science and Engineering, Doshisha University, 2.Faculty of Sustainability Studies, Hosei University, 3.Central Research Institute of Electric Power Industry, 4.Pasco Co.)

Keywords:the eastern marginal fault of the Nara basin, Obitoke fault, Tenri fault, paleoseismic trenching, borehole data, paleoseismic activity

The eastern marginal fault of the Nara basin consists of a series of east-dipping reverse faults at the topographic boundaries between mountains and hills on the east and basins and lowlands on the west. This fault zone poses high seismic hazards to the central Kinki area but the probability of earthquake occurrence in the next 30 years is very loosely constrained. Since the 2019 academic year, we have conducted active fault mapping, slip rate estimation, and obtaining paleoseismic data of the fault zone under a 3-year research project, "Comprehensive Research Project for the Nara-bonchi-toen Fault Zone." In 2020, we conducted paleoseismic investigations at a few localities within the Nara basin. At Imaichi, Nara City, we carried out a GPR survey, trenching, and borehole drilling to identify geologic evidence for recent faulting on the Obitoke fault. At Yashima, Nara City, we excavated pits to radiometrically date faulted and un-faulted geomorphic surfaces by the Tenri fault. Besides, geologic cross-sections based on the "Database for Boreholes in the Kansai Area" clearly illuminate the deformation of lower terrace deposits by the Obitoke fault, which gives a dip-slip rate of 0.3-0.4 mm/yr of the fault.