[S-SS10] Active faults and paleoseismology

[SSS10-P10] An analysis of the distribution of the Obitoke and Sahoda faults in the Nara basin based on airborne LiDAR and GPR surveys

*Shozo Kawashima¹, Hiroyuki Tsutsumi², Haruo Kimura³ (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, 3.Central Research Institute of Electric Power Industry)

Keywords:the eastern marginal fault of the Nara basin, Obitoke fault, Sahoda fault, ground-penetrating radar (GPR), airborne LiDAR

The eastern marginal fault of the Nara basin consists of east-dipping reverse faults running along the topographic boundary between eastern hills and mountains and western basins from the south-eastern rim of the Kyoto basin to the rim of the Nara basin. Paleoseismic activity of this fault system is not clarified sufficiently, therefore the estimated possibility of occurrence of earthquake along this fault system is very loosely constrained. The distribution of the Obitoke fault and the Sahoda fault, which run inside the Nara basin has not been revealed precisely because of unclear displaced fault landforms. We conducted surveys for obtaining paleoseismic data of the fault zone and fault mapping under a 3-year research project from the 2019 academic year, "Comprehensive Research Project for the Nara-bonchi-toen Fault Zone."

To investigate near-surface structures to decide a site of paleoseismic trenching, we carried out ground penetrating radar (GPR) profiling across the Obitoke fault at Imaichi and Ikeda, Nara City. The length of the GPR survey lines were 128 m at Imaichi and 144 m at Ikeda. The GPR data were collected by 50 MHz GPR system (pulseEKKO PRO made by Sensors and Software Inc.) The depth converted GPR sections after data processing showed near-surface structures above a maximum depth of about 2 m. At Imaichi, the GPR section showed structures interpreted as flexures, which suggested that they recorded accumulation of deformation at a point of an extension of a scarp formed by the Obitoke fault. At Ikeda, we found structures that were supposedly faults.

To map faults on the north-eastern part of the Nara basin, we made a digital elevation model from airborne LiDAR data obtaind by Doshisha University. As a result, the trace of the Obitoke fault was verified that it extends for 8 kilometers or more. The trace of the Sahoda fault was confirmed to be running for about 4 kilometers and back-tilting topography was found on the uplifted side.