[S-SS12] Active faults and paleoseismology

[SSS12-P06] Shallow subsurface structures of the Obitoke fault in the Nara basin based on ultra-rapid-stacking GPR profiling

*Shozo Kawashima¹, Haruo Kimura², Kaoru Taniguchi³, Toshimichi Nakanishi⁴, Hiroyuki Tsutsumi⁵ (1.Division of Science of Environment and Mathematical Modeling, Graduate School of Science and Engineering, Doshisha University, 2.Central Research Institute of Electric Power Industry, 3.Pasco Corporation, 4.Museum of Natural and Environmental History, Shizuoka, 5.Department of Environmental Systems Science, Faculty of Science and Engineering, Doshisha University)

Keywords:the eastern marginal fault of the Nara basin, Obitoke fault, ground-penetrating radar (GPR), ultra-rapid stacking, shallow subsurface structure, reverse fault

The eastern marginal fault 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 paleoseismic activity of this fault system is not clarified sufficiently, therefore the estimated possibility of earthquake occurrence along this fault system is very loosely constrained. 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."
We carried out ground penetrating radar (GPR) profiling across the Obitoke fault in a dissected valley at Imaichi, Nara City. The GPR data were collected by 25 MHz and 50 MHz ultra-rapid-stacking GPR system (pulseEKKO Ultra made by Sensors and Software Inc.). This system can conduct over 32000 to 65000 stacks, in contrast to 64 stacks by conventional system (pulseEKKO PRO). This increases the signal-to-noise ratio and enables to see the more GPR signal at greater depths.
At Imaichi, we carried out trenching and arrayed boring from 2020 to 2021 (MEXT and DPRI, 2021). Prior to these research, we attempted to investigate shallow subsurface structures across the Obitoke fault by GPR surveys, although the obtained profile has a maximum depth of about 2 m. Therefore, we have conducted GPR profiling with depth of 15 m or more by using a high-stacking GPR system to supplement the result of arrayed boring. The obtained profile of the 25 MHz survey shows distinct reflectors from about 6 m to 12 m depth. We inferred that these reflectors display a flexure formed by the Obitoke fault.