Small fault scarp or" shishigaki", which is it?:a case study on the Neodani fault

概要

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Pit excavation across a small uphill-facing fault scarp on a mountain slope could be a paleoseismological tool for active faults without suitable sites for traditional trenching. We tested this method on the Neodani fault, which is one of causative faults for the 1891 M 8.0 Nobi earthquake. We found an unnatural small uphill-facing scarp on a mountain slope just along the trace of the fault and decided that the scarp should have tectonic origin. However, pit excavation across it, radiocarbon ages from the pit, and following additional investigation on historical documentations reveal that it is probably one of ruins of a" shishigaki" (man-made protection wall against damage to crops by wild animals), which was built during the late Edo period (1804-1810 A. D. ). With the knowledge of a shishigaki, we would have previously recognized that the scarp was not a tectonic one: key observations are presence of level land along the scarp that used to be a paddy field (now woods), and common lack of recognizable vertical offset of a mountain slope across the scarp. As ruins of a shishigaki are reported to be widely distributed in all Japan except for Hokkaido and Tohoku regions, great care should be exercised with them in recognizing small fault scarps on a mountain slope.