Paleoearthquakes recorded by deformed fluvial deposits on the Boshima fault, Arima-Takatsuki fault zone, southwest Japan

Abstract

The Arima-Takatsuki fault zone (ATFZ) consists of active intraplate strike-slip faults that extend for about 42 kilometers in southwest Japan. The ATFZ is thought to have ruptured in a historic earthquake in A. D.1596. A trench opened for this study across the right-lateral Boshima fault reveals evidence for Holocene seismic events. We surveyed this trench wall excavated along trend with late Pleistocene to Holocene fault scarps and offsets of stream channel walls. A fault-perpendicular trench exposes mainly coarse-grained fluvial deposits, as well as liquefied silt, that define the main fault zone. Analysis of stratigraphic relations and radiocarbon datings in Holocene fluvial deposits indicate at least four events. Among them period of the most recent earthquake postdates the fifth to sixth century humic soil. Although the uppermost section of strata is artificially disturbed, our result indicates the faulting event along the ATFZ occurred in the historical time.