Rupture history of the Sakaitoge fault during the Holocene in the northern part of the Sakaitoge-Kamiya fault zone, central Japan

Abstract

The Sakaitoge fault is a left-lateral strike-slip active fault trending NNW-SSE to NW-SE direction in western Nagano Prefecture. Seven trenches were excavated in Nagawa and Kiso Villages; they are SA and SB trenches at Sogurazawa site, YA and YB at Yoriaido site, and HA, HB and HC at Hososhima site. In six trenches except HB trench we observed the evidence of paleoseismic activities. We observed two discrete horizons of upper terminations of the faults on the walls of YB trench. This means at least two faulting event occurred after the beginning of the deposition of the soil layers. The calibrated radiocarbon dates show that the age of the last event of the Sakaitoge fault is between 2910 BC (possibly 1670 BC) and 220 AD. The age of the penultimate event is between 5725 BC (possibly 5710 BC) and 4700 BC. The vertical slip-rate of the Sakaitoge fault is estimated to be 0.2−0.3 m/10³ yrs from deformation of the terrace surface at the Sogurazawa site. The net slip-rate is larger than this considering that the fault has left-lateral strike-slip component.