Reappraisal for recent faulting history of the Hakushu fault, the southern part of the Itoigawa

Co-seismic faulting activity of the Hakushu fault in the southern part of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan was re-appraised by using newly obtained paleoseismological data for the last several hundred thousands years. The Hakushu fault, 8 km long, consists of intermittently continuous, short fault traces from northwest to southeast direction. Two parallel fault scarps (eastern and western scarps) occur in the southern part of the Hakushu fault. We conducted a trench excavation at the Yokote site across the western scarp. These excavations revealed evidences for the latest, at least two surface-rupturing events. The events were present sometimes between 1695 and 2720 cal y BP and between 7325 and 8360 cal y BP. The timing of the penultimate event is roughly coincident with the latest event (6650 and 7000 cal y BP) at the Daibo site on the eastern fault scarp. The recurrence interval of time and slip rate between these two events is approximately estimated as 5000 years and 0.4-0.6 mm/yr in vertical-slip, respectively. A total vertical-slip rate estimated only from the trenching on both fault scarps is 0.4-0.8 mm/yr. On the other hand, another vertical-slip rate estimated from a detailed correlation of reference deposits including peripheral deformation of fault scarp is 1.0-1.8 mm/yr. These slip-rates suggest that the net-slip rate for co-seismic faulting of the Hakushu fault will be over 1.0 mm/yr. A set of steeply/moderately dipping fault planes on the Yokote trench walls showed a palmtree-like structure as a whole deformation. This finding indicates that the faulting includes a strike-slip component. The strike-slip occurrence is evidently distinct from a character of low-angle thrust fault as suggested by previous studies. However, it is rather consistent with a character of sinistral slip on the Houohzan fault adjacent to the southern edge of the Hakushu fault.