We conducted drilling survey to re-examine near surface geometry and a rate of the vertical deformation on the Kamishiro fault, northern part of the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL), central Japan. 40- and 45-m-long core samples, extracted from the hanging wall of the Kamishiro fault, consist of fluvial sediments (alternation of sand-mud and sand-gravel layers). In the two cores, bedding plane is almost horizontal at uppermost part, gently sloped at upper to middle part, upstanding at lower part, and mildly sloped or horizontal at lowest part, respectively. The core samples enable us to estimate horizon of penetration of the Kamishiro fault on the basis of change with sharp boundary from upstanding strata older than 30,000 cal BP to underlying mildly sloped or horizontally laminated strata younger than 30,000 cal BP. Geologic cross-section on the basis of correlation of both the two cores with previously known stratigraphy, indicates that estimated fault is a reverse fault with dip of about 30 degrees. Altitude of facies boundaries and over fifty radiocarbon ages show cumulative vertical displacements of 14-20 m during the past 10,000 years, indicating average vertical displacement rate of 1.4-2.0 mm/yr. And, the net slip rate is estimated to be 2.8-4.0 mm/yr by using 30 degree dip of fault. The average vertical displacement rate in this study is smaller than that in the previous study where data of lithofacies and radiocarbon age is insufficient on the upthrown side. Because the Kamishiro fault is associated with drag folding near the surface, previously reported net slip rate during the past 10,000 years is considered to be underestimated. The largeness of net slip rate in this study relative to previously estimated rate is consistent with the forecast that the previous estimation is underestimated. The lowness of Holocene average vertical displacement rate, compared with previously estimated rate during the past 28,000 years, suggests decrease in activity of the Kamishiro fault during the past 10,000 years. And, the Holocene average vertical displacement rate shows that average recurrence interval is 150-360 years assuming that faulting with vertical displacement of 0.3 to 0.5 m as same as that at 2014 Nagano-ken-hokubu earthquake has repeated. This interval is shorter than that obtained from previous paleoseismic study, implying unknown paleoseismic event with small vertical displacement can be detected through reconsideration of paleoseismic survey.
