Revisited timing of paleoearthquakes and long-term slip rate along the Shimotsutaki fault, the Itoigawa-Shizuoka Tectonic Line, central Japan

The Itoigawa-Shizuoka Tectonic Line active fault system in central Japan extends for ca. 150 km and it is one of the most active faults on land. It consists of east-dipping reverse faults, north-west trending left-lateral strike-slip faults and west-dipping reverse faults. The Shimotsutaki fault consists of the southeastern portion of the central fault segment. We performed trenching and Geoslicer survey at Shimotsutaki site. The site is located in depression where fine sediments have continuously deposited since several thousands years. The trenches and sections of Geoslicers exposed several high-angle faults cutting debris-flow deposit near the trench bottoms and thick humic layer covered with cultivated soil at the surface. We identified three faulting events and estimated the timing of faulting based on radiocarbon ages. The most recent event postdates 1500 yBP and the timing is consistent with previous estimation. The penultimate and ante-penultimate events may have occurred between 2000 and 3300 yBP, and 3300 and 5500 yBP, respectively. Judging from these data, the Shimotsutaki fault has activated at the interval of less than 3500 year and possibly at the average of ca. 1800 year. This recurrence interval is shorter than previous estimation. Additionally, we could constrain the left-lateral slip-rate of 5.5 mm/yr as the maximum value, based on the age of debris-flow deposit exposed on the trench walls and evolution of fluvial terraces related to offset river. Such refinement of paleoseismological data would give us to understand fault segmentation and contribute more precise evaluation of seismic hazard along the Itoigawa-Shizuoka Tectonic Line active fault system