Paleoseismic trenching at Ikeda Kemi-kita site, Ikeda Town, Nagano Prefecture, reveals the mid-to-late Holocene surface-rupturing history of the Matsumoto-bonchi-toen fault (MBTF), a constituent fault of the eastdipping reverse-faulting dominant northern section of the Itoigawa-Shizuoka Tectonic Line active fault system(ISTL). The trench exposed shallowly eastward dipping thrust faults that displace mid-to-late Holocene fluvial terrace and alluvial fan sediments. The sediments contain evidence for two surface rupturing events with an about 1 m vertical offset individually in the past 6,000 years. Plenty of 14C dates allow to place bounds on timing of each event; AD 690 to AD 1170 (possibly constrained between AD 900 and AD 1120) for event 1 (the latest event) and 2280 BC to 1770 BC for event 2. 14C wiggle matching applied to a buried tree that is interpreted to be toppled down in association with event 2 estimates the date of outermost ring to be between 2330 BC and 2250 BC. The age range for the latest event overlaps with those previously reported in the north section and the Gofukuji fault to the south, which constitutes the left-lateral strike-slip-faulting dominant middle section.The latest event may be correlated with either historically documented AD 762 or AD 841 earthquake, or a historically unknown large earthquake, depending on choice of the 14C dating materials. Timing of two events suggests the possibility that the rupture on MBTF was less frequent compared to the Gofukuji fault since the middle Holocene.
