Recent Surface Faulting Events along the Middle Section of the Itoigawa-Shizuoka Tectonic Line

Trenching Survey of the Gofukuji Fault near Matsumoto, Central Japan

Abstract

The middle section of the Itoigawa-Shizuoka Tectonic Line (Middle ISTL) is an active fault system that extends NW-SE for 50km from Matsumoto to Kobuchizawa, in central Japan. The Middle ISTL is characterized by high average slip-rate reaching 8 to 10mm/yr during the Late Pleistocene and Holocene. This is one of the highest slip-rate reported from active faults on land in Japan. Empirical relation between slip-rate and recurrence time indicates that the Middle ISTL may rupture more than once a thousand year. The previously known recurrence time estimates of 3500 to 5000 years were significantly longer than the expected recurrence time. The last faulting event on the Middle ISTL most likely occurred in 841 A. D. according to historic record and paleoseismological works. Since the elapsed time is about 1152 years, the estimation of recurrence time is critical to evaluate the potential of next earthquake. In order to know the history of recent faulting events, we excavated the Gofukuji fault, the northernmost segment of the Middle ISTL at Namiyanagi, south of Matsumoto. Investigation of 6 trenches, 3 test pits and topography around the trenches brought following results. The ages of three most recent faulting events are 445-1386 A. D., 150-334 A. D., and 839-189 B. C. The recurrence time is between 111 and 1236 years. Assuming the correlation of the last event with the 841 A. D. event, the recurrence time is estimated to be 338 to 1172 years. Average recurrence interval in this case ranges 515 to 840 years. The elapsed time of 607 to 1152 years is probably longer than the recurrence time. The average slip rate of the fault since c. a. 6000 B. C. is 9.4±4.5mm/yr left-lateral. The left-lateral coseismic slip during the last event is estimated as 7.5±1.5m.