Holocene faulting activity of the Ibigawa fault-Trenching study in Fujihashi Village, Gifu Prefecture, central Japan

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The Ibigawa fault is a left-lateral strike-slip active fault trending NW-SE direction in northwestern Gifu Prefecture. This fault runs parallel to the surface ruptures associated with the 1891 Nobi earthquake, however, no surface faulting was observed along this fault during the 1891 earthquake. The Active Fault Research Center, GSJ, AIST carried out a trenching survey on the Ibigawa fault to assess the activity of this fault and to make clear the interaction and segmentation of active faults in this area. Three trenches were excavated at Tsuka area in abandoned Tokuyama Village (Fujihashi Village at present) to reveal the paleoseismic activity of this fault. There are three terrace surfaces formed in the Latest Pleistocene to Holocene around the trench site. We divided these terraces into the Lower I, Lower II and Lower III terraces. Trench A is located on the Lower III terrace surface, and trenches B and C are located on the Lower I terrace surface. High-angle faults cutting the terrace deposits and undeformed surface soil layers were observed on every trench walls. The calibrated radiocarbon date from the deformed gravel bed in trench A is 2,000-1,880 cal y BP, and the age of the surface soil covering the fault zone in trench C is 1,170-980 cal yBP. These dates show that the age of the last event of the Ibigawa fault is about 2,000 to 1,000 cal y BP. The penultimate event seems to have occurred in about 3,900 to 2,700 cal y BP. The left-lateral offset of the base of the Lower III terrace deposit in trench A is estimated to be about 6.5 m although it is still uncertain how many times of faulting produced this offset.

Holocene faulting activity of the Ibigawa fault-Trenching study in Fujihashi Village, Gifu Prefecture, central Japan

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