中央構造線活断層系三野断層の最新活動時期

The Median Tectonic Line (MTL) active fault system is one of the most active intraplate faults in Japan. The fault system, which is more than 300 km long, is a right-lateral strike-slip fault with an average slip rate of 5-10 mm/y in east Shikoku. The 13.5-km-long Mino fault of the MTL active fault system is located at the western part of Tokushima Prefecture in east Shikoku. We carried out trench excavation surveys of the Mino fault at Ueno in Mino Town and Ikenoura in Mima Town. Both sites are situated at fault depressions formed on the middle and lower terrace surfaces by the activity of the Mino fault. Fault depression deposits consist of younger and finer grained layers with abundant 14C dating samples. We inferred the dates of faulting events from upward fault terminations on the trench walls.
At Ueno, it is recognized that the latest rupture event occurred between 1, 295-1, 390 cal A.D. and 1, 660-1, 950 cal A.D. (190 ± 50 years B.P.). Multiple faulting events are also suggested after K-Ah ash fall (about 5, 200 B.C.), because the ash layer steepens to near vertical. At Ikenoura, the latest rupture event is estimated to have occurred after 1, 525-1, 660 cal A.D.
Taking account of the previous reports on the eastern extension at Chichio and Zunden faults, we conclude that the MTL active fault system in east Shikoku ruptured in the 16th century A.D. or later. The liquefactions, which occurred between the latter half of the 16th century A.D. and the beginning of the 17th century A.D., are recognized at archaeological sites at Maruyama and Ogaki in the western part of Tokushima Prefecture. These sites are located immediately south of the Mino fault and the Ikeda fault to the west. This fact suggests that the liquefactions were caused by faulting of the MTL active fault system in east Shikoku.