Characteristics and Activity of a Southwestern Part of the Tokusa-Jifuku Fault and the Northeastmost Part of the Western Mt. Kido Fault in Central Yamaguchi Prefecture, Southwest Japan

Abstract

We conducted a topographical survey, an investigation of fault outcrops, a trench survey, and/or an array drilling survey at the Shinome and Tashiro areas, Ato Town, central Yamaguchi Prefecture, in order to clarify characteristics and activity of the Tokusa-Jifuku and Western Mt. Kido faults. In the Shinome area, an excellent exposure of the Western Mt. Kido fault is found on a slope excavated during a road cutting work. In this outcrop, a gravel bed is cut clearly by a fault accompanied by white fault gouge approximately ten centimeters wide. By 14C age dating of samples from slope sediments in a footwall of the fault, it is judged that the Western Mt. Kido fault moved during past 5,400 years. In contrast, a fault with approximately 70 centimeters of a south-side-up displacement was identified by the trench excavation for the Tokusa-Jifuku fault in the Shinome area. Based on 14C age dating of samples obtained from the gravel bed with clay and sand matrix overlaying the fault, it is pointed out two possibilities of the last movement of the Tokusa-Jifuku fault: the one is no movement during the past 7,600 years; and the other is the movement during the past 6,300 years. In the Tashiro area, 14C age dating of a silt sample collected in gravel bed overlaying the fault by the trench excavation reveals that no movement occurred on the Tokusa-Jifuku fault during the past 5,200 years. The array drilling indicates that the basement rock developing in the southeast side from the fault is displaced upward about 30 centimeters on a presumed fault. By the