ABSTRACT1 Yokohama National University, Yokohama, Japan Terr. Atmos. Ocean. Sci., Vol. 18, No. 1, March 2007 56 for horizontal and ca. 5 m for vertical) than those (2.7 m for horizontal and ca. 2 m for vertical) of the 1999 earthquake. The age of the penultimate earthquake at the Fengyuan site is the oldest earthquake recorded among the known penultimate penultimate events on the Chelungpu fault so far and suggests that this part of the Chelungpu fault may be a fault segment independent of the central and southern parts of the fault.

We investigated the geological structure and hydrogeological properties of the Omagari Fault in Neogene siliceous sedimentary rocks of the Horonobe area, northern Hokkaido, by geological mapping, borehole investigations, reflection seismic survey, and audio-frequency magnetotelluric survey. As a result of the investigation, the-D distribution of the Fault was clarified and the following were shown. In addition, the magnetotelluric survey in particular was effective for mapping of the-D distribution and hydrogeological properties of the fault. The Omagari Fault has a fault zone, about m wide, that consists mainly of the damage zone, and has a permeable structure. The magnetotelluric survey shows several high-resistivity zones, one of which corresponds to the Omagari Fault inferred from the reflection seismic surveys. The high resistivity zones are correlative with the concentration zones of low-saline water, which suggests infiltration of ground water through the permeable Omagari Fault zone. The Omagari Fault over-steps at the earth surface and at the same time is converging in subsurface zones.

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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