The active Sakota-Ikumo and Tokusa-Jifuku faults of the Lake Ohara-W. Yauneyama fault system extending in northeast-southwest direction are distributed in parallel in northeastern Yamaguchi Prefecture. The ENE-WSWtrending Chomonkyo fault belonging to the fault system is situated at a section, from Nakanotoro, Kawakami, Hagi City to Watarigawa, Ato-Ikumo-Higashibun, Yamaguchi City, between the two active faults, and has been pointed out to be active because it is accompanied by valleys and ridges bending to the right, but the timing of its movement remains unknown. We conducted an outcrop exploration, and paleoseismic trench and boring surveys to clarify the timing of paleoearthquakes.

　 We found an outcrop of the Chomonkyo fault, which strikes ENE-WSW and dips nearly vertical, cutting unconsolidated sediment layers. The fault is accompanied by a fault zone consisting of cataclasite and fault gouge, and approximately horizontal slickenlines can be recognized on the fault plane. We suggest that the motion sense of the fault changed from sinistral to dextral because the cataclasite and the fault gouge have composite planar fabrics indicating sinistral and dextral strike-slip faultings, respectively.

　 The trench and boring surveys conducted near the active-fault outcrop indicated that the Chomonkyo fault slipped twice after 4.5 ka and also at least twice between 130 and 6.8 ka. Therefore, it is possible that a recurrence interval of the fault before 6.8 ka is longer than that after 4.5 ka. However, it is more probable that a recurrence interval of the fault after 130 ka is constant because the fault may have slipped significantly more than twice between 130 and 6.8 ka.