Abstract
This paper presents the distribution, characteristics, and activity of faults which constitute the Lake Ohara fault system, by the combination of new fault-outcrop investigations, trench excavation survey, and borehole drillings, with previous studies for the fault system. The fault system is composed of 7 active faults; the Lake Ohara, Western Mt. Kido, North-western Yamaguchi Basin-Marginal, R. Yoshiki, Shimogo, Eastern Ube, and R. Niho faults. In the first part of this report, based on a review of previous studies, geomorphologic and geologic data about the fault system and constituent active faults are compiled. Second, results of the investigation performed in this study are described in detail. It is pointed out that no movement has occurred on the Lake Ohara fault in the past 3, 200 years. In contrast, the Western Mt. Kido fault might move approximately 3, 500 years ago, simultaneously with the Shimogo fault. It is supposed that these two active faults move at a recurrence interval of-21, 000 to>24, 000 years. Although each constituent fault has a different average slip-rate of a vertical component, it ranges from 0.01 to 0.08m/103 years.
