The Okamura fault, a segment of the Median Tectonic Line active fault system, is a master active fault along a northern range front of the Shikoku Range with well-defined geomorphic features related to its late Quaternary faulting. We have excavated five sites across the Okamura fault at Iioka, Saijo City, to date the times of recent faultings and surface-rupturing earthquakes. This paper summarizes geological characterisitics and late Holocene activity of the Okamura fault by comparing individual trenching studies, which have been reported by previous several papers.
The Okamura fault trends N80°E and dips with high angle to almost vertical fault plane. This fault offsets all sediments except for artificially modified and cultivated soil immediately below the ground surface. Multiple V-shaped flower structures are recognized in the sediments near the top of the fault zone, and these are interpreted to have been formed by filling coseismic ground fissures associated with surface rupturing earthquakes. The most recent faulting on the Okamura fault is estimated to have occurred during the 4th to 7th century A. D. and several older events are also recognized in these outcrops by trench excavations. The recurrence interval is about 1000 years or slightly longer.
A silty layer exposed on the north and south sides of the fault in trench ifi is offset rightlaterally 5.7meters. This lateral slip is associated with the latest event. The angular gravels of the Cretaceous Izumi Group which crops out in the upper reach of stream“a”southwest of the study area, are contained only at lower horizons on the north side of the fault. This observation suggests cumulative right-lateral displacement along this fault. The sedimentary environment of alluvial fan area has progressively changed with the lateral movement and largely different faces of sediments on the both sides of the fault were caused by the cumulative lateral offset