Holocene Activity and Timing of the Latest Faulting Event of the Torigoe Fault, West of Nagaoka City, Central Japan

Abstract

The western margin of the Echigo Plain, central Japan, is marked by a set of reverse faults trending NNE-SSW. These faults are assumed to be very active ones. However, it is difficult to know the recent rupture history of the faults because of thick pile of unconsolidated sediment. The Torigoe Fault vertically dislocates the late Pleistocene fluvial terrace surfaces over several meters, west of Nagaoka City, southern part of the Echigo Plain. We identified a gentle, broad slope on Holocene lowland as the distinct faulted topography on the basis of interpretation of large-scale aerial photographs and leveling of topographic profiles. The fault displays the evidence for its recent reactivation about 2m high. Trenching and drilling surveys across the deformed zone reveal the vertical displacement associated with faulting, the timing of latest faulting event and average recurrence interval of the Torigoe Fault. The most recent faulting on the Torigoe Fault probably occurred since 12^th-13^th century. The ground surface was vertically dislocated about 2m by reverse slip. The net-slip on fault plane should be over 2m. Although great shaking might attack the area around the fault, there is no historical record of heavy damage since 17^th century. The fault vertically offsets the gravel layer dated at ca. 5, 600-5, 700 BC by 10-12m. The amount of displacement represents the accumulative result of five to six faulting events. The younger sediment (1, 300-2, 400 BC) may be offset by 5-6m. If there have been five to six faulting events since ca. 5, 600-5, 700 BC, the average recurrence interval of faulting is estimated to be 1, 100-1, 900yrs.