Geologic evidence for late Quaternary repetitive surface faulting on the Isurugi fault along the northwestern margin of the Tonami Plain, north-central Japan

摘要

Our detailed field investigation, paleoseismic trenching, and airborne light detection and ranging (LiDAR)-derived topographic data provides the first direct evidence for late Quaternary repetitive surface faulting on the northeast-striking Isurugi fault along the northwestern margin of the Tonami Plain in the Hokuriku region of north-central Japan. This fault has been interpreted previously by different researchers as both inactive and active, owing to a lack of geologic evidence and a failure to identify fault-related geomorphic features. Our mapping of LiDAR topography revealed a series of northeast-trending warped fluvial terraces, about 1.5km long and 170m wide, with an age of 29ka. We interpreted these geomorphologic features to represent an active pop-up structure bounded to the southeast by the northwest-dipping main thrust of the Isurugi fault and to the northwest by a southeast-dipping backthrust that splays off the main thrust in the shallow subsurface. Paleoseismic trenching across the northwestern part of an elongate terrace exposed a series of southeast-dipping backthrusts and associated northwest-verging monoclines. The deformation and depositional age of the strata provide evidence for repetitive surface rupturing on the backthrusts since the latest Pleistocene; the latest of these events occurred in the Holocene between about 4.0 and 0.9ka. Despite the poor preservation of the surface expression of the Isurugi fault, repetitive scarp-forming faulting in the late Quaternary and the proximity of the Oyabe River and its tributaries to the fault trace suggest that there may be an extension of the Isurugi fault to the northeast and southwest beneath the Tonami Plain that makes the fault long enough to generate a large earthquake (Mw6.8) accompanied by surface rupture.