Geologic evidence for late Q uaternary repetitive surface faulting on the I surugi fault along the northwestern margin of the T onami P lain, north‐central J apan

Our detailed field investigation, paleoseismic trenching, and airborne light detection and ranging ( LiDAR )‐derived topographic data provides the first direct evidence for late Q uaternary repetitive surface faulting on the northeast‐striking I surugi fault along the northwestern margin of the T onami P lain in the H okuriku region of north‐central J apan. 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 1.5 km long and 170 m wide, with an age of ≤ 29 ka. 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 P leistocene; the latest of these events occurred in the H olocene between about 4.0 and 0.9 ka. Despite the poor preservation of the surface expression of the I surugi fault, repetitive scarp‐forming faulting in the late Q uaternary and the proximity of the O yabe 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 T onami P lain that makes the fault long enough to generate a large earthquake ( M w ≥ 6.8) accompanied by surface rupture.