LIDAR DATA, GEOLOGIC MAPPING, AND PALEOSEISMIC TRENCHING REVEAL LATE QUATERNARY FAULT RUPTURES IN THE CASCADIA FOREARC OF SOUTHWESTERN BRITISH COLUMBIA

High-resolution lidar topography, structural and geomorphic mapping, and paleoseismic trenching reveal new evidence for late Quaternary activity on several onshore faults in the Cascadia forearc of southwestern British Columbia, Canada. Two new paleoseismic trenches excavated across an uphill-facing scarp along the Leech River fault, a ~60-km-long fault in the forearc of the northern Cascadia subduction zone on Vancouver Island, confirm at least three late Quaternary surface-rupturing earthquakes, each with ~1 m or more of vertical displacement. The western trench, excavated within a faulted debris-flow channel incised into a steep colluvial hillslope, exposed Jurassic Leech River Complex in fault contact with deformed Holocene (≤10 ka) loess and colluvium. The eastern trench, excavated across the adjacent interfluve, exposed latest Pleistocene till faulted against Holocene scarp-derived colluvium. Preliminary OxCal models based on >25 radiocarbon dates from scarp-derived colluvial units bracket the most recent earthquake to ~1500-1700 years BP. From detailed field mapping, structural analyses and geophysical profiles across the eastern 30 km of the Leech River fault we interpret that the trenches exposed a fault strand that is part of a steeply-dipping, 500-m-wide by 30-60 km long fault zone that accommodates right-lateral oblique transpression. The orientation of the Leech River fault suggests it continues offshore to the east, <5 km from Victoria, Canada, and may merge with the Darrington-Devils Mountain fault and/or the Southern Whidbey Island fault zone (Washington state) across the Strait of Juan de Fuca. Recent field mapping based on newly acquired lidar data indicate that two other prominent forearc faults on Vancouver Island, the San Juan and Beaufort Range faults, may also have hosted large-magnitude late Quaternary earthquakes. Both of these structures have faulted channel networks developed in late Quaternary colluvial or alluvial units, with surface offsets on the order of 2-8 m. Collectively, these observations are the first to conclusively document permanent strain accumulation along active forearc faults in northernmost Cascadia, and suggest that this active fault network should be considered in future seismic hazard assessments for southwest British Columbia and northwest Washington.