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Characterizing the hazard associated with Quaternary-active faults in the forearc crust of the northern Cascadia subduction zone has proven challenging due to historically low rates of seismicity, late Quaternary glacial scouring, and dense vegetation that often obscures fault-related geomorphic features. We couple lidar topography with paleoseismic trenching across the Leech River Fault on southern Vancouver Island to produce the first detailed surface rupture history of an onland forearc fault in British Columbia, Canada. The results indicate that this fault produced three surface-rupturing earthquakes in the last similar to 9 kyr and is therefore capable of producing large (M-w > 6) earthquakes in the future. We provide new constraints on the fault's length (similar to 130 km) and Holocene slip rate (>= 0.2-0.3 mm/year) that, together with the earthquake ages, should be incorporated into new seismic hazard assessments and building code practices relevant to urban centers in southwestern British Columbia (Canada) and northwestern Washington State (United States).

Plain Language Summary In this study we document the first detailed history of earthquake surface rupture for an onland fault within the Cascadia subduction zone of British Columbia, Canada. We show that the Leech River Fault on southern Vancouver Island has produced three surface-rupturing earthquakes in the last similar to 9,000 years and is therefore capable of producing large (M > 6) earthquakes in the future. We provide new constraints on the fault's behavior that should be incorporated into new seismic hazard assessments and building code practices relevant to urban centers in southwestern British Columbia (Canada) and northwestern Washington State (United States). Nearby regions of SW British Columbia likely contain similar active faults that could host damaging earthquakes.