Quaternary rupture of a crustal fault beneath Victoria, British Columbia, Canada

The seismic potential of crustal faults within the forearc of the northern Cascadia subduction zone in British Columbia has remained elusive, despite the recognition of recent seismic activity on nearby fault systems within the Juan de Fuca Strait. In this paper, we present the first evidence for earthquake surface ruptures along the Leech River fault, a prominent crustal fault near Victoria, British Columbia. We use LiDAR and field data to identify >60 steeply dipping, semi-continuous linear scarps, sags, and swales that cut across both bedrock and Quaternary deposits along the Leech River fault. These features are part of an ~1-km-wide and up to >60-km-long steeply dipping fault zone that accommodates active forearc transpression together with structures in the Juan de Fuca Strait and the U.S. mainland. Reconstruction of fault slip across a deformed <15 ka colluvial surface near the center of the fault zone indicates ~6 m of vertical separation across the surface and ~4 m of vertical separation of channels incising the surface. These displacement data indicate that the Leech River fault has experienced at least two surface-rupturing earthquakes since the deglaciation following the last glacial maximum ca. 15 ka, and should therefore be incorporated as a distinct shallow seismic source in seismic hazard assessments for the region.This research was supported by an NSERC Discovery grant to KM and NSF EAR IRFP Grant #1349586 to CR