Paper No. 117-11
Presentation Time: 4:15 PM
EVIDENCE FOR LATEST QUATERNARY SURFACE RUPTURE ALONG THE LEECH RIVER FAULT NEAR VICTORIA, BRITISH COLUMBIA, CANADA
New surficial and bedrock mapping and paleoseismic trenching of the Leech River fault provide the first evidence for Quaternary surface-rupturing earthquakes in southwestern British Columbia, Canada. The Leech River fault extends ~60 km across southern Vancouver Island, from Victoria, British Columbia to the Pacific shoreline and is a terrane-bounding structure separating the Pacific Rim Terrane from basalts of the Eocene Crescent Terrane. The fault is not currently listed in the active fault catalogue for Canada, and post-Eocene-Oligocene slip had not been documented prior to this study. However, based on new field mapping aided by lidar topography, we identify >60 individual, sub-parallel, linear scarps, sags and swales occurring in semi-continuous, en echelon arrays that offset bedrock and late Pleistocene-Holocene deposits. Field observations of these scarps confirm that they are not the result of anthropogenic, glacial or landslide processes, and in several places the scarps are located above exposures of faulted bedrock with brittle fracture networks and gouge. At a site ~5 km west of Leechtown, British Columbia, we estimate ~6 m of dip-slip reverse displacement of a post-Last Glacial Maximum (<~15 ka) colluvial surface and ~4 m of displacement of intervening channels. Two paleoseismic trenches at this site reveal (1) Jurassic Leech River Schist in fault contact with latest Pleistocene loess and colluvium, and (2) latest Pleistocene till thrust over post-glacial colluvium. These trenches preserve a record of at least three, and possibly four, earthquakes since the Last Glacial Maximum, each with ~1 m vertical displacement. We interpret the active Leech River fault as a 500–1000 m-wide, steeply dipping fault zone that accommodates transpression across the northern Cascadia forearc. The onshore trace of the Leech River fault may continue offshore to the east, south of Victoria, and may be kinematically linked to active faults in western Washington (e.g., Devils Mountain and Southern Whidbey Island faults). The Leech River fault is likely one of several active crustal faults that should be considered in seismic hazard assessments for southern British Columbia and northwestern Washington.