Lidar data, geologic mapping, and paleoseismic trenching reveal late Quaternary surface ruptures and increased seismic hazard in southwestern British Columbia, Canada

Kristin D. Morell, Christine A. Regalla, Colin B. Amos, Scott E. Bennett, Audrey Graham, Lucinda Leonard, Emerson Lynch, & Nicolas Harrichhausen

Published August 11, 2017, SCEC Contribution #7510, 2017 SCEC Annual Meeting Poster #148

Lidar topography, structural-geomorphic mapping, and paleoseismic trenching reveal new evidence for late Quaternary activity on three faults on Vancouver Island, British Columbia, that were previously thought to be inactive since the Eocene. Two paleoseismic trenches excavated across a strand of the Leech River fault, a ~60-km-long fault zone in the forearc of southern Vancouver Island, confirm at least three late Quaternary surface-rupturing earthquakes, each with ~1 m or more of vertical displacement. Preliminary OxCal models based on >25 radiocarbon dates from scarp-derived colluvial units in the trenches bracket the two most recent earthquakes on the Leech River fault to ~1,500-1,700 and ~3,200-3,400 years BP, while the earlier event remains poorly constrained. Field mapping, structural analyses and GPR and resistivity profiles across the eastern 30 km of the Leech River fault show that the fault strand exposed by the trenches is part of a near-vertical, 500-m-wide by >30 km long positive flower structure that accommodates right-lateral transpression. Recent field mapping based on newly-acquired lidar data indicates that two other prominent forearc faults on Vancouver Island, the San Juan and Beaufort Range faults, may also have hosted surface-rupturing earthquakes in the late Quaternary. Both of these structures exhibit 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 late Quaternary activity on terrestrial faults on Vancouver Island, and suggest that more work should be undertaken to assess the seismic hazard that these crustal faults pose to population centers and infrastructure in southwest British Columbia and northwest Washington.

Citation
Morell, K. D., Regalla, C. A., Amos, C. B., Bennett, S. E., Graham, A., Leonard, L., Lynch, E., & Harrichhausen, N. (2017, 08). Lidar data, geologic mapping, and paleoseismic trenching reveal late Quaternary surface ruptures and increased seismic hazard in southwestern British Columbia, Canada. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology