Evidence for a Quaternary-Active Fault Network in the Forearc of Southwestern British Columbia

Date: 4/24/2019

Time: 08:45 AM

Room: Puget Sound

The seismic hazard posed by potentially active faults in the forearc crust of the northern Cascadia subduction zone has proven difficult to constrain because of relatively low rates of seismicity, recent glacial activity, and thick vegetation. Here we present results from lidar topography, structural-geomorphic mapping, and paleoseismic trenching that reveal evidence for a Quaternary-active fault network in southwestern British Columbia on southern Vancouver Island. Three paleoseismic trenches across the Leech River Fault indicate that this terrane-bounding structure has hosted at least three surface-rupturing earthquakes in the last ∼9 kyr, with a Holocene slip rate of ≥0.2–0.3 mm/year, and is therefore capable of producing large (Mw > 6) earthquakes. Recent structural and geomorphic mapping on newly acquired lidar, and high-resolution topographic surveys along the San Juan and Beaufort Range Faults, two other prominent faults on southern Vancouver Island, indicate that these structures may have also hosted large-magnitude late Quaternary earthquakes. Both faults exhibit faulted channel networks developed in late Quaternary colluvial or alluvial units, with displacements on the order of ~2-6 m. These observations collectively argue for latest Quaternary activity along structures previously considered inactive since the Paleogene, and highlight the need for new seismic hazard assessments pertinent to residents of southwestern British Columbia and northwestern Washington State.

Presenting Author: Kristin Morell

Authors