PALEOSEISMIC TRENCHING AND LIDAR DATA REVEAL A PREVIOUSLY UNRECOGNIZED QUATERNARY-ACTIVE FAULT IN THE CASCADIA FOREARC OF SOUTHERN VANCOUVER ISLAND, CANADA

New paleoseismic trenching and lidar data document Holocene surface rupture of a previously unrecognized active fault in the northern Cascadia forearc. The Elk Lake fault, located in suburban Saanich, ~10 km north of downtown Victoria, British Columbia, Canada, was first recognized in bare-earth lidar data, expressed as a NW-striking, ~2-4 m-high topographic scarp that offsets late Quaternary glacial landforms and deposits. The fault is approximately coincident with a previously-mapped, unnamed bedrock fault that was not known to be active in the Quaternary. Paleoseismic trenching across the NE-facing scarp revealed very fine-grained, thinly-laminated glacio-marine sediments characterized by alternating dark (clay-rich) and light (silt-rich) beds. These beds thin upwards and are deformed by a gently SW-dipping thrust fault in the core of a fault-propagation fold. Trishear forward modeling and resistivity tomography of the trench site suggest that 3.2 m of slip on a steeper fault at depth produced the NE-vergent fault-propagation fold. Oxcal models using radiocarbon ages of charcoal collected from deformed sediments and an overlying, undeformed colluvial wedge suggest at least one surface-deforming earthquake occurred between 4.7 ka and 2.3 ka. The fact that this earthquake age does not immediately follow the last deglaciation at ~11 ka suggests the earthquake was unrelated to glacial isostatic rebound. Future earthquakes on the Elk Lake fault may therefore pose a significant seismic hazard to the local population and infrastructure. These data further emphasize the importance of using lidar mapping, paleoseismic trenching, and other techniques to characterize the longer-term geomorphic and stratigraphic record of past earthquake ruptures and to assess the seismic hazard of potentially active upper plate faults in a region that is otherwise characterized by relatively low background seismicity.