Subduction forearcs are subject to seismic hazards from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of these approaches in a highly populated region of Vancouver Island, Canada, by combining lidar remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the XEOLXELEK - Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar-derived bare-earth digital terrain models and historical air photos show a ∼2.3 m-high fault scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse-slip earthquake produced a fault-propagation fold above a blind southwest-dipping fault. Five geologically plausible OxCal models of radiocarbon-dated charcoal from deformed deposits and one colluvial wedge constrain the likely earthquake age to between 4.7 and 2.3 ka (68% confidence interval). Fault-propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest-dipping fault can reproduce the observed deformation. Fault scaling relations suggest a M 6.3–7.4 earthquake with a 15 to 53 km long surface rupture may be responsible for the modeled slip. An earthquake near this magnitude in Greater Victoria could result in major damage, and the results of this study highlight the importance of using both remote sensing and field studies to identify and characterize active faults in forearcs.
|Date made available||May 19 2023|