Discovery of an Active Forearc Fault in an Urban Region: Holocene Rupture on the XEOLXELEK-Elk Lake Fault, Victoria, British Columbia, Canada

Subduction forearcs are subject to seismic hazard 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 a combined approach in a densely populated region of Vancouver Island, Canada, by combining 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, similar to 10 km north of the city of Victoria. Lidar-derived digital terrain models and historical air photos show a similar to 2.5-m-high 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 chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault-propagation fold modeling indicates similar to 3.2 m of reverse slip on a blind, 50 degrees southwest-dipping fault can reproduce the observed deformation. Fault scaling relations suggest a M 6.1-7.6 earthquake with a 13 to 73-km-long surface rupture and 2.3-3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments. Plain Language Summary Faults occurring in the upper plate above a subduction zone are often located near densely populated coastal areas, but their hazard is often underappreciated due to their low deformation rates. In the northern Cascadia forearc on the west coast of North America, high-resolution topography and geologic mapping show a similar to 2.3-m-high scarp across a similar to 14,000 year-old land surface 10 km north of downtown Victoria, British Columbia, Canada. This newly identified fault, the XEOLXELEK-Elk Lake fault (XELF), crosses Saanich Peninsula within Greater Victoria and poses a hazard to the region's similar to 400,000 inhabitants. Therefore, determining whether it produced recent large earthquakes is important for updating regional earthquake hazard models and increasing earthquake preparedness. To study the earthquake history of the fault, we used shallow geophysical techniques and excavated a trench across the scarp to examine the sedimentary record of deformation. These combined methodologies determined a single large earthquake, of magnitude 6.1-7.6, likely occurred on the XELF between similar to 4,700 and 2,300 years ago. A similar future earthquake on the XELF Lake fault could cause major damage to the Greater Victoria area. Thus, our results can improve future earthquake hazard assessments.