Holocene earthquake rupture of the XEOLXELEK–Elk Lake fault in the Greater Victoria area, 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 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 ~ 2.5 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 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 6.1–7.6
earthquake with a 13 to 73 km long surface rupture and 2.3 to 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 the results of this study highlight the
importance of using both remote sensing and field studies to identify and
characterize active faults in forearcs and regions with recent glaciation.