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Discovery of an active forearc fault in an urban region: Holocene rupture on the XEOLXELEK-Elk Lake fault, Victoria, British Columbia, Canada

December 1, 2023
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, ∼10 km north of the city of Victoria. Lidar-derived 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 chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. 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 2.3–3.2 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.
Publication Year 2024
Title Discovery of an active forearc fault in an urban region: Holocene rupture on the XEOLXELEK-Elk Lake fault, Victoria, British Columbia, Canada
DOI 10.1029/2023TC008170
Authors Nicolas Harrichhausen, Theron Finley, Kristin D. Morell, Christine Regalla, Scott E.K. Bennett, Lucinda J. Leonard, Edwin Nissen, Eleanor McLeod, Emerson M. Lynch, Guy Salomon, Israporn Sethanant
Publication Type Article
Publication Subtype Journal Article
Series Title Tectonics
Index ID 70261203
Record Source USGS Publications Warehouse
USGS Organization Geology, Minerals, Energy, and Geophysics Science Center
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