Using geophysical methods to constrain the Quaternary activity of the eastern San Juan fault
Date
2026
Authors
Irwin, Danny
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Abstract
Upper plate faults of southern Vancouver Island have hosted Quaternary earthquakes, posing a significant risk to the densely populated regions of Greater Victoria, Vancouver, and Seattle. The San Juan fault lies adjacent to several faults that have been proven active in the Quaternary, yet its activity remains uncertain due to lack of study. I use a recently available wealth of lidar data to carefully map the eastern part of the fault, looking for intersects between fault lineaments and glacial morphological features. I find a site that is easily accessed by vehicle, where a suspected fault scarp is clearly visible in the lidar data, situated in glacial deposits. Either this scarp formed before the till was deposited, or the scarp formed since the deposition, and is due to either dip-slip and/or strike-slip motion. I perform electrical resistivity tomography, relative gravity, and total magnetic field surveys across the scarp at this location to test these hypotheses, with the resistivity profile showing the most decisive results. Fault structures are observed, and disruptions appear in the uppermost layer, with little visible vertical offset in the till bases, consistent with fault rupture in the Quaternary, with the motion being mostly strike-slip. These findings, combined with similar findings from previous studies further west on the San Juan fault, make a compelling case for further work on the fault, and this location, with its ease of vehicular access, presents the ideal setting.
Supervisors: Lucinda Leonard and Edwin Nissen
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Keywords
fault, geophysical surveying, Quaternary, earthquake, San Juan fault, Vancouver Island