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A Comprehensive Search for Evidence of Active Faulting in the Southern Coast Mountains of British Columbia, Canada: Progress and Preliminary Results

The southern Coast Mountains of British Columbia are situated in the inner northern forearc of the Cascadia Subduction Zone. They are traditionally viewed as the static backstop for the northward moving Oregon coast block. While many faults have been mapped in the Coast Mountains, conclusive evidence of Holocene displacement has not been observed. The recent discoveries of surface ruptures along the Leech River, Beaufort Range, and XEOLXELEK-Elk Lake faults on neighboring Vancouver Island underscore the importance of reexamining the faults in the southern Coast Mountains, which could pose a significant hazard to densely populated areas including Metro Vancouver. However, this effort is challenged by glaciated, mountainous geomorphology, and dense vegetation.

We collated digital elevation models of varying coverages and scales, from 1-arc-second satellite data to meter-scale lidar data. Particular attention was paid to locations of previously mapped faults and seismicity. We identified escarpments related to regional-scale lineaments at Bowen Island, Alouette Lake, Coquitlam Mountain, and Nahatlatch Lake, in addition to known fault systems such as the Fraser River Fault. We conducted field surveys in 2022 and 2023 to collect ground observations where the linear features appear to intersect Holocene sediments. In some areas, we deployed a UAV lidar scanner to obtain ultra-high resolution topographic data. At one site in the Britannia Shear Zone, we observed a 20-cm offset in the soil profile on opposite sides of a fault exposure, but the UAV lidar did not reveal a scarp extending from the outcrop. In addition to vegetation cover and glacial erosion, we have been challenged by unpredictable weather, difficult access to remote sites, and a lack of sufficiently fine-grained Holocene sedimentary deposits to potentially record sub-meter-scale displacements. Future plans include helicopter-based lidar surveys of remote areas and sub-bottom profiling in lakes that may better preserve offsets in Holocene stratigraphy. The results of this project will be incorporated into Canada’s national seismic hazard and risk models.

Session: Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults [Poster Session]

Type: Poster

Room: Exhibit Hall

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Raul Benjamin Mendoza

Student Presenter: Yes

Additional Authors