Presentation
Lidar and paleoseismic trenching reveal first documentation of late Quaternary onshore faulting in the forearc of southwestern British Columbia
AGU Fall Meeting
(2016)
Abstract
Two new paleoseismic trenches excavated across the Leech River fault, a ~60-km-long fault in the forearc of the northern Cascadia subduction zone on Vancouver Island, confirm at least three surface-rupturing earthquakes have occurred in southwest British Columbia since the latest Quaternary. Analyses of high-resolution lidar-derived topography were used to identify the trench sites, where a 2-4 m-high, E-W-striking fault scarp offsets a latest Pleistocene colluvial surface. The western trench, excavated across one of several faulted debris flow channels incised into this surface, exposed schist of the Jurassic Leech River Complex in fault contact with latest Pleistocene loess and colluvium. The eastern trench, excavated across an adjacent interfluve, exposed latest Pleistocene till faulted against scarp-derived colluvium that post-dates the Last Glacial Maximum (<~15 ka). Colluvial wedge stratigraphy provide evidence for ≥ 3 large earthquakes since the latest Pleistocene, each with ~1 m of vertical displacement. Restoration of surface displacement across the scarp suggests ~6 m of dip-slip has occurred since development of the colluvial surface while only ~4 m of displacement has occurred since channel incision, which further supports the occurrence of repeated surface-rupturing events within the past ~15 ka. Our field mapping, aided by lidar-derived topography, indicates that this scarp is part of a steeply-dipping, 500 m-wide fault zone that is continuous for more than 30-50 km and likely accommodates regional transpression together with other active faults in western Washington (e.g., Devils Mountain fault). This study reinforces the need for lidar when conducting paleoseismic studies in this steep and densely vegetated terrain. These observations provide the first evidence of late Quaternary onshore faulting in the forearc of southwestern Canada and suggest the region is host to active structures that should be considered in seismic hazard assessments.
Keywords
- Paleoseismic trenching,
- Surface-rupturing earthquakes
Disciplines
Publication Date
December 14, 2016
Location
San Francisco, CA
Citation Information
Morell, K., C. Regalla, C.B. Amos, S.E.K. Bennett, L. Leonard, A. Graham (2016) Lidar and paleoseismic trenching reveal first documentation of late Quaternary onshore faulting in the forearc of southwestern British Columbia, Eos Trans. AGU, Fall Meet. Suppl., Abstract T33D-02.