An improved earthquake catalogue in southern British Columbia supplies new constraints on the fault responsible for the 1946 M 7.3 Vancouver Island earthquake

Abstract:

We employ the automated detection and location algorithm REST of Comte et al. (2019) to seismic waveform data from the Vancouver Island region (northern Cascadia forearc) to identify earthquakes not present within the Geologic Survey of Canada (GSC) catalogue. Over twice as many hypocenters are determined for the time period 1995-2020 in the new catalogue, with a significant augmentation in the number of events and traveltime picks in both the onshore and offshore regions. Improved geographic coverage supplied by new earthquake sources will enable higher resolution studies of velocity structure and seismotectonics in southwest British Columbia, where uncertainties remain surrounding the identity of fault structures responsible for large earthquakes of the 20^th century. We highlight, for example, the considerable uncertainty regarding the fault responsible for the M 7.3 1946 Vancouver Island event, the largest crustal earthquake recorded in northern Cascadia in modern times. In this region, our new catalogue includes 395 microseismic (M_L≤1) events absent from the GSC catalog in the upper 30 km of the crust, aligned parallel to and immediately northeast of the NW-trending surface trace of the Beaufort Range fault. This structure was hypothesized by Rogers (1987) as a potential host for the 1946 event and coincides with the downdip limit of tremor in this region. We speculate that seismicity is promoted by fluids, derived from the subducting Juan de Fuca plate, that migrate upward into the overlying forearc crust. The upward migration of fluids along pre-existing forearc structures may promote the reactivation of faults in response to the modern Cascadia stress regime. Future work will focus on precise earthquake relocation, velocity structure inversion and focal mechanism analysis to better define the tectonic context in which these events occur.