Considering a Seismically Active Leech River Valley Fault Zone in Southwestern British Columbia

Abstract

The transpressional reverse Leech River fault (LRF) extends across the southern tip of Vancouver Island and beneath the city of Victoria, British Columbia, Canada. New paleoseismic studies suggest at least three surface-rupturing earthquakes have exceeded a moment magnitude (M) of 6 within a proposed Leech River Valley Fault Zone (LRVFZ) within the last 9,000 years. We examine the impact of an active LRVFZ to predicted earthquake ground motions for Victoria. In a probabilistic formulation considering the likelihood of all earthquake sources, LRVFZ earthquakes will contribute the most to high-frequency ground motions (≥ 10 Hz) in Victoria. The Canadian seismic design ground motions for Victoria increase on average by 4 – 23% at 10 Hz depending on the selection for the magnitude-recurrence rate associated with the LRVFZ. In a deterministic formulation considering rupture complexities for a suite of M 7 LRVFZ scenario earthquakes, predicted low-frequency (\u3c 0.5 Hz) ground motions in Victoria vary between 1 cm/s (weak shaking) and 19 cm/s (strong shaking) depending on the scenario. The highest ground motions in Victoria are generated by an eastward-rupturing large magnitude LRVFZ earthquake with maximum slip at shallow depth near the city