Contribution of the Leech River-Devil's Mountain fault system to seismic hazard in Victoria, B.C.

The Devil’s Mountain fault is an active fault included in the 2014 USGS hazard model for Washington State. Recent neotectonic investigations have suggested that a west-northwestwards extension of the fault (the Leech River fault) has sea-bottom and onshore evidence pointing to recurrent young offsets. Accordingly, a logic tree model for the Leech River – Devil’s Mountain fault system (LRF-DMF) incorporating various fault lengths, slip rates of 0.25 mm/yr with upper and lower alternatives of 0.15 and 0.35 mm/yr, and interactions between the faults was developed and added to Canada’s 6th Generation seismic hazard model. The LRF was given a 50% chance of being active. Although the slip rate is low for an active tectonic region, the fault system passes through greater Victoria, British Columbia, and contributes to the overall seismic hazard for southernmost Vancouver Island. We calculate the hazard in greater Victoria with and without the LRF-DMF in order to estimate its effect. The hazard in downtown Victoria is already high (coming mainly from in-slab sources at short periods and the Cascadia subduction zone at long periods) and decreases slowly northwards. The hazard increment due to the LRF-DMF is quite small, even very close to the fault, and as expected its contribution to the hazard decreases away from the fault so that in Sidney at ~25 km distance it is insignificant. The importance would have been very different in a lower hazard region, or if the slip rate on the LRF-DMF had been considerably higher.