Tephra fall probabilistic hazard maps for VEI 3 and VEI 4 eruptions at Mt. Garibaldi (Nch’kay’) volcano, British Columbia, Canada

Abstract Following explosive volcanic eruptions, hazard from tephra fall can be widespread. Based on the geological record of past eruptions and risk to population and infrastructure, Mt. Garibaldi volcano ( Nch’ k ay’ ) in British Columbia has been assigned the highest threat ranking of any Canadian volcano. However, until this contribution no hazard map exists for the volcano. In our study, we formed a government-university partnership and modelled tephra dispersion for Volcanic Explosivity Index (VEI) 3 and VEI 4 scenarios for Mt. Garibaldi to produce probabilistic tephra fall hazard maps. We found significant tephra impacts (loads ≥ 0.1 kg m − 2 which can cause skid risks on roads and disrupt airports) extend over 810 km 2 for the VEI 3 scenario and 9360 km 2 for VEI 4 (with 50% probability of exceedance). Tephra loads that may cause infrastructure damage (≥ 1 kg m − 2 ) cover 250 km 2 and 1670 km 2 for the VEI 3 and VEI 4 scenarios, respectively. We combined our results into a single tephra ground accumulation hazard map and information brief. This is the first probabilistic volcanic hazard map using well-constrained source parameters for a Canadian volcano. Further work with emergency managers, will build on this map, toward consideration of appropriate mitigation measures.