Assessments of water-driven mass flows at snow and ice-covered volcanoes are often based on the assumption that the most impactful flows will happen during eruptions (lahars). However, as eruptions are a relatively low-probability event, non-eruptive debris flows tied to precipitation or glacial outburst floods may be more common. This is especially relevant at Mount Shasta, one of the southernmost glaciated stratovolcanoes in the Cascade volcanic arc, where past hazards assessments focused on the possibility of larger flows triggered by eruptive activity. Recent years (2021–2025) at Mount Shasta, however, have seen smaller non-eruptive lahars (locally called ‘debris flows’), which have had significant impacts on the communities and infrastructure surrounding the volcano, disrupting major transportation routes and threatening municipal water supplies. Given Mount Shasta’s current quiescence, it is prudent to focus volcanic hazard assessment on these non-eruptive lahars. In this study, we expand the historical record of debris flow occurrence and determine whether there is a discernible link between debris flow frequency and broader, meteorological forces. We classify debris flows into two broad groups based on associated meteorological parameters: (1) flows triggered by rainfall during storms, and (2) flows without associated precipitation occurring during hot summer days in years with widespread drought, including below average snow accumulation in the previous winter. These findings are consistent with historical anecdotal observations by local residents – that debris flow occurrence outside of precipitation is more frequent in hot, dry summers – and provide important information for evaluating which meteorological conditions may lead to an increase in debris flow occurrence. We also highlight the fact that debris flows may be a hazard at Mount Shasta even when there is no precipitation. These results provide important data for evaluating debris flow hazards in the face of ongoing changes in weather patterns across northern California and near other snow-and-ice-clad volcanoes worldwide.
The Corbetti volcanic system, located in the central part of the Main Ethiopian Rift, has no recorded eruptions in living memory but shows evidence of past activity ranging from highly explosive Plinian eruptions to dominantly effusive events. Approximately 1.1 million people live within 30 km of the volcano and over 12 million within 100 km, alongside critical infrastructure such as highways, Hawassa Airport, and major towns including Hawassa and Shashemene. A geothermal power plant is also under development within the caldera. Despite significant advances in understanding Corbetti through recent fieldwork, hazard assessment remains limited due to challenges in distinguishing deposits, poor lateral continuity, and scarce dateable material. This study combines new and existing field data with expert judgment to develop probabilistic tephra-fall footprints for four plausible eruption scenarios and a worst-case scenario. These are integrated with exposure data to assess potential impacts and eruption frequency. Results indicate an average recurrence interval of 520 years for any explosive eruption, with low-to-moderate volume events ten times more likely than large Plinian eruptions. Modelling shows tephra dispersal predominantly westward, potentially exposing over 750,000 people, 3,400 km of roads, and 1,600 km2 of cropland to tephra loads exceeding 1 kg m− 2 ( 1.6 mm) during a Plinian-type event. In the same scenario, more than 37,000 people could experience > 100 kg m− 2 ( 16 cm) of tephra within 20 km of the vent. Even smaller eruptions could significantly impact agriculture and infrastructure, underscoring the need for preparedness and risk management in a region with no historical memory of explosive volcanism.
Volcanic ashfall has only recently been recognized as a destabilizing hazard for small craft, as evidenced by vessel capsizing during the 2011–2012 Cordón Caulle eruption in Argentine Patagonia. However, the ash loads required to compromise vessel stability remain poorly constrained. This study presents the first operational methodology to quantify capsizing risk under volcanic ash loading through an analytical vulnerability framework. Using the Maxsurf software suite for numerical hydrostatic and stability modelling, large-angle static and dynamic simulations were conducted for three vessel types representative of those operating in Patagonian lakes: a rigid inflatable boat, a motor sailboat, and a small passenger vessel. Simulations combined three Beaufort-scale environmental forcing conditions with fourteen deck-accumulation thicknesses, two bulk densities, and an additional snow-loading scenario documented during the motivating case studies. Reductions in key stability indices were quantified and used to derive vessel-specific vulnerability curves. Additionally, three damage states (DS0-DS2) were defined by translating international intact-stability criteria into discrete categories of progressive stability loss, enabling identification of ashfall thresholds for each state. The resulting vulnerability model was then evaluated for applicability and predictive performance against documented incidents from the 2011–2012 Cordón Caulle eruption, using a combined metric-based and spatially informed retrospective validation approach. Predicted impacts show strong agreement with observed damage, demonstrating that documented ashfall accumulations exceeding 150 mm can plausibly induce vessel capsizing under realistic combinations of ash-density, snow-loading, and sea-state conditions. Minor biases were identified, including slight under- and overestimation for larger and smaller vessels, respectively, while intermediate damage states (DS1) remained less constrained because of rapid transitions from safe conditions (DS0) to failure (DS2). By resolving the coupled controls on vessel stability through systematic pre- and post-processing, the framework yielded ashfall thresholds broadly consistent with previously proposed generic limits, demonstrating its potential as a transferable basis for interpreting past incidents and informing risk assessment for vessels exposed to volcanic ashfall.
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 km2 for the VEI 3 scenario and 9360 km2 for VEI 4 (with 50% probability of exceedance). Tephra loads that may cause infrastructure damage (≥ 1 kg m− 2) cover 250 km2 and 1670 km2 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.
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’kay’) 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 km2 for the VEI 3 scenario and 9360 km2 for VEI 4 (with 50
