Lava flow hazard modeling during the 2021 Fagradalsfjall eruption, Iceland: applications of MrLavaLoba

Abstract. The 6-month-long effusive eruption at the Fagradalsfjall volcano in 2021 is the most visited eruption site in Iceland to date (June 2023), and it needed intense lava flow hazard assessment. In this study we document how strategies for lava flow modeling were implemented using the stochastic model MrLavaLoba to evaluate hazards during this effusive event. Overall, the purposes were threefold: (a) pre-eruption simulations to investigate potential lava inundation of critical infrastructure, (b) syn-eruption simulations for short-term (2-week time frame) lava flow hazard assessment and (c) syn-eruption simulations for long-term (months to years) hazard assessments. Additionally, strategies for lava barrier testing were developed, and syn-eruption topographic models were incorporated into simulations in near real time. The model provided promising results that were shared regularly at stakeholder meetings with the monitoring personnel, scientists and civil-protection representatives helping to identify potential short-term and long-term lava hazards. This included evaluation of the timing of barrier overflow and the filling and spilling of lava from one valley to another. During the crisis the MrLavaLoba model was updated to increase functionality such as by considering multiple active vents. Following the eruption, the model was optimized substantially, decreasing the computational time required for the simulations and speeding up the delivery of final products.