Operational dispersion modeling of 2023 Shishaldin Volcano eruption: complications from unknown granularity and mafic co-PDC plumes

Abstract The 2023 eruption of Shishaldin Volcano consisted of 13 explosive events, each resulting in drifting ash clouds that disrupted aviation and fallout that affected Aleutian communities and mariners. Shishaldin Volcano is monitored by the Alaska Volcano Observatory (AVO) with a local seismic network, regional infrasound stations, lightning detection networks, web cameras, and low-latency geostationary satellite data. These data streams allow a rapid estimation of critical eruption source parameters for operational ash dispersion modeling, such as the onset time of ash emissions and estimates of plume height. Although most of these dispersion model forecasts were consistent with subsequent reported fallout and satellite observations, ash plumes generated by syn-eruptive pyroclastic density currents (PDC) sometimes diverged from model forecasts. These co-PDC plumes had broad sources along the flanks of the volcano with lower plume heights than vent-derived plumes. Additionally, vent-derived plumes had coarse grain size distributions that led to inconsistencies with fallout model forecasts due to the more rapid settling of particles from the eruption cloud. Due to the remote location of Shishaldin Volcano, sampling tephra fallout to determine the typical grainsize was not typically possible, however combining a single field visit and community provided samples during the eruption provided data to develop a model for vent and co-PDC derived plumes. In this study, we present a reanalysis of the sequence of explosive events using the Ash3d dispersion/sedimentation model and the implications of this reanalysis on operational modeling of explosive mafic eruptions.