Scoria Fallout Modeling and the 3 March 2015 VEI-2 Eruption of Villarica Volcano, Chile

Abstract:

In March 2015, Villarrica volcano erupted a spectacular 1.5 km lava fountain and 6-8 km plume, depositing a thin (several mm or cm) layer of scoria tens of km toward the east and southeast. We show results of numeric models (the advection-diffusion equation solver Tephra2, and particle-tracking models) informed by NOMADS atmospheric data used to model this fallout. Models show strong winds (up to 25 m/s) toward the east and southeast concentrating the narrow deposit in those directions, and the vertical variation of wind direction predicts particle sorting along the wind-transverse direction. Both of these were observed in the field. We discuss the challenges faced by fallout models of scoria: because of its irregular shapes, high and variable porosity, and propensity to break apart on impact, aerodynamic properties are difficult to assess by physical observations. This introduces ambiguity when comparing models to observations. Finally, we demonstrate how short-term hazard predictions can benefit from the integration of fallout models with weather forecasts up to several days in advance, and how hazard communication to the public can benefit from snapshots and animations showing zones subject to tephra fall and time delays from eruption to impact.