On the impact of the effusion rate trend for the assessment of lava flow hazards

Lava flows are recurring and widespread hazards that affect areas around active volcanoes, having the potential to cause significant social and economic loss. In the last decades, physics-based models of lava flows have been proven effective and powerful tools to forecast and assess the hazard posed by effusive events. These models require different input parameters, such as the physical properties of the fluid (e.g., melt compositions, water content, rheological law, thermal properties) and the topography of the terrain. A critical parameter in physical-mathematical modelling is the effusion rate, i.e. the rate at which lava is discharged. Lava effusion rate is variable in time, strongly controlling the emplacement and run-out distance of lava flows. Nevertheless, both for assessing long-term hazards and for monitoring efforts during on-going eruptions, effusion rate is assumed to be constant or to have a bell-shaped time-dependent behavior. Here we present an analysis of the time-averaged discharge rates (TADRs, i.e. the effusion rate averaged over given periods) estimated for recent flank eruptions at Mt. Etna volcano (Italy) in order to define a possible generalized effusion rate trend to be used for the physical modeling of lava flows. The temporal series of TADRs, derived from field measurements and satellite thermal imagery, were normalized in order to obtain homogeneous curves in duration and sampling times, reducing redundancies and improving data consistency. Our analysis indicates that most of the effusion rate curves for flank eruptions of Etna are characterized by a fast waxing phase with the peak occurring between the 0.5 and 29% of the total eruption time, followed by a progressive decrease in the waning phase. By using the median values associated to the occurrence of effusion peaks and to the slope variations of descending curves in the waning phase, we estimated an averaged curve that was used to run numerical tests by means of the physics-based GPUFLOW model. Different tests were performed considering how the “characteristic effusion rate curve” could impact single vent scenarios, as well as on short- and long-term hazard maps. Statistics on the final emplacements revealed variations up to 20%, confirming the key role of the effusion rate in controlling the development of lava flow fields.