Inundation and evacuation of shoreline populations during landslide-triggered tsunami: An integrated numerical and statistical hazard assessment

Abstract. The volcanic island of Stromboli (Southern Tyrrhenian sea, Italy) is renowned for its persistent, periodic, low-intensity explosive activity, whose spectacular manifestations attract tens of thousands of tourists every year. However, sporadic more intense major explosive and effusive eruptions, and paroxysms, pose serious threats to the island. In addition to direct hazards, granular slides of volcanic debris and pyroclastic avalanches, which can rapidly reach the sea potentially generating tsunamis, are often associated with such unpredictable eruptive activity. Due to the very fast propagation of the tsunami around the island, and the consequent short tsunami warning time (ranging from less than a minute to only a few minutes) mitigation efforts and evacuation from the Strombolian coast must be carefully planned. In this paper, we describe a new, GIS-assisted procedure that allows us to combine the outputs of an ensemble of 156 pre-computed landslide-generated tsunami hazard scenarios (with variable landslide volume, position, and density), statistical exposure data (i.e., the number of inhabitants and tourists) and digital geographic information, to obtain a quantitative (scenario-based) risk analysis. By means of the analysis of the road network and coastal morphology, we develop a model with routes and times to reach a safe area from every pixel in the inundated area, and appraisal for the time needed to escape versus the wave arrival time. This allows us to evaluate and quantify the effectiveness of potential risk mitigation by means of evacuation. The creation of an impact score linking the predicted inundation extent and the tsunami warning signals is intended, in the long term, to predict the intensity of future tsunamis, and to adapt evacuation plans accordingly. The model, here applied to Stromboli, is general, and can be applied to other volcanic islands. Evacuating an island hosting several thousand tourists every summer with very little warning time supports the absolute necessity for such mitigation efforts, aimed at informing hazard planners and managers, and all other stakeholders.