Genesis of tsunami waves generated by Pyroclastic flows and the Early-Warning system.

Pyroclastic flows are a common product of explosive volcanism and have the potential to generate tsunami wave when dense flows encounter bodies of water. Tsunami generated by large sub-aerial mass may constitute a serious hazard for coastal population and environment, as documented by several examples in recent history and by numerous geological traces of paleo-events. On July 3rd and August 28th 2019 paroxysms at Stromboli volcano produced a few km-high eruption columns, huge tephra fallout and pyroclastic density currents propagating along the Sciara del Fuoco at a mean speed of 70 m/s. These latters generated two moderate high tsunami waves characterized by a period of ~30 s which were recorded at a sampling rate of 125 Hz in the very near-field by two elastic beacons located a distance of 0.3 (PDC) and 1.5 (PLB) Km from the location where the mass flow entered the water. Video camera images and arrival times indicate that tsunami near the coastline was propagating at a mean velocity of 22 m/s. This gave the unique opportunity to detect the tsunami at the source providing an unprecedented record of the tsunami wave when is forming. The tsunami record provides an important constraint for the modelling and it has a large impact on the understanding of the tsunami generated by pyroclastic flows dynamics. The record of the two tsunami is also offering the possibility to test our ability to automatically recognize those long period waves and to trigger the acoustic alarm network installed in Stromboli and Panarea islands and in Milazzo (Sicily). We show how the Early Warning system was able to detect the tsunami providing the alert some minutes before (~ 2 minutes) the waves were reaching Stromboli island providing for the first time an efficient tool to mitigate the tsunami risk associated to the explosive volcanism. Moreover, the high quality of these data recorded in the very near-field is suggesting the possibility of using simple analytical equation to quickly estimate the source parameters of the tsunami in terms of volume per unit width, maximum speed of sub-aerial flow and submerged travel time.