A ground motion model for volcanic areas in Italy

In Italy, earthquakes caused by volcanoes are of primary importance in the evaluation of seismic hazard, since several volcanoes seriously endanger densely populated areas (e.g. Catania and surroundings, Campi Flegrei, Vesuvius). On the other hand, there are very few models for the prediction of ground motion induced by volcanic events, mostly because observations are scarce and volcanic earthquakes less frequent than crustal events. Following the recent earthquakes in the Etna area (mainshock 26/12/2018 M w = 4.9) and in the island of Ischia (mainshock 21/08/2017 M w = 3.9), it was possible to increase the number of recordings for volcanic areas in Italy and, in particular, close to the epicentre. The data available after the recent events revealed the limitations of previous models and especially their inadequacy to predict the ground motion observed in the near source, that can be unexpectedly high. We calibrate a new empirical model to predict the amplitudes of several intensity measures for volcanic areas in Italy. The most relevant aspect in the proposed model is the different attenuation with the distance between shallow and deep events, with discerning focal depth fixed at 5 km. The equations are valid for the geometric mean of horizontal components of PGA, PGV and acceleration response spectra ordinates at 5% damping (in period range T = 0.025–5 s). The range of validity in magnitude is 3.5–4.9 and the hypocentral distance range is 1–200 km.