A common source for the destructive earthquakes in the volcanic island of Ischia (Southern Italy): insights from historical and recent seismicity

The island of Ischia, located in the Gulf of Naples, represents an unusual case of resurgent caldera where small-to-moderate magnitude volcano-tectonic earthquakes generate large damage and catastrophic effects, as in the case of 4 March 1881 (Imax-VIII-IXMCS) and 28 July 1883 (Imax X-XI MCS) historical earthquakes, and of the recent 21 August 2017 MW = 3.9, event. All these earthquakes struck the northern area of the island. With about 65,000 inhabitants, Ischia is a popular touristic destination for thermals baths, hosting more than 3,000,000 visitors per year, thus representing a high seismic risk area. Assessing its seismic potential appears a fundamental goal and, to this end, the estimate of the magnitude of significant historical events and the characterization of their source are crucial. We report here a reassessment of historical data of damage of 1881 and 1883 earthquakes to evaluate the main source parameters of these events (obtained with the BOXER and EXISM software) and quantitatively compare, for the first time, the results with the source characteristics, obtained from instrumental data, of the recent 2017 earthquake. The results allowed us to assess the location, as well as the possible dimension and the related maximum magnitude, of the seismogenic structure responsible for such damaging earthquakes. Our results also provide an additional framework to define the mechanisms leading to earthquakes associated with the dynamics of calderas.