Contribution to knowledge regarding the sources of earthquakes on the island of Ischia (Southern Italy)

This paper analyses data regarding the seismicity and ground deformations of the island of Ischia. The goal is to describe these phenomena as a space–time process, exploring the mechanism as a whole in order to answer questions concerning why and how historical earthquakes occurred and the geological constrains at the time of the earthquake of 21 August 2017. According to our analysis, the genesis of earthquakes in the island may be due either to regional tensile tectonics or to increased pressure in a shallow magma source. The eruptive rest following the 1302 Arso lava flow, the seismic silence from 1883 and the ground sinking recorded at least from the end of the nineteenth century suggest a decline of pressure in the shallow magma source and in the action of the stress field generated by the spreading of the Tyrrhenian Basin. The mechanism that can generate the observed ground deformation field and seismicity is the gravitational and thermal loading of the island, because the load of the massif of Mt. Epomeo onto its hot substratum may induce the gravitational spreading that would have produced its sliding southwards and its rotation northwards. The surface of detachment plunges towards the south and emerges at a high angle at the northern edge of Mt. Epomeo, where coseismic ground effects and the greatest damage have been observed. This mechanism that generated the 21 August 2017 earthquake could be extended to historical seismicity.