The volcano-tectonic 2017 earthquake at Ischia island (Italy): geophysical and geological constrains

On 21 August 2017, a shallow earthquake of Md 4.0 struck the Casamicciola Terme village in the north of Ischia volcanic island (Italy). It caused two fatalities and heavy damage in a restricted area of a few square kilometers. The damaging earthquakes at Ischia Island are no exception. The earthquakes repeat with similar characteristics over the historical time window (1796, 1828, 1881, and 1883) and their epicentral location is systematically confined to the same area of few square kilometers at the base of the northern flank of Mt Epomeo. Our study aims to show the vantages of multidisciplinary analysis based on integrated geophysical investigation and geological investigations for constrain the characteristics of the Casamicciola active/capable fault system of Ischia Island responsible of 2017 volcano-tectonic earthquake. We performed a) a geophysical survey with use the impulsive electromagnetic method of the Ground Penetrating Radar (GPR); b) SAR data analysis; c) two new stratigraphic boreholes in the hangingwall and footwall of the Holocene graben. For constrain the geological model of the E-W seismogenetic fault hypothesized in the literatures we have investigated the piedmont belt of the N slope of Mt. Epomeo where was mapped coseismic surface ruptures following the 2017 earthquake. We conducted a geophysical investigation with use the impulsive electromagnetic method of the Ground Penetrating Radar (GPR) that through low-frequency antennas perform deep stratigraphic analyzes up to about 100 m. With GPR we have identified the stratigraphic contacts in depth and the presence of synthetic and antithetic faults systems bordering the Holocene Graben. Moreover, we realized two new stratigraphic boreholes in the hangingwall and footwall of the Holocene graben for constrain the stratigraphic unit involved into displacement. Through satellite interferometric techniques we assess the space and time evolution of ground deformation rate and model the displacement field.