The damaging seismic event of August 21, 2017, on Ischia Island (southern Italy): constraints on the source mechanism

Abstract

On August 21, 2017, Ischia was struck by a seismic event of MD 4.0 that provoked significant shaking and severe damages (maximum intensity reached in Casamicciola village; Azzaro et al., 2017) including 2 victims. The incongruity between damage and magnitude cannot be explained only by local site effects or especially vulnerable constructions, but may be influenced by particular characteristics of the seismic source. The first automatic hypocenter location at INGV was off-shore at a standard crustal depth of 10 km in the area between Ischia island and the Italian Peninsula. Relocation of the event confirmed a shallow hypocenter (ISIDe, 2016), in proximity of the maximum damage intensities observed. The calculated magnitudes of ML 3.6, Mw 3.8 and MD 4.0 seemed to be at odds with the high macroseismic intensity (hereafter reported as IEMS – the Intensity of the European Macroseismic Scale). The disagreement between the observed maximum macroseismic intensity and seismic energy seems to be related to the particular seismotectonics and geological situation of the volcanic island. This situation recalls what happened in 28 July 1883, when Ischia was struck by an earthquake that caused more than 2300 deaths and severe damage in the northern part of the island, near the small town of Casamicciola (Mercalli, 1884; Cubellis and Luongo, 1998; Carlino et al., 2010). In this paper we analyze the 2017 Ischia earthquake, trying to account for the disagreement between the high maximum macroseismic intensity and the relatively low magnitude, and to clarify the contradictory information on the source parameters calculated. To this end, we relocate the hypocenter by a single-station method using an ad hoc velocity model and calculate moment magnitude and source mechanism by inversion of the full moment tensor.