Seismological Constraints on the Source Mechanism of the Damaging Seismic Event of 21 August 2017 on Ischia Island (Southern Italy)

Abstract

On 21 August 2017, anMD 4.0 earthquake struck Ischia island in the Tyrrhenian Sea off the coast of Naples, Italy. In spite of its modest magnitude, the earthquake caused two deaths and severe building damage on the northern side of the island. Initial hypocenter locations based on arrival times were highly uncertain and several proposed moment tensor solutions were inconsistent. These contradictory observations prompted a new calculation of the earthquake parameters using alternative methods. Our new approach, based on the determination of P-wave particle motion, evaluation of rotated spectra, and accurate calculation of S-minus-P travel time, yields a hypocentral depth of 2 km and a location in the same area as the devastating seismic event that struck Ischia in 1883. We invert the moment tensor for a best-fitting double couple (DC), obtaining anMw 3.9 with a normal mechanism and an optimal depth of 8 km. Calculation of the full moment tensor results in (1) 36% negative isotropic component and 26% negative compensated linear vector dipole (CLVD) components, (2) a better fit at a shallower source depth than for the corresponding DC, and (3) a magnitude estimate of Mw 4.1. Modeling of the waveform and the first motion recorded in Ischia’s station IOCA predicts, however, a negative polarity, which is in disagreement with the observation. We therefore suggest a complex rupture process, with an initial shallow normal-faulting event that triggered a subsequent shallow underground collapse.