The 2017, Mw 4, Ischia Earthquake (South Italy): Source mechanism and rupture model from the inversion of a near-source strong motion record

Abstract:

The moderate earthquake, occurred in the volcanic island of Ischia located in south-west of Naples, generated a widespread strong ground shaking that caused several buildings collapse, two victims and several tens of injured people. The event has been well recorded by a three-component accelerometric station (IOCA) operated by INGV and located a few hundred meters distance from the event epicenter. The station IOCA has measured a relatively large amplitude ground shaking and long-lasting S-wave signal longer than expected for 3.9 earthquake. To investigate the event rupture complexity and its radiated wave field, we use a finite-fault model to invert the near-source velocity records at IOCA station and search for the best-fit kinematic rupture parameters. First, using the point source approximation, the initial hypocenter location and focal mechanism i.e., dominant right-lateral strike-slip, have been further constrained and confirmed by the analysis of P-arrival times and polarities at IOCA and two additional seismometric stations (CAI and F09) deployed at the island and moment tensor determination from the regional earthquake records, as well. The kinematic rupture modelling of IOCA records showed that the rupture nucleated at about 600 m west of IOCA and 1.1 km depth, along a 1 km, NW-SE striking fault, with a rupture velocity 0.8 km/s (90% of the shear wave velocity at the same depth). The retrieved rupture model coupled with multi-path reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, explain the observed ground motion duration at IOCA and strong shaking amplitudes recorded all over the island.