Assessment of site effects in volcanic areas: results from Seismic Microzonation studies in the Island of Ischia (Naples, Italy)

The Seismic Microzonation (SM) of Level 3 allows quantifying numerically seismic amplification effects on microzones, previously introduced after detailed geological reconstructions of local subsoil and geophysical-geotechnical characterization of bedrock and cover terrains. Major goals of the Level 3 SM studies are, indeed, the calculation and mapping of Amplification Factors (AF) at ground for different period intervals of engineering interest, and the defining of acceleration response spectra for each microzone. According to the Italian standards (Gruppo di Lavoro MS, 2008; SM Working Group, 2015), this can be achieved through 1D and 2D numerical simulations of the ground motion performed respectively on stratigraphic logs and on engineering-geological cross sections, with litothypes parametrized in terms of Vs and G/G0-? and D-? curves. The obtained results are basic for correct Urban Planning Strategies and Seismic Risk Mitigation Policies. It is here presented the case of Level 3 SM studies concerning the municipalities of Casamicciola Terme, Lacco Ameno and Forio (Island of Ischia, Naples, Italy), affected by the 21 August 2017 earthquake (Mw 3.9, http://iside.rm.ingv.it/en/event/16796811), which caused two victims and heavy damages to a large number of buildings. Ground acceleration was 0.28g at IV.IOCA station, EW component. Soon after the event, it was issued the OCDPC 476/2017 Decree by the Italian Civil Protection Head to promote studies on these territories for: 1) defining the engineering geological setting, and 2) collecting geophysical data (312 seismic noise measurements, 20 MASW tests, 4 passive 2D arrays), both preparatory for SM. The synthesis of borehole and literature data on the complex stratigraphy and volcano-tectonics of the island, coupled with results from the geophysical investigations, allowed to detect in a preliminary way the boundary depth between bedrock and cover units (Cavinato et al., 2018, with references therein). The following Decree 1/2018, by the Government Commissioner for reconstruction of damaged territories of the Island of Ischia, established purposes, time schedule (January-July 2019) and roles of participants to Level 3 SM studies on those areas. The studies, aimed at defining the AF and response spectra, were conducted by professionals, constantly and scientifically supported by a team of researchers from CNR (IGAG and ISMAR), INGV, and University of Naples "Federico II" (DICEA). In particular, the scientific community was responsible for the integration and validation of the study results.