GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 421 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. Methods. An integrated approach implying iterative and interactive elaborations of geological, geophysical and geotechnical data is necessary for SM studies. In the area of interest, a huge number of seismic noise measurements (84), elaborated with HVSR technique, and MASW tests (34) were conducted by the professionals, while 11 passive 2D arrays were performed by the support team to obtain 6 deep vertical Vs profiles trough inversion of Rayleigh dispersion curves with ellipticity. Four boreholes were drilled for Down Hole test (from 35 to 50 m deep) and for collection of 12 undisturbed core samples; on the latter laboratory tests were conducted to obtain geotechnical parameters: grain size distributions and other basic properties, and G/G 0 -γ and D-γ curves. A set of thematic maps (at scales 1:3.500 or 1:5.000) was produced for each municipality, after field surveys and data collection from past and new campaigns of investigations, following the ICMS guidelines (Gruppo di Lavoro MS, 2008): Geological-technical Maps (Fig. 1) with engineering - geological cross sections; Maps of investigations; Maps of fundamental frequency (F 0 ; F r ); Maps of Seismically Homogeneous Microzones (SM of Level 1). Seismic Microzonation Maps of Level 3 were elaborated combining results from 1D and 2D numerical modeling; they show the areal distribution of AF for three intervals of period of engineering interest (0.1-0.5s, 04.-0.8s, 0.7-1.1 s). Seven time histories of acceleration were selected and scaled using the software InSpector (Acunzo et al. , 2014) for the modelling. The elastic target response spectrum is built on the indication of NTC 2018, considering the coordinates of the site with the highest reference hazard. The return period is fixed to 475 years (probability of exceedance of 10% in 50 years). Numerical modelling was performed through 1D (STRATA; Kottke and Rathje, 2008). and 2D (QUAD4M; Hudson et al. , 1994) codes. Seven engineering - geological cross sections were selected for 2D simulations which are representative of the most relevant geomorphological and stratigraphic-structural patterns of the island; the 1D simulations were conducted on the whole set of microzones. All collected vector and raster data were subject to compliance evaluation with respect to ICMS guidelines: Land use guidelines for areas affected by coseismic landslide (Commissione Tecnica per la Microzonazione Sismica, 2017), National Data Archiving and Representation (Commissione Tecnica per la Microzonazione Sismica, 2015). Data collection (data base) and thematic maps for seismic microzonation was made using the MzTools software, Python QGIS Plugin (Cosentino et al. , 2018). Results. The whole territory is subdivided into stable zones prone to local amplification and instability-prone zones. Microzones are related to structural-stratigraphically and morphologically well-defined blocks: i.e. graben and horst structures bounded by high-angle normal faults, such as the Piazza Maio graben and the Grande Sentinella horst (Casamicciola), and the Panza blocks (Forio); pyroclastic and lava centres and complexes (Rotaro-Tabor Mts;