Seismic microzonation in a complex volcano-tectonic setting: the case of northern and western Ischia Island (southern Italy)

On August 21, 2017, the volcanic island of Ischia was struck by a Mw 3.9 (http://cnt.rm.ingv.it/event/16796811) earthquake which caused two victims and heavy damages on wide portions of the Casamicciola Terme, Lacco Ameno and Forio municipalities. Following the Decree 1/2018 of the Government Commission for the reconstruction of damaged areas of Ischia Island, a Grade 3 Seismic Microzonation study was committed by each municipality to private consultants, who were scientifically supported by a multidisciplinary team of geologists, geophysicists and geotechnical engineers from CNR, INGV and University of Naples “Federico II”. The existing data from previous geological and geotechnical investigations were integrated with the results of new geo-lithological surveys and geophysical measurements (HVSR, MASW, Down hole tests) carried out by the consultants. The whole amount of data was quality checked, homogenized and collected in a SQLite/SpatiaLite geodatabase using a QGIS open source software by the support team. A geo-lithological map of northern and western sides of the island was then produced, and the results of routine geophysical tests were coupled with those of seismic 2D ambient vibration arrays, to obtain shear wave velocity profiles (several hundreds of meters deep) down to the seismic bedrock, made of lithic tuffs and lavas, and to define its buried morphology. This in turn enabled to draw a map of seismically homogeneous microzones of the entire study area. Cyclic torsional shear tests were carried out on 10 undisturbed soil samples, mainly taken from epiclastic cover deposits, to characterize their non-linear behavior to be adopted for seismic response analyses. These latter were carried out along 1D profiles characterizing each microzone and along selected 2D cross sections running from the Mount Epomeo to the coast. The dynamic analyses allowed to detect and map areas characterized by variable amplification factors, of spectral accelerations throughout different ranges of periods (0.1-0.5s, 04.-0.8s, 0.7-1.1 s). The zones most prone to site amplification are those corresponding to localized graben-like depressions at the toe of Mount Epomeo, such as Maio neighborhood of Casamicciola, where thickened epiclastic covers are entrenched into blocks of uplifted bedrock and where a combination of 1D stratigraphic amplification and 2D reflection and diffraction effects is expected.