Influence of geological complexities on local seismic response in the municipality of Forio (Ischia Island, Italy)

Abstract

Seismic response studies carried out in the Municipality of Forio on Ischia (NA), southern Italy, following the 21st August 2017 earthquake allowed to detect local effects related to specific geological-structural settings that reflect the complexity of the volcanic context which characterises the entire island of Ischia and, more specifically, the western sector of Mt. Epomeo and the adjacent coastal plain of Forio. In particular, the following features have been observed: i) polarization and amplification effects in the proximity of tectonic elements that dissect the Zaro promontory, where volcanic deposits from massive to stratified widely outcrop; ii) stratigraphic resonances on significantly variable frequency values, changing within distances of a few hundred meters, which can be related to the juxtaposition of landslide debris (such as debris-/rock-avalanches and lahar) in correspondence with the town of Forio; iii) seismic amplification in the sector involved in the ongoing gravitational deformation of Mt. Nuovo even in the absence of polarization of the particle motion. The peculiarities of the geological contexts analysed in the Forio Municipality allow to apply different interpretative schemes that vary from stratigraphic resonance (one-dimensional model of resonant column) mostly controlled by thickness and wave velocity in soft soils onto a stiff bedrock, to the resonance of jointed rock masses which are not completely released from the adjacent bedrock so avoiding typical free vibrations with normal modes (three-dimensional oscillating mass model), or to the interaction in the ear surface with physical discontinuities responsible for modifying the physical properties of surface waves polarizing and amplifying them. The collected evidence of local seismic response in Forio exemplifies how not conventional interpretative keys for seismic zoning can be proposed to identify sectors whose response schemes do not necessarily fall within the standard of stratigraphic or topographical amplification adopted by current national guidelines for Seismic Microzonation (SM) studies. In particular, the evidence of local seismic response collected for the Forio Municipality were taken into account in the SM and relative products that were realised in the following years.