Characterization of rockfall mechanisms and run-out in active volcano-tectonic areas: a case study from Ischia Island, Southern Italy

Abstract

Ischia (Southern Italy) is a volcanic island of the Phlegrean Volcanic District that was recently affected by multiple geological hazards, including floodings, landslides, rockfall and earthquakes.In this study, rockfall stability is analysed, assuming as a case study a 400m-wide cliff made of Green Tuff and located on the western area of Mt. Epomeo. The two outcrops studied are located at 280 and 420 m a.s.l., above the site of Frassitelli, Forio d’Ischia, which is an area of high residential, touristic and agricultural importance. The former is a high-angle outcrop affected by tens of meters-long faults, whereas the latter is characterised by high-dip pinnacles.We analysed the fracture systems affecting the examined formation to compute the kinematic analysis of the potential rupture mechanisms and to perform numerical simulations of potential rockfall scenarios. The data acquisition was carried out by means of classical geological field surveys and structural analysis on Virtual Outcrop Models (VOM) obtained from images acquired by drones. The VOMs were analysed with ‘CloudCompare v2.10.2’ and ‘OpenPlot’ software. The former allowed the automatic digitalisation of the exposed discontinuities by applying the ‘Facets’ plugin, based on a least-square fitting algorithm (Fernández, 2005). ‘OpenPlot’ enabled the extraction of the geostructural information from the VOM, by computing the best-fit planes of the polylines manually drawn along the interference between the geological surface and the outcrop topography (Tavani et al., 2011).The measured and the extracted features were classified following their attitude. Three main sets were defined, striking N-S, NW-SE and NE-SW. The fracture dataset was used to perform a kinematic analysis with ‘DIPS’ software on the surface discontinuities extracted from ‘Facets’ plugin. The 'wedge sliding' resulted the most critical potential rupture mechanism to occur on the analysed outcrops. Successively, numerical simulations of rockfall scenarios were computed based on the acquired structural information. The latter permitted us to identify the maximum run out of the potential blocks and draw some consideration on the rockfall hazard of the area.