Three-dimensional simulations of rockfalls in Ischia, Southern Italy, and preliminary susceptibility zonation

Ischia island is a volcano--tectonic horst in the Phlegrean Volcanic District, Italy. We investigated rockfalls in Ischia using STONE, a three--dimensional model for simulating trajectories for given detachment locations of blocks. We propose methodological advances regarding the use of high--resolution LiDAR elevation data, the localization of possible detachments sources, and the inclusion of scenario--based seismic shaking as a trigger for rockfalls. We demonstrated that raw LiDAR data are useful to distinguish areas covered by tall vegetation, allowing realistic simulation of trajectories. We found that the areas most susceptibile to rockfalls are located along the N, N--W and S--W steep flanks of Mt. Epomeo, the S and S--W coast, and the sides of some steep exposed hydrographic channels located in the South sector of the island. A novel procedure for dynamic activation of sources depending on ground shaking, in the event of an earthquake, helped inferring a seismically--triggered source map and the corresponding rockfall trajectories, for a scenario with 475 y return time. Thus, we obtained preliminary rockfall suceptibility in Ischia both in a "static" (trigger--independent) scenario, and in a seismic shaking triggering scenario. They must not be considered a risk map, but a starting point for a detailed field analysis.