An integrated approach to earthquake-induced landslide hazard zoning based on probabilistic seismic scenario for Phlegrean Islands (Ischia, Procida and Vivara), Italy

In this study we present an integrated approach to assess earthquake-induced landslide hazard at the source area of the slope instability process. The method has been applied to the case study of Ischia, Procida and Vivara islands that represent an integral part of the Campi Flegrei, a densely populated, active volcanic area, located at the NW margin of the Naples Bay, Italy. The proposed method follows a stepwise procedure including: 1) Probabilistic Seismic Hazard Analysis (PSHA); 2) assessment of site and topographic effects; 3) input of the PSHA outputs into a classic sliding rigid block analysis for slope instability (Newmark's approach); 4) construction of landslide frequency - magnitude curves for the estimate of the slope failure probability as a function of defined Newmark's threshold values under different probabilistic seismic scenarios; 5) construction of earthquake-induced landslide hazard maps at the source area, based on the integration of the probabilistic approach and the geological, morphological and geotechnical database available for the study area. The Probabilistic Seismic Hazard Analysis (PSHA) is aimed at the definition of the seismic input with different annual exceedance frequency. PSHA results, expressed in terms of Peak Ground Acceleration (PGA) at the bedrock, are calculated for 14 return periods (T) ranging from 10 to 2000 yr. PGA values have been corrected for the site effect associated with geological and morphologic conditions for each selected return period. Secondly, the corrected PGA values have been used as an input for the classic sliding rigid-block Newmark's approach, implemented in a Geographic Information System (GIS) to assess the relative potential for slope failure (landslide susceptibility) both in static (Factor of Safety, FS) and dynamic (Critical acceleration, a(c)) conditions. The combination of T-dependent, site-corrected PGA with the critical acceleration allowed for the calculation of the expected Newmark's displacements (D-N) under different probability of exceeding or return periods (probabilistic seismic scenarios). As a further step, in order to estimate the earthquake-induced landslide hazard, we defined three D-N threshold values that have considered capable to trigger shallow seismic-induced landslides in the regional context, and mapped the sectors with D-N values exceeding such thresholds. On this basis, we constructed frequency-magnitude curves to estimate the probability of slope failures at the source areas, as a function of D-N, by correlating the annual probability of landslide occurrence with the number of terrain cells associated with D-N values greater than the selected threshold. Finally, based on the estimated annual landslide frequency of the seismic triggering event for each terrain cell, we implemented a 1:5000 scale map of Earthquake-induced Landslide Hazard for Ischia, Procida and Vivara Islands. The map reports the zoning and ranking of study area into sub-zones, on a pixel basis, according to the degree of the potential hazard from landslides derived by the frequency of the triggering event.