A review of submarine landslides in the Ischia Island offshore (Tyrrhenian Sea)

Abstract A revision of the submarine landslides occurring in the Ischia offshore has been carried out, considering theoretical approaches, modeling, and seismo-stratigraphic data, focusing on the Ischia Island example. Based on the literature data, marine hazards and tsunami assessment have been considered. Although sidescan sonar data demonstrating the hummocky facies in the northern Ischia offshore have not been published, morpho-bathymetric data from earlier studies have been reinterpreted. Only a portion of the seismo-stratigraphic data has been reevaluated, and newly interpreted seismic sections are displayed. Major debris-avalanche deposits are preserved in the offshore areas in the north, west, and south, whose chronostratigraphic age is Holocene. Deposits from Northern Ischia exhibit blocky acoustic facies and hummocky/mounded morphology. Multiple generations of mass-transport deposits are suggested by the geological interpretation of seismic profiles. The main implication for the hazard evaluation is highlighted by the presence of significant underwater deposits off the northern, western, and southern coasts, suggesting that Ischia is characterized by a marked susceptibility to landslides, some of which may have reached speeds that made them potentially tsunami-generating and a risk to the local population and infrastructure. The factor of safety has been evaluated based on literature data. With a factor of safety ranging from 2.01 to 3.69, the majority of coastal sectors exhibit extremely low levels. This is also true for Forio (western Ischia) and Casamicciola (northern Ischia), where significant debris avalanche deposits are found in the nearby offshore. This makes these locations less likely to slip. On the contrary, the southern part of Ischia has a very high factor of safety (less than 1.01), particularly in the Maronti and S. Angelo sectors, which may be vulnerable to slides. Seismic sections on Ischia’s southern slope, which exhibit significant slope gradients and the predominance of rocky lithologies (rocks and tuffs; Fig. 4), reinforce this geological interpretation.