R6-Worst-Case Tsunami Approach Applied to Catania (Eastern Sicily)

The Worst-case Credible Tsunami Scenario Analysis (WCTSA) is a methodology aiming at assessing the tsunami hazard for a target site based on a limited number of scenarios. These are defined starting from the events hitting the selected area in the past, and from potential events elaborated from the knowledge provided by geology, geomorphology, geodynamics and seismology of the studied zone. In this paper, the recipe related to the WCTSA is described through its application to the town of Catania (Sicily, south Italy), based on the paper by Tonini et al. (2011). The purpose here is not to discuss the details of the application to Catania itself and the results obtained for that specific site, but rather to outline the strategy in its key aspects, highlighting advantages and drawbacks. In general, the steps constituting a WCTSA are the sources identification, the modelling of the generation, propagation and inundation phases of the tsunami for each source, the vulnerability and risk assessment. Coherently with the results illus-trated in Tonini et al. (2011), here only the first three phases related to the hazard assessment will be discussed, while some results of the whole workflow including also vulnerability and risk assessment can be found in Tinti et al. (2010). The WCTSA approach should be regarded as an alternative to PTHA but not in oppo-sition to it. First, it is particularly applicable to regions where seismic sources are not the predominant contributors to tsunami generation, or where they are negligible or absent: it is well known that landslides, volcanoes and meteorological forcing can produce local relevant waves and that well-established PTHA approaches are presently lacking for these types of sources (for instance, because of the difficulties or even the impossibility to establish a magnitude frequency distribution). Secondly, especially local sources can produce large effects in limited coastal portions, often in conjunction with peculiar coastline morphologies, that can only be captured through very high-resolution grids, which perhaps can make the computational cost for a PTHA prohibitive.