Earthquake-Triggered Tsunamis: Impact of the Uncertainty in the Rupture Slip Distribution on the Resulting Tsunami Wave Heights and Flow Velocities

The estimation of expected damage and losses from natural hazards requires that uncertainties in the modelling and knowledge of future events be quantified and taken into consideration. This is true not only in a fully probabilistic context but also in future scenario calculations, particularly when looking at two or more cascading hazards in which the link between them is not univocal. An offshore earthquake that triggers a tsunami would be one such case. Even if the moment magnitude and rupture size and location of the earthquake were fully defined, it is not possible to know a priori the slip distribution along the rupture and the subsequent co-seismic topographic displacements. Many feasible slip distributions can be associated with the same moment magnitude and dimensions of the rupture, and these lead to a distribution of subsequent topographic displacements and, with that, a diversity of tsunami outcomes. Exactly how much variety exists in the resulting tsunamis, in terms, for example, of maximum wave height or maximum flow velocity at points of interest, and, ultimately, damage to buildings and losses, is the question driving the present study, which is part of the “risk workflow for CAScading and COmpounding hazards in COastal urban areas” (CASCO) project. The ultimate objective is to understand the relevance of this uncertainty and whether it needs to be modelled in the whole damage/loss calculation chain.

To investigate this, 500 realisations of stochastically generated rupture slip have been produced for the 1908 Mw 7.1 Messina earthquake, whose rupture source is taken from the Italian Database of Individual Seismogenic Sources (DISS). The subsequent realisations of ground surface deformation (at the bottom of the sea and on land) were used as input to run realisations of the resulting tsunami in the Strait of Messina, eastern Sicily and western Calabria with the TsunAWI software. Maximum wave heights, maximum absolute velocities and maximum flux can vary significantly for selected observation points along the coast and within the Messina Strait. While a weak correlation has been identified between these tsunami outputs and inputs such as the maximum initial co-seismic vertical displacement, a stronger correlation has been observed with respect to the distance to the centroid of rupture slip. So far, results indicate that the uncertainty in the co-seismic slip along the rupture and the subsequent vertical displacements has a relevant impact on the resulting tsunami, suggesting that this source of uncertainty should not be entirely neglected in models. Using these tsunami outputs to estimate damage to buildings in the area allows us to understand the ultimate final impact on damage and loss calculations, and to develop and test strategies to reduce the resulting computational demand.