Numerical modelling of the September 8, 1905 Calabrian (southern Italy) tsunami

Abstract

This paper presents a study of the tsunami following the disastrous earthquake that occurred on the 8th of September 1905 in Calabria, southern Italy. The shock caused devastation in many towns and villages leading to more than 500 victims. According to coeval sources the tsunami was not catastrophic, but was large enough to inundate low lying lands in several coastal segments and to affect boats. No specific study was ever devoted to this tsunami in the literature. Our analysis is carried out by means of numerical modelling and aims at filling this gap. Since the source fault of the earthquake has not yet been identified, we study three tsunamis produced by likely potential sources, according to macroscismic data and the seismotectonic knowledge of the region, namely the Capo Vaticano (CV) fault, the Vibo Valentia (VV) fault and the Lamezia (LA) fault. We compute tsunami waves under the hypotheses that the sea floor deformation is caused by a shear double-couple dislocation taking place over a rectangular fault and that sea waves propagate according to the shallow water approximation. The three tsunamis cause either a pure initial depression (VV and LA) or a predominant initial depression (CV) of the sea surface in the gulf of St. Eufemia. One very relevant finding is that tsunami energy is trapped within a narrow channel along the Calabrian coast to the north of the source with the consequence that wave amplitudes computed here are much larger than the values expected in the case of an ordinary energy decay law with distance; energy trapping along southern coastal segments is less efficient. Only a small fraction of tsunami energy penetrates the Straits of Messina to the south. Propagation to the west is affected by the Acolian islands that produce a quasi-shadow zone on the side opposite to the tsunami impact. The LA tsunami is found to be too weak to be compatible with observations. A sensitivity analysis concerning some parameters of the faults (namely length, width and slip magnitude) has been performed in order to explore their influence on the tsunami features. The main result of this paper is that we cannot discriminate between the CV and VV sources; both are in agreement with some historical observations, whilst they do not match others. We believe that a better tuning of the source mechanism over these faults can improve the agreement between model computations and historical observations.