Numerical modeling of hydro-acoustic waves for a tsunami early warning system

Abstract

Tsunamigenic fast movements of the sea-bed generate pressure waves in weakly compressible sea water, namely hydro-acoustic waves, which travel at the sound celerity in water (about 1500 m/s). These waves travel much faster than the counterpart long free-surface gravity waves and contain significant information on the source. Measurement of hydro-acoustic waves can therefore anticipate the tsunami arrival and significantly improve the capability of Tsunami Early Warning Systems (TEWS). However, applications to real cases require detailed numerical modelling in order to clearly define the time series at point A due to a source at point B. Three-dimensional models are straightforward to use, but require unrealistic computational times when applied to large-scale geographical areas, i.e. they cannot be used for a systematic investigation on an oceanic scale of prediction. The problem is further complicated by the effects of compressible viscous sediment layer at the sea bottom, which have a deep inuence on the hydroacoustic waves propagation over large distances. The present work provides the derivation of two numerical models suitable for hydro-acoustic waves simulation and the application of model on large scale domain. The Mild-Slope Equation in Weakly Compressible uid (Sammarco et al., 2013 and Abdolali et al., 2014) is presented first. It reduces the computational problem from three to two dimensions, hence reducing dramatically the computational costs. Then the capabilities of the model is extended by including the effects of a sediment layer at the bottom (Abdolali et al., 2015b). Finally, two applications to real bathymetries are presented. In the first, the model is applied to simulate the hydro-acoustic wave propagation in the central and eastern Mediterranean Sea, generated by two main destructive historical earthquakes: the 365 AD Crete event and the 1693 Sicily event (Abdolali et al., 2014 and Cecioni et al., 2015). In the second application the model is used to reproduce the 28 October 2012 7.8 Mw earthquake occurred off the West coast of Haida Gwaii archipelago, Canada. For this event deep water field measurements are available for comparison Abdolali et al., 2015a). On the basis of the numerical results for these real cases, several conclusions on the possible use of hydro-acoustic waves as support to Tsunami Early Warning Systems can be drawn.