Source identification of extreme event deposits in the Lesser Antilles using forward numerical modeling of tsunami wave propagation and sediment transport

Abstract:

In the arc of the Lesser Antilles, born from the subduction of the Atlantic oceanic plate under the Caribbean plate, the tectonic processes can initiate Mw8 or more megathrust earthquakes. The associated tsunamis can reach the closest coasts within a few minutes with amplitudes of several meters. Such kind of destructive events can be considered as a great threat for the population. During the last centuries, four tsunamigenic earthquakes have been identified in this region with tsunami waves of up to 0.5-m for the smallest in 2004 and 10-m for the strongest in 1867. Tsunamigenic earthquakes in this region are considered rare, and the available data are qualitative only. Clues of a much older event were recently identified in Anegada island with paleoseismological data, which might have generated more than 10-m high waves. The island, the easternmost of the Virgin Islands, is flat, uninhabited and surrounded by coral reef. Recent field surveys revealed the presence of 500 years old coral boulders and marine deposits several kilometers inland. No tsunami or earthquake has ever been reported from that time and the hypothesis of a big storm cannot be excluded. In this study, we use forward numerical models of tsunami and boulder transport in order to discriminate the origin of those deposits. The model calculates the particle displacement generated by a flow at each time step, previously validated with experimental results. Laboratory experiments were conducted using a 10-m long and 0.4-m width channel, modeling the tsunami wave by a dam break and analyzing boulders trajectories. This transport model is used in the simulation of tsunami scenarii offshore Anegada island, providing a range of tsunami intensities necessary to trigger the displacement of the observed boulders and to reproduce their traveled distance. From the tsunami intensities and the knowledge of the tectonic and fault systems of the area, we can deduce a range of potential earthquakes sources and magnitudes.