Source identification of Middle Age extreme event deposits in the Lesser Antilles using forward numerical modeling of tsunami

The arc of the Lesser Antilles is one of the most quiet subduction zone in the world. In this region, the convergence of the Atlantic and the Caribbean plates is low (few mm/year) and most of the seismicity is an intraplate and crustal seismicity. Among the Mw>7 earthquakes recorded in the historical catalog (1690 near Barbuda, 1843 near Guadeloupe, 1867 near the Virgin Islands, 1839 offshore Martinica, 1969 offshore Dominica, 1974 near Antigua), only the 1839 and 1843 events are suspected to be interplate earthquakes. The 1867 Virgin Island earthquake generated an important tsunami with waves of 10m that devastated the closest islands. A tsunami followed the 1843 earthquake but without much damage. These two events are the only known damaging tsunami in this region, but another older one might be added to the list. Indeed, an increasing number of tsunami deposits have been identified in the recent years on several islands of the arc, all of them being around 500 years old (~1450 AD). These deposits are all located in the northern segment of the arc, between Antigua and Puerto-Rico, in Anegada, St-Thomas (Virgin Islands), Anguilla and Scrub islands. There is unfortunately no record and no testimonies of an extreme event at that time.

The northern segment of the arc is particularly complex because located at the transition between the Greater Antilles and the Lesser Antilles. It is crossed by the Anegada Passage, a series of faults and basins cutting through the arc, which defines the limit between the Puerto-Rico micro-plate and the Caribbean plate. This passage and the numerous intra-arc fault systems present between the islands are active and likely compensate for the plates motion. The very low slip deficit detected with GPS measurements at the subduction contacts of Puerto-Rico and the Lesser Antilles indicates that the interface from Guadeloupe to Puerto-Rico can be considered as totally uncoupled or holding the characteristics of a very long seismic cycle. A tsunami generated by an extreme event 500 years ago in this region could be related to intra-arc, outer-rise, intraplate or interface fault rupture. The identification of the source would enable a better understanding of the seismic cycle and the dynamic of this part of the arc.

This study lists and set models of all the potential faults that could trigger an earthquake in the area encompassing the three islands : Anguilla, Anegada and StThomas. We have created high-resolution bathymetric grids and performed tsunami simulations for each fault model. We uses run-up models to compare the simulated wave heights and run-up distance to all the deposits heights and positions. The magnitudes of our fault models range between 7 and 9, but very few of them generate a strong enough tsunami to match the observed deposits.