Forearc crustal faulting and estimated worst-case tsunami scenario in the upper plate of subduction zones. Case study of the Morne Piton Fault system (Lesser Antilles, Guadeloupe Archipelago)

Abstract

Abstract. In this study, alternatively to the megathrust, we identify upper plate normal faults orthogonal to the trench as a possible tsunami source along the Lesser Antilles subduction zone. We study the Morne Piton Fault system, a trench-perpendicular upper crustal fault affecting the Lesser Antilles forearc at the latitude of Guadeloupe. By the means of seismic reflection, high resolution bathymetry, Remotely Operated Vehicle images and dating, we reassess the slip rate of the Morne Piton Fault at 0.2 mm.yr-1 since fault inception (i.e. 7 Ma), dividing by five previous estimations and thus increasing the earthquake time recurrence and lowering the associated hazard. We evidence a metric scarp with striae at the toe of the Morne Piton Fault system suggesting a recent fault rupture. We estimate a fault rupture area of ~ 450–675 km2 and then a magnitude range for the seismic event around Mw 6.5 ± 0.5. We present results from a multi-segment tsunami model representative for the worst-case scenario which gives an overview of what could happen in terms of tsunami generation if the whole identified Morne Piton Fault segments ruptured together. Our model illustrates the potential impact of local tsunamis on the surrounding coastal area as well as local bathymetric controls on tsunami propagation as (i) shallow water plateaus act as secondary sources and are responsible for a wrapping of the tsunami waves around the island of Marie-Galante, (ii) canyons are focusing and enhancing the wave height in front of the most touristic and populated town of the island, (iii) a resonance phenomenon is observed within Les Saintes archipelago showing that the waves’ frequency content is able to perturbate the sea-level during many hours after the seismic rupture.