­­Tsunami hazard induced by an Early Holocene landslide in the Tagus delta off Lisbon (Portugal)

Submarine landslides are a major source of tsunamis, with waves highly dependent of the properties of both the landslide and the marine domain. This study investigates the tsunamigenesis of a shallow-water submarine landslide, recently mapped at the mouth of the Tagus delta River, off Lisbon, Portugal. The Tagus delta landslide is estimated to be 8 ky old, with a depositional maximum thickness of ~ 20 m and a volume of approximately 0.27 km ³ , extending over more than 9 km in length and 3 km in width. We assess the tsunamigenic potential and hazard of this possible early Holocene landslide using numerical simulations over high resolution reconstructed paleo-bathymetry, and project a similar future landslide scenario under the present-day conditions. Results show that both paleo- and future landslide scenarios are tsunamigenic, posing a significant tsunami hazard to the nearest coasts of Lisbon and surrounding areas. At the source region, the formed tsunami reached heights of 0.8 m (paleo-scenario) and 1.45 m (future-scenario), increasing to approximately 2 m (both scenarios) as it propagated towards Lisbon’s coastline. This study is a first attempt to address the tsunamigenic potential of a submarine landslide occurring in a shallow water environment of the Portuguese margin domain. The results reinforce the need to include these small, but frequent, events and their induced tsunamis in marine geo-hazard assessments of the region.