Identification of tsunami-induced deposits using numerical modeling and rock magnetism techniques: A study case of the 1755 Lisbon tsunami in Algarve, Portugal

Storm- and tsunami-deposits are generated by similar depositional mechanisms making their discrimination hard to establish using classic sedimentologic methods. A promising approach is to use rock magnetism techniques to search for new physical benchmarks of tsunami deposits and to integrate them into a multi-disciplinary study. To test our method, we investigate the tsunami deposit of the Boca do Rio estuary generated by the 1755 earthquake in Lisbon which is well described in the literature. We use concentration (MS, SIRM) and grain size (χarm, ARM, B 1/2 , ARM/SIRM) sensitive magnetic proxies coupled with SEM microscopy to unravel the magnetic mineralogy and the depositional mechanism of the tsunami deposit. In order to study the connection between the tsunami deposit and the different sedimentologic units present in the estuary, magnetic data were processed by multivariate statistical analyses. Finally, we test the 1755 tsunami scenario using a numerical inundation model. Our results show that the tsunami deposit is constituted by a mixture of sands, mostly issued from the abrasion of the littoral dune, and reworked materials. Detrital titanomagnetite and, in lesser content, detrital titanohematite are the principal magnetic carriers. Multivariate statistical analyses indicate that the magnetic mineralogy of the tsunami deposit correspond to a mixture of material issue from a mixture of the underlying layers. Our numerical simulation shows a large inundation of the estuary with flow depths varying from 0.5 to 6 m and runup of ~7 m. In conclusion, all data suggest that most of the material constituting the tsunami deposit come from the erosion of the littoral sandy dune and from the reworking of the sedimentological units previously deposited in the estuary. We also propose that our multi-disciplinary approach is a suitable method to identify paleotsunami in beach embayment environments. Keywords: tsunami deposit, beach embayment, rock magnetism, numerical modelling.