The 1755 Lisbon Tsunami: Tsunami Source Determination and its Validation

Abstract

The Lisbon Earthquake of November 1, 1755, one of the most catastrophic events that has ever occurred in Portugal, Spain, or Morocco, caused severe damage and many casualties. The tsunami generated by this earthquake is well documented in historical accounts, it was reported throughout the North Atlantic Ocean, as it reached not only Portugal, Spain, and Morocco, but also the Madeira and Azores Archipelagos, England, Ireland, and the Caribbean. In spite of the importance of this event, the source of the tsunami remains unknown. In this paper, the authors reevaluate some of the historical tsunami travel times obtained by previous authors. Based on these times, wave ray analysis is used to determine the location of the tsunami source area. These results, combined with turbidites obtained by previous authors at the Tagus and Horseshoe Abyssal Plains, lead to the conclusion that the source of the 1755 Lisbon Tsunami could be located in the area of the Gorringe Bank. Then, a hydrodynamic simulation is carried out with this area presupposed as the source. The numerical model results provide good agreement when compared with both historical and sedimental records. However, in the past, the Gorringe Bank has been dismissed as the source of this tsunami for several reasons. Therefore, these issues are discussed and discredited. As a consequence of all these facts, it can be concluded that the origin of the 1755 Lisbon Earthquake and Tsunami could be located in the area of the Gorringe Bank.

The Historical Review of the 1755 Lisbon Tsunami

“…Several scholars have been trying to compile and understand the physical characteristics of this tsunami, as discussed by [1]. The main obstacle has been the language, since the reports are written in different languages: In Portugal they are mostly written in Old Portuguese, contemporary from the 18th century, while in Spain the accounts are written mostly in Spanish and in Morocco in Arabic.…”

“…Therefore, in order to provide the full tsunami descriptions in all the affected areas by this historical event, [4] compiled and translated to English all the available data. Then, for the first time ever, significant developments in the comprehensive analyses of this historical tsunami were possible: By estimation a magnitude 8.7 earthquake, the most probable tsunami source area was pointed out at the Gorringe Bank [1]; has a consequence, and by the use of the tsunami numerical model, it was possible to conduct a tsunami hazard assessment in Portugal, by the combination between travel times and maximum water level [5]; the most probable time of the earthquake was calculated has 10:15-10:16 UTC [5]. Furthermore, by considering a scenario similar to the 1755 Lisbon Tsunami, the calculation of the inundated areas at the municipality scale in Portugal, and by using the tsunami numerical model, allowed a more detailed analysis of the impact of this king of natural disaster: In Figueira da Foz the tsunami would reach as far as 10 km upstream de Mondego River [6]; in Setubal the tsunami would inundate the urban area of the city as far as 400 m…”

Territorial Vulnerability Assessment Supporting Risk Managing Coastal Areas Due to Tsunami Impact

“…In addition, at Vila do Bispo the tsunami run-up was extreme, being the highest values with at the cliffs. As we move away from the source (eastern and northern Portuguese coastal zones), the tsunami travel times increase and tsunami run-up decreases, as confirmed by the tsunami numerical model [2,13]. (1) damage/fatalities due to earthquake, fire and tsunami.…”

“…Based on the tsunami numerical model carried out by [2,13] at the regional scale, the study uses the inundated areas calculated by [1] at Figueira da Foz, and the outputs model upgrades at Setubal and Vila do Bispo.…”

“…An analysis by vulnerability class shows that most buildings at this site have low vulnerability. The majority of buildings (13) are exclusively residential, one has a commercial function and the other is an outbuilding. Of the 13 buildings that are exclusively residential, eight exhibit low vulnerability and only two are highly vulnerable.…”