The 1755 Lisbon tsunami; evaluation of the tsunami parameters

Abstract

The tsunami generated by the 1755.01.11 earthquake affected mainly the coasts of the Iberian Peninsula and Northwest Morocco and was observed all over the North Atlantic coasts. The catastrophic dimensions of that phenomenon had a tremendous impact on the city of Lisbon and on several villages along the south coast of Portugal. The earthquake was felt all over Europe and the seismic intensity was estimated as X–XI (Mercalli Intensity Scale) at Lisbon and Southwest Portugal (Cape S. Vicente). The most destructive waves were observed along the coast of Portugal, specially in Lisbon, in the area of the S. Vicente Cape, along the Gulf of Cadiz and Northwest Morocco. Throughout historic times, earthquakes have periodically affected the city of Lisbon causing severe damage and casualties. In spite of that, the city kept growing, so the extension of damage and the loss of human lives in 1755, was quite impressive. The down town of Lisbon was flooded by the rising of the waters of the river Tagus and most historical documents reported waves of 6 m height. At Cape S. Vicente (Southwest Portugal) the run-up height, evaluated from historical data, is greater than 15 m. The eye witness accounts from Spain and Morocco reported wave heights greater than 10 m and large flooded areas along the Gulf of Cadiz and in several harbours in Morocco, e.g. Safi and Agadir. In the city of Lisbon, the number of casualties due exclusively to the tsunami, is estimate around 900, and the penetration of the waters is evaluated to be 250 m. Most of the available literature concerning the 1755 earthquake is based on the compilation of Pereira de Sousa (1919) and, sometimes, incorporates both well established historical records and non reliable information. As the 1755 event evaluation is crucial to a quantitative approach of the tsunami hazard and risk assessment in Portugal, a new examination of the historical records was needed before the establishment of reliable tsunami parameters that can be used both in numerical models of tsunami propagation and in geodynamic studies. In this paper, we present a new compilation of almost all the available historical data from the countries affected by the tsunami. In the analysis of these records, the following tsunami parameters are inferred: travel time, polarity of the first movement, maximum run-up height, period, number of waves, duration of the sea disturbance and extent of flooding.