THE 1755 LISBON TSUNAMI - EVALUATION OF THE TSUNAMI PARAMETERS

The tsunami generated by the 1755.01.11 earthquake affected mainly the coasts of the Iberian Peninsula and Northwest Morocco and was observe d all over the North Atlantic coasts. The catastrophic dimensions of t hat phenomenon had a tremendous impact on the city of Lisbon and on se veral villages along the south coast of Portugal. The earthquake was f elt all over Europe and the seismic intensity was estimated as X-XI (M ercalli Intensity Scale) at Lisbon and Southwest Portugal (Cape S. Vic ente). The most destructive waves were observed along the coast of Por tugal, specially in Lisbon, in the area of the S. Vicente Cape, along the Gulf of Cadiz and Northwest Morocco. Throughout historic times, ea rthquakes have periodically affected the city of Lisbon causing severe damage and casualties. In spite of that, the city kept growing, so th e extension of damage and the loss of human lives in 1755, was quite i mpressive. The down town of Lisbon was hooded by the rising of the wat ers of the river Tagus and most historical documents reported waves of 6 m height.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 witnes s accounts from Spain and Morocco reported wave heights greater than 1 0 m and large flooded areas along the Gulf of Cadiz and in several har bours in Morocco; e.g. Safi and Agadir. In the city of Lisbon, the num ber of casualties due exclusively to the tsunami, is estimate around 9 00, and the penetration of the waters is evaluated to be 250 m. Most o f the available literature concerning the 1755 earthquake is based on the compilation of Pereira de Sousa (1919) and, sometimes, incorporate s both well established historical records and non reliable informatio n. As the 1755 event evaluation is crucial to a quantitative approach of the tsunami hazard and risk assessment in Portugal, a new examinati on of the historical records was needed before the establishment of re liable tsunami parameters that can be used both in numerical models of tsunami propagation and in geodynamic studies. In this paper, we pres ent a new compilation of almost all the available historical data from the countries affected by the tsunami. In the analysis of these recor ds, the following tsunami parameters are inferred: travel time, polari ty of the first movement, maximum run-up height, period, number of wav es, duration of the sea disturbance and extent of flooding. (C) 1997 E lsevier Science Ltd.