Probably the most destructive tsunami in Europe during historical times occured on 1 November 1755. An earthquake took place offshore circa 200 kilometres WSW of Cape St Vincent, on the Gorringe Bank on the sea floor west of Portugal and attained a magnitude estimated at 8.5Ms. The epicentre of the earthquake was in an area along the Azores-Gibraltar plate boundary that forms the western part of the lithosphere boundary between the Eurasian and African plates (Moreira, 1985). The eastern section of the Azores-Gibraltar plate boundary (which includes the Gorringe Bank) is an zone of active plate compression and in this area faults tend to have a large source component that results in high-magnitude and deep-seated tsunamigenic earthquakes.

The considerable destruction that took place in Lisbon, in addition to widespread fires, was mostly attributable to three tsunami waves estimated to be between 5 and 13 metres high that took killed 60,000 people in Portugal alone. There are also numerous reports of tsunami flooding and fatalities on a large scale along the Algarve coast and on the coastline of Morocco (Andrade, 1992).

In England, contemporary observations by Borlase (1755; 1758) describe the arrival of the tsunami in Mounts Bay, Cornwall. Borlase noted '... the first and second refluxes were not so violent as the 3rd and 4th (tsunami waves) at which time the sea was as rapid as that of a mill-stream descending to an undershot wheel and the rebounds of the sea continued in their full-fury for fully 2 hours... alternatively rising and falling, each retreat and advance nearly of the space of 10 minutes until five and a half hours after it began'. Reconstructed tidal changes for this day for the Isles of Scilly show that the time of high tide coincided approximately with the arrival of the first tsunami wave (Foster et al., 1991) some five hours after the first shocks were reported on the Portuguese coast. There are no known reports of the progress of the tsunami northeast along the Channel but the coastal flooding effects must have been considerable.

There is some evidence to indicate that the 1755 Lisbon tsunami was not solely caused by a sea bed fault. Recently a large submarine slide complex has been identified on the seafloor adjacent to the Gorringe Bank and tentatively dated to 1755. This discovery raises the possibility that the tsunami was partly generated by an earthquake-triggered fault on the seabed and partly by submarine sediment slumping.