Effects of the 28 February 1969 Cape Saint Vincent Earthquake on Ships

Abstract

In the interval between the El Asnam earthquakes of 1954 and 1980, both tsunamigenic and recorded on tide gauges of the western Mediterranean coasts, the earthquake and tsunami of 28 February 1969 took place. The epicentre was located in the Atlantic Ocean, southwest of Cape St. Vincent, and tsunami waves were recorded on tide gauges of the Atlantic coasts of Portugal, Spain and Morocco. This means there were three tsunami events over a 26-year period in the current NEAMTWS coverage area. There is no evidence of land sightings of tidal wave arrivals, except for two cases in Gijon (Spain) and near the mouth of the Bou Regreg River (Morocco), which will be discussed later. The behaviour of the sea in the coastal areas was apparently normal considering the presence of a storm that was moving from the southwest of the Iberian Peninsula, accompanied by very strong rainfall in Morocco, moderate to strong rainfall in Andalusia and southern Portugal, and weak rainfall or very cloudy skies in the rest of the area. However, on the high seas, both the earthquake and the tsunami effects were clearly felt by six ships of various tonnages at different distances and azimuths from the epicentre, even with the risk of shipwreck. The crews of these ships, which at the time of the earthquake were sailing across the Atlantic near the epicentral area, reported violent vibrations, very high waves and specific damages, which call into question the belief that seaquakes and tsunamis are not perceptible on deep waters, inviting us to analyse the risk posed by earthquakes with submarine epicentres for maritime navigation.