A damaging seaquake

Abstract

The term seaquake is used to mean shaking caused exclusively by an earthquake but felt on board a vessel at sea, excluding effects from tsunamis. Thousands of such occurrences are known which are occasionally listed in the casualty reports of Lloyd's List and in other sources, causing considerable concern to mariners but rarely serious structural damage to seaworthy vessels. Seaquakes, because the sensation they often create is one of the ship running aground, have been responsible for some of the fictitious reefs and shoals shown in early navigation charts. This research note brings to attention a little known case of a damaging seaquake which is of interest for the study of the vulnerability of offshore engineering structures and marine vessels, particularly submarines, as well as containers for the disposal of nuclear waste, and also provides information on the earthquake source responsible for the seaquake, that with the data available cannot be obtained by normal seismological methods. On the 28th February 1969 the motor tanker 'Ida Knudsen', a 32,000-tonne vessel built in 1958, was sailing in ballast from Lisbon to the Persian Gulf when it experienced a 'violent vertical shock'. This happened at about 02 h 45 min (GMT) when the ship was at a position 36-12N-10.70W in 2,700 fathoms of water. At the time the general state of the sea in this part of the Atlantic was 3 to 4 with moderate swell, and the windforce was 4 to 5.