Underwater Volcanoes and Earthquakes
	Volcanoes Certain parts of the oceans are subject to volcanic activity and where these are known they are shown on charts and mentioned in Sailing Directions so that ships may avoid them. An example of an underwater volcano in intermittent eruption was that observed by the Japanese weather ship Chikubu Matu in September 1952, near the Nanpo Shoto chain of islands, in about 31° 55' N, 140° 00' E. A strong smell of sulphur was noticed, and a column of white smoke was seen to be rising out of the sea. The column of smoke became mixed with steam, but was then suddenly darkened by black smoke accompanied by flames from a violent explosion, and sprang to a height of 5000 m. Almost simultaneously the sea below the column rose bodily in the form of a dome about 800 m in diameter. Volcanic ash soon began to fall from the great column of smoke. Three days later a small Japanese survey vessel, sent to investigate, was lost with all hands when an even more violent eruption occurred. It is estimated that another dome of water was thrown up, rising about 10 m above the surrounding sea and nearly 2.5 miles in diameter. After another two days, the volcano again erupted but less violently. Earthquakes In some parts of the oceans earthquakes sometimes occur. When one occurs in the vicinity of a vessel, the signs which can be expected depend on the violence of the earthquake, the distance of the ship from the epicentre and the depth of water she is in. For example, in February 1969 an underwater earthquake occurred with its epicentre about 115 miles WSW of Cabo de Sao Vicente, Portugal. Ships in the vicinity at the time felt the shock with different degrees of intensity. One ship, about 100 miles NE of the epicentre and in a depth of 450 m was set into violent vibrations for about one minute, while another about the same distance NW of the epicentre and in a depth of 3650 m felt a severe vertical shock, as if the vessel was lifting out of the water: neither of these ships suffered damage. The motor tanker Ida Knudsen (32,000 grt), however, which was within 15 miles of the epicentre, was lifted bodily upwards, slammed violently back, and experienced very heavy vibrations: the damage was such that she was condemned as a total loss.