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A tsunami in Belgium?

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Tsunami was a word rarely heard before the powerful seaquake off southeastern Asia and the tsunami in Sumatra. But the question 'Can this also happen in the North Sea?' is on many lips since.

History has proven that this is possible indeed. The North Sea and the English Channel are not as quiet as one might think, even if their seismic activities are not so intense or frequent as in the classic risk areas, such as Japan, Chile or California.

This knowledge of the past is not only confirmed by historical sources, but also by geological data covering the fairly recent period of 'only' 8000 years, during which the North Sea took its present day appearance. A unexpected layer of sand is found along the complete eastern coast of Scotland as far as northern England. Scientific evidence confirmed that this layer was deposited by a tsunami triggered by an enormous submarine landslide. This happened 7900 years ago along the continental slope of the Norwegian Sea, situated halfway to Norway in what is known today as the Storegga slide area. The sea level used to be 14 metres lower then. The slide speed and the displaced volume were such that they produced a megatsunami. Its consequences are not only noticeable in Scotland, but also along the coasts of Iceland, Norway, the Faroe Islands and Shetland. There is evidence that the tsunami reached a height of 25 metres in Shetland. This gigantic tidal wave is known as the Storegga Tsunami. No traces of it have been found in the Belgian coastal region (yet). The continental slope of the Norwegian Sea is still unstable...

Storegga slide in the Norwegian Sea

The Storegga slide that caused a tsunami about 8,000 years ago. Red circles indicate where tsunami deposits have been located; the minimum height of the tsunami is indicated on several places (according to Bondevik, 1997).

The southern North Sea and the English Channel do not have a continental slope, but earthquakes do occur. On 21 May 1392 an earthquake hit mainly Kent and Flanders; its epicentre was in the southern North Sea. There was no mention of a tsunami. More recently, and thus better documented, was the earthquake of 6 April 1580. It measured between 5.3 and 5.9 on the Richter scale and had its epicentre 30 to 25 km deep in the English Channel. A tsunami inundated Calais and caused floods as far as Boulogne. The next day, a second tsunami struck Dover and seemed to have reached the Mont Saint Michel. A sudden sea swell arose in the Channel, sinking 20 to 30 vessels. A survivor reported that the waves rose more than 15 m high. The 1931 Dogger Bank earthquake, in the southern North Sea, had a magnitude of 6.1 and caused a tsunami that especially hit Britain. Excavations for a sea inlet in the dunes (which must allow the sea water to pour into the land) exposed a layer of nothing but shells. Although further research has to be carried out, it is almost sure that this pile of shells is not the result of a storm, but more likely that of a tsunami.

Seismic activities in our regions are closely monitored. The impact of a possible tsunami on the densely populated coastal areas along the southern North Sea is unknown. This short inventory is meant as a plea for more consciousness-raising, and to emphasise the role of geology in this matter.

Picture of a wave

© Cecile Baeteman

Musson R.M.W. 1994. A catalogue of British Earthquakes. British Geological Survey.
Smith D.E. 2005. Tsunami: a research perspective. Geology Today.
Smith D.E. et al. 2004. The Holocene Storegga tsunami in the United Kingdom. Quaternary Science Reviews 23.

 
Last modified : July 26, 2013