Thursday, 23 June 2016

A tsunami in Belgium?

In the wake of a series of devastating tsunamis in the Indian Ocean, Chile and Japan, Cecile Baeteman was asked whether such an event could also happen in the North Sea. Prof. Baeteman is an expert in sea-level change and coastal processes at the Royal Belgian Institute of Natural Sciences. Here are her thoughts on whether tsunamis pose a threat to coasts of the UK, France and, in particular, Belgium:

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...

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 in several places (according to Bondevik et al., 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 under 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 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. 

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.

A quick final word from me (Ed Garrett): Many have previously assumed that shallow water depths would reduce the energy and height of tsunami waves to the point that they would no longer be damaging by the time they hit the populated coastlines of the southern North Sea. However, recently published evidence from Denmark suggests that this is not the case. Fruergaard et al. (2015) identified a layer of marine sand in a former freshwater lake in southwestern Denmark that can be linked to the Storegga tsunami, suggesting that little if any of the coastline of the North Sea is immune from the threat of future tsunamis. 

This article was originally published on the website of the Royal Belgian Institute of Natural Sciences. 

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