Professor David Smith, University of Oxford
Until relatively recently, tsunami were unheard of in Shetland. However, discovery of the Holocene Storegga Slide tsunami in 1988 changed everything and since that time tsunami activity around Shetland’s coasts has been actively considered. Since the Fourth Biennial Flood Report, detailed studies of the anatomy of the Storegga tsunami by groups from Norway and the United Kingdom have provided detailed evidence of the event. It is now recognised to have taken place in the autumn of a year around 8000 BP; inundated the heads of firths up to a minimum of 25m above the High Water Mark of the time (an inundation higher than anywhere else in the United Kingdom); and refracted around islands, sometimes reaching higher levels on coasts in the lee of the approaching waves. In Shetland, evidence for this event is widely displayed along peat cliffs and low-lying valleys, demonstrating that tsunami can reach into many areas even of a crenulated coastline and strike with considerable force. The greatest force of the tsunami was probably experienced on exposed headlands, but the greatest inundation was in inlets, where today coastal populations are concentrated.
It has sometimes been suggested that the Storegga event was a one-off, unlikely to happen again. However, recent studies are beginning to indicate that tsunami may have been more frequent visitors to the shores of Shetland and for that matter the coastlines of NW Europe as a whole. Since the mid 1970s, an increasing number of seismic surveys of the continental margins of the N Atlantic, and especially around NW Europe, have been undertaken. They have disclosed a large number of submarine slides like Storegga. Although dates for these events as yet are few, the slides appear to have been developed in glacial sediments, and the form of the slides shows that, whilst many occurred shortly after the ice sheets melted, many have also occurred in more recent times. Along the continental margin north and south of Shetland there are a number of slides, including along the Faroe-Shetland channel, where detailed studies undertaken by the British Geological Survey have disclosed repeated re-mobilisation of the slide sediments. Although the generation of a tsunami by submarine slides depends upon a number of coincidences, including a sufficiently rapid movement and of course a sufficiently large amount of sediment, these circumstances can occur. In addition, the slide does not have to be as large as the Holocene Storegga Slide. In 1946, the Unamak slide offshore southern Alaska, which covered an area on the sea bed of about 1200 km2, less than a twentieth that of the Storegga event, generated waves 35m high – higher than those yet suggested for Storegga. Along Shetland’s coasts there are several locations where sand horizons similar to those deposited during the Storegga event, occur, notably at Basta Voe, on Yell, indicating that tsunami, possibly generated by the movement of one of these slides, may well have occurred more recently than the Holocene Storegga Slide tsunami.
Whilst we should not over-emphasise the tsunami threat, it is worth remembering that even relatively small tsunami waves are far more likely to inundate coastal areas than storm waves because they involve landwards translation of the whole water column, rather than the surface of the water. Plainly, their effect will be exacerbated if they strike at high tide.