Sources of Tsunami Hazard

Introduction

In the section an account is given of the various sources of tsunamis for the North Atlantic Ocean. Firstly, an account is given of trans-Atlantic tsunamis, these being tsunamis of sufficient magnitude to travel from their point of generation on one side of the Atlantic to the other. Second, a description is given of the mid-Atlantic ridge as a possible tsunami source. Third, a discussion is presented on possible tsunami sources along the continental slope of NW Europe (Figure 1).

Trans-Atlantic Tsunami Sources

There is the possibility that a tsunami generated, for example off the eastern coast of Canada may result in a large tsunami striking the coastlines bordering the NE Atlantic. The evidence, however, is conflicting. For example it is known that a large earthquake and underwater slump took place near a submarine canyon off the Grand Banks of Newfoundland in November 1929. The earthquake produced a tsunami that locally reached altitudes of 30m above sea level along parts of the Newfoundland coastline. Although the tsunami waves also undoubtedly were propagated eastwards towards Europe, no tsunami of any significance was recorded as having struck the shores of Britain. By contrast, the destructive tsunami associated with the Great Lisbon earthquake of November 1755 AD produced a tsunami that was observed among the Caribbean Islands. Although the tsunami was locally up to 20m high along the coastlines of Portugal and Morocco, the flood waves that reached the Caribbean amounted to no more than a gentle swell.

In the case of the Second Storegga Slide of circa 7,100 years ago one might imagine that tsunami waves reached the coastline of eastern Canada yet stratigraphic investigations of lake sediments locally close to sea level in Newfoundland have not revealed any trace of the tsunami (MacPherson, pers. comm.). It is worthy of note that of the three tsunamis described above the only one known to have traversed the Atlantic Ocean (Lisbon 1755 AD) is the one generated by an offshore earthquake, the other two both having been principally due to underwater slumping and sliding. This difference may either represent an artefact arising from the general lack of observational and measurable data or it may hint at a difference in the way in which earthquake- and underwater landslide-generated tsunamis are generated and propagated.

The Mid-Atlantic Ridge as a Possible Tsunami Source

In theory it is possible that tsunamis may be generated as a result of volcanic activity and related land movements in the area of the mid-Atlantic Ridge (Figure 1). To all intents and purposes it is almost impossible to determine whether or not the mid-Atlantic Ridge has functioned as a source for tsunamis in the recent past. The reason for this difficulty lies in attributing particular tsunamis to specific source mechanisms. For example, in the absence of instrumental or documentary earthquake data, a tsunami striking western Shetland may originate from a wide variety of possible sources. It can be stated, however, that tsunamis arising from underwater landslides are unlikely to derive from the mid-Atlantic Ridge since the seabed sediment cover is extremely thin. In addition, it is perhaps worth commenting that geological activity along the mid-Atlantic Ridge is most commonly associated with the release of molten volcanic rock. Under such circumstances, if a major period of igneous activity took place at a location along the mid-Atlantic Ridge, the geological processes are most likely to take place at a sufficiently slow rate not to enable any tsunamis to be generated. Support for this statement is provided by the creation of new volcanic islands emerging from the sea during historical times south of Iceland (e.g. the island of Heimay, 1970). During the creation of this new volcanic island, no tsunami is known to have been generated.

Possible Tsunami Sources Along the Continental Slope of NW Europe

Inspection of Figure 1 shows very clearly that although the Storegga Slides represent the principal area of sea floor slumping along the continental slope margin of NW Europe, there are many other locations where geological surveys have demonstrated the presence of large numbers of small slump and slide deposits. For the most part these represent accumulations of sediments deposited during the Quaternary Ice Age. The features indicated on Figure 1 are those identified by Kenyon (1987). None of them however are of a magnitude similar to the Storegga Slides or to the newly-discovered slide north of the Faeroe Islands (Van Weering et al., 1998). To date, none of the ages of the slides and slumps are known. In order to accomplish this it is necessary to drill into the sediment accumulations and use geological methods to date the age of the sediments that directly rest upon the respective slide deposits. This has not been done and accordingly it is impossible to determine if any tsunamis were generated by these slumps.