Tsunamis – so-called ‘tidal waves’ generated by major submarine earthquakes, landslides, or volcanic eruptions – are headline-grabbers. The major tsunamis of Boxing Day 2004 off Sumatra and March 2011 in Japan between them killed hundreds of thousands of people. But, tsunamis have occurred through the lifetime of our planet, since long before humans had the means to record them, and research into historic events could give us clues towards dealing with those which may occur in the future.
Prehistoric Tsunamis: the Evidence
There is a rich oral tradition which tells of huge waves coming in from the sea. Such events are widely-known around the Pacific (research quoted by the Oregon Department of Geology and Mineral Industries cites evidence from Japan and in the traditional tales of the Native Americans) as well as in classical literature. But not all tsunami events are recorded – and it isn’t possible to gain much solid information from those which are.
So, how do geologists find out about major historic and prehistoric tsunami events? Like earthquakes, tsunamis leave imprints on the landscape. Although the popular image is of a series of huge waves arriving and then receding, those transient waves leave lasting marks in the geological record. By drilling cores into the soil, scientists can identify typical tsunami event profiles from the patterns they leave behind.
A typical layer in a sediment core which represents a tsunami event would include the sudden termination of a long period of sedimentation (characterised by an uneven surface) followed by a rapidly-deposited layer of materials, including larger clasts (stones or rocks or pebbles) which have typically been transported from some distance away.
Using such drilling techniques, teams of scientists are able to identify (and date) the occurrence of prehistoric tsunamis. A study led by Klaus Reicherter, of Aachen University, for example, identified deposits from at least three tsunami events in the North Aegean Basin during the Holocene period (i.e. in the 10,000 years since the last Ice Age).
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