Last month's tsunami tragedy, shocking as it was, had ample historical precedent. On 1 November 1755, for example, a fire following an earthquake destroyed two-thirds of Lisbon, Portugal. In panic, the population sought shelter near the shoreline, only to be hit by waves said to be as high as houses. More than 60,000 people died.

Devastating tsunamis are known in historical times to have affected the populated coasts of Papua New Guinea, Japan, Hawaii, Crete, Sicily and the Crimea — to name just a few. In the Pacific region, where 80% of all tsunamis occur, a 1947 analysis indicated that seismic sea waves higher than 7.5 metres occur on average every 15 years1. Records going back to 684 BC refer to four Pacific tsunamis higher than 30 metres.

Outside the Pacific, tsunami frequencies have been studied in some detail only for the Aegean and Black Sea regions. Records there reveal that the coastal and surrounding areas of Turkey have been affected by more than 90 tsunamis over the past 3,000 years2.

For most other areas, information concerning the return periods of tsunamis is scarce. A rough comparison of tsunami frequencies in different parts of the globe was done in 2000 by the London-based Benfield Hazard Research Centre, as part of its Tsunami Risks Project. The resulting risk analysis estimates the return periods of 10-metre waves to be about 1,000 years for the North Atlantic and Indian oceans, southern Japan and the Caribbean, 500 years for the Philippines and the Mediterranean Sea, 250 years for Alaska, South America and Kamchatka in eastern Siberia, and less than 200 years for Hawaii and the southwest Pacific.

The south Asian disaster will have a “huge effect” on instigating more thorough risk assessments, predicts Bill McGuire, a volcanologist and director of the London research centre, as well as encouraging preventive measures in threatened regions.

→ http://www.nerc-bas.ac.uk/tsunami-risks