10 Mar 08

Calculation of the sea wave (tsunami) caused by the AD 365 earthquake at 30 minutes after the earthquake. Red and blue shades correspond to peaks and troughs of about 50 cm.

Studying an ancient earthquake has enabled Oxford University researchers to quantify the likelihood of a tsunami in the Eastern Mediterranean.

They estimate that a ring of faults around the south of Greece and the Aegean Sea generates tsunami earthquakes approximately once every 800 years and, because the last such earthquake took place in 1303, the probability of a tsunami affecting the region is much higher than had been thought.  

The Oxford researchers – working with colleagues from the Universities of Cambridge, Nice and Imperial College London – identified the cause of an earthquake that generated a tsunami that destroyed Alexandria on 21 July AD 365. Reporting in Nature Geoscience, the group describe how they tracked down the origin of this ancient quake to a fault beneath western Crete. Very precise radiocarbon dates of uplifted shorelines show that western Crete was lifted by about ten metres within a few decades of AD 365, and the shape of the uplifted shorelines is diagnostic of distortion of the land surface by an earthquake.

The researchers then used GPS stations to take very precise measurements of how the Earth’s surface is being slowly compressed all around the southern Aegean Sea today. From these measurements they predict that the energy being built up will be released in tsunami-earthquakes somewhere along this fault approximately every 800 years.

Professor Philip England, Head of Oxford’s Department of Earth Sciences and a co-author of the paper, said: ‘The AD 365 event is important because it is the only earthquake in the Mediterranean where the evidence can be studied on land, rather than being hidden under the ocean. It was one of the most devastating events in the ancient world: destroying cities and drowning thousands of people in coastal regions from the Nile Delta to modern day Dubrovnik.’

Calculations suggest that, as it crossed the open ocean, the wave height of the AD 365 tsunami was similar to that of the 2004 Sumatra tsunami – around one metre high. This leads the researchers to believe that, when it hit the shore, this sea wave would have been highly destructive.

‘This is our first real stab at quantifying the risk of a tsunami event in the Eastern Mediterranean,’ said Professor England. ‘What we can say for sure is that if the AD 365 earthquake were to be repeated it would have a devastating impact on the Mediterranean region.’

This work was supported by the UK’s Natural Environment Research Council (NERC) and by Oxford University.