A tsunami could strike again
2008-03-10 14:01
Paris - "The sea was driven back, and its waters flowed away to such an extent that the deep sea bed was laid bare and many kinds of sea creatures could be seen," wrote Roman historian Ammianus Marcellus, awed at a tsunami that struck the then-thriving port of Alexandria in 365 AD.
"Huge masses of water flowed back when least expected, and now overwhelmed and killed many thousands of people... Some great ships were hurled by the fury of the waves onto the rooftops, and others were thrown up to 3km from the shore."
Ancient documents show the great waves of July 21 365 AD claimed lives from Greece, Sicily and Alexandria in Egypt to modern-day Dubrovnik in the Adriatic.
The tsunami was generated by a massive quake that occurred under the western tip of the Greek island of Crete, experts believe.
Titanic quakes
Until now, the main thinking has been that this quake - as in the Indian Ocean tsunami of December 26 2004 - occurred in a so-called subduction zone.
A subduction zone is where two of the Earth's plates meet. One plate rides over another plate which is gliding downward at an angle into the planet's mantle.
But as the rock becomes brittle and deformed at greater depths, these zones can also deliver titanic quakes, displacing so much land that, when the slippage occurs on the ocean floor, a killer wave is generated.
The 365 AD quake occurred at a point on the 500km long Hellenic subduction zone, which snakes along the Mediterranean floor in a semi-circle from southwestern Turkey to western Greece.
University of Cambridge professor Beth Shaw carried out a computer simulation of the quake, based especially on fieldwork in Crete where the push forced up land by as much as 10m.
Tsunami-generating quake could strike
They estimate the quake to have been 8.3-8.5 magnitude and that its land displacement - of 20m on average - puts it in the same category as the 9.3 temblor that occurred off Sumatra in 2004.
They conclude the slippage occurred along 100km on a previously unidentified fault that lies close to the surface, just above the subduction zone.
The quake happened at a depth of around 45km - around 30km closer to the surface than would have been likely if the slip had occurred on the subduction fault itself.
After the 365 AD quake, the fault is likely to remain quiet for around 5 000 years.
But if the tectonic structure along the rest of the Hellenic subduction zone is similar, a tsunami-generating quake could strike the eastern Mediterranean in roughly 800 years, the scientists estimate.
"Repetition of such an event would have catastrophic consequences for today's densely populated Mediterranean coastal regions."