Earthquakes liquefy cities

Three Egyptian cities met near-biblical fates, as the ground beneath their buildings turned liquid and swallowed them, writes Steve Farrar.

A team of archaeologists and scientists have pieced together the grisly fate of the cities after under-water investigations of the ruins.

They believe the shock of earthquakes in the first millennium AD liquefied clay under the foundations of heavy structures, sending them up to 5m downwards.

The results of the investigation are to be announced at the American Geophysical Union meeting on Sunday.

The cities - Menouthis, Herakleion and the eastern suburb of Canopus - were found beneath the Mediterranean sea in the Bay of Aboukir, some 25km east of Alexandria. Remains included temples, homes and bridges, all now up to 5m below the surface.

Franck Goddio, of the European Institute of Submarine Archaeology in Paris, said that the archaeologists were puzzled as to why ruins in the city of Canopus usually appeared only within hollows in the seabed.

"Every time we found a column or granite block, it was in a sort of small basin," he said.

While the whole area had clearly fallen victim to gradual subsidence and rising sea levels, it appeared that the effect was far worse in some spots than in others.

The team's scientists discovered the geomagnetic signatures of faults directly below the ruins and concluded that the cities had been built on top of soft, organic-rich mud.

In normal circumstances, the clay could support the weight of the buildings. However, when the area was violently shaken by an earthquake, the normally chaotic clay particles - which resemble microscopic disks - would have lined up with each other and the large water content between them could have oozed out.

The result was that the ground on which the Egyptian's built ports and cities simply liquefied beneath them, probably in about AD 741-742.

"It would have been very sudden, the buildings collapsing several metres in a few seconds," said Goddio.

"The fact that there was weight on a particular spot could have been the trigger that caused the liquefication."