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Ancient Mediterranean Tsunami May Strike Again

Mason Inman
for National Geographic News
March 10, 2008
 
Tsunamis like the the one that devastated ancient Alexandria in A.D. 365 may hit the Mediterranean relatively often, a new study argues.

Scientists say they have pinpointed the geological fault—off the coast of the Greek island of Crete—that likely slipped during a huge quake and caused the ancient tsunami.

Massive earthquakes—greater than magnitude 8—may strike the Mediterranean roughly every 800 years, the research suggests.

But other scientists say that not enough is known about these faults to predict how often such quakes might strike. They argue that the 365 disaster may have been unique.

The authors of the new study measured the remains of corals, algae, and other sea life that run in a band along the coast of Crete.

"The ancient shoreline resembles a bathtub ring high above the sea on the cliff face," said Beth Shaw of the University of Cambridge in England, lead author of the study.

The findings, published in Nature Geoscience, fix a date to when western Crete was suddenly lifted up, strengthening the tie between this event and the tsunami (see a map of Crete).

The team narrowed down the date of the uplift to within a few decades of 365, bolstering the idea that the upward shift happened in one sudden jolt.

"Ten meters [33 feet] of uplift is quite astonishing," Shaw said. "Unexpectedly, our results confirm that all [of the] uplift did happen in the 365 A.D. earthquake."

(See photos of an island that was lifted by an earthquake in 2007.)

Ancient Devastation

For at least 150 years, scientists have recognized that something unusual happened along western Crete.

When Capt. Thomas Pratt mapped the Aegean Sea in the 1850s, he noticed an ancient Roman harbor on the island sitting high and dry, 20 feet (6 meters) above the sea.

Later research linked this uplift to the 365 earthquake and tsunami.

Archaeologists found skeletons in collapsed buildings with coins dating to this period. Other signs of destruction were found from this time in far-flung areas, from Cyprus to Libya (see a map of the Mediterranean).

Mollusk shells from the former shoreline of Crete had also been dated to roughly the same time.

The ancient earthquake and tsunami made a strong impression on people of the time, records show.

The historian Ammianus Marcellinus documented the devastating effects of the event in Alexandria, Egypt.

"The solidity of the whole earth was made to shake and shudder, and the sea was driven away," he wrote. "The mass of waters returning when least expected killed many thousands by drowning.

"Huge ships … perched on the roofs of houses …and others were hurled nearly two miles [3.2 kilometers] from the shore."

Separate accounts tell of earthquakes and tsunamis hitting other cities around the Mediterranean at roughly the same time.

New Fault Discovered

Shaw and colleagues used a computer model to estimate the size of tsunami waves that would have sloshed through the Mediterranean Sea in ancient times.

Their results show that some coasts would have been hit harder than others, and the findings matched the historical and archaeological evidence of damage around the region.

The team used measurements of the Crete shoreline to determine which fault caused the lifting.

They traced the earthquake to a previously unrecognized fault along an underwater rift called the Hellenic Trench.

The newfound fault is near a larger fault known to run along the region, where two continental plates run into each other (see a map of Earth's shifting plates).

"One fault is lubricated, slipping quietly without earthquakes," Shaw said. "The other slips infrequently in large earthquakes, which can cause tsunamis."

Using global positioning system (GPS) measurements of the movement of the continental plates, the team estimated how much energy builds up in the "sticky" fault and estimated it could slip and cause a massive earthquake about once every 800 years.

A Unique Disaster?

Roger Bilham is a geophysicist at the University of Colorado in Boulder who was not involved in the new study.

He said that the study presents "bad news"—namely, that any of a handful of faults in the area "could slip in megaquakes at any time.

"That the Mediterranean, with its growing coastal population in excess of 130 million … could host a large tsunami at any moment is cause for considerable unease," he added.

(Read related story: "Major Quake, Tsunami Likely in Middle East, Study Finds" [July 26, 2007].)

Paolo Pirazzoli of the National Center for Scientific Research in Meudon, France, led the first study linking the uplift in Crete to the A.D. 365 earthquake.

He is unconvinced by the new estimate of earthquake frequency in the region, arguing that the ancient quake is "enigmatically unique."

He points to ancient shorelines higher up the cliffs in Crete that show the island had lifted little over the past 125,000 years, except for during the 365 event.

For their part, Shaw and colleagues disagree about the age of these earlier shorelines. Their measurements put them at about 40,000 years old.

Also, Shaw added, "Earthquakes tend to repeat along the same fault or set of faults. So it would be highly unusual to have one unique event."

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