The findings are published in the August issue of the journal Geology.
Back on land the team found additional evidence to link this fault to the A.D. 551 earthquake. A "staircase" of platforms rising from present-day sea level shows how the land had moved upward each time the thrust fault moved.
Each time the thrust fault ruptured it lifted the coastline by around three feet (a meter), Elias said.
When the platforms were at sea level they were colonized by mollusks. But as soon as they were thrust out of the water by an earthquake the mollusks died.
By dating the mollusk shells on the raised platforms, Elias' team could determine when the thrust fault moved.
At least four earthquakes similar to the A.D. 551 quake have occurred over the past 6,000 to 7,000 years, the team found—suggesting a 1,500- to 1,750-year recurrence time for destructive quakes.
From the length of the thrust fault and the amount of uplift of the platforms on land, Elias and his colleagues estimate that the A.D. 551 earthquake must have had a magnitude of about 7.5 on the Moment magnitude scale, a more modern form of measurement than the Richter scale. (What is an earthquake?)
When the fault ruptured in A.D. 551, part of the the seafloor collapsed by around 5 to 10 feet (1.5 to 3 meters). This drop triggered a surging tsunami, which gained height rapidly as it pushed toward land.
Stewart of the University of Plymouth said the study is interesting, but remains cautious about blaming the newfound thrust fault for the A.D. 551 event.
"The Mediterranean has a lot of big earthquakes and there are lots of benches [platforms] everywhere. It is hard to link those benches to a particular fault," he said. (Related: "Ancient Tsunami Smashed Europe, Middle East, Study Says" [December 4, 2006].)
Nonetheless, he believes that the risk of another big earthquake occurring is very high, and should be taken seriously.
"In the past this area has had a lot of big earthquakes and tsunamis, but in modern history it has been quite quiet," Stewart said.
"We have been lulled into a false sense of security, just like we were in the [2004 Indian Ocean earthquake]."
Some of the many historical records from the time of the A.D. 551 earthquake describe the complete ruin of Berytus (Beirut), Jewel of Phoenicia, and the sea retreating one to two Roman miles, or 4,921 to 9,842 feet (1,500 to 3,000 meters) from shore, enough to ground mooring ships and uncover sunken ones.
More than 30,000 people died in Beirut alone. (Who were the Phoenicians?)
"If this earthquake and tsunami were repeated today, it would be a disaster of enormous proportions," said Sanford Holst, an author and expert on ancient Phoenicia.
More than 70 percent of Lebanon's roughly 4 million people live along the coast. The seaport of Beirut has a population of 1.5 million.
What's more, much of the country's infrastructure is also located along the coast. Major highways, electrical power stations, airports, and economic centers are all next to the sea, Elias said.
To prepare for the next big quake, many of the tall buildings that line the coast need to be reinforced to withstand earthquakes. New buildings need to be built with large earthquakes in mind. And people need to be informed.
"We need an earthquake and tsunami alert system and proper emergency plans," Elias said.
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