for National Geographic News
In A.D. 551, a massive earthquake spawned huge tsunamis that devastated the coast of Phoenicia, now Lebanon.
Now a new underwater survey has finally uncovered the fault likely responsible for the catastrophe and shown that it rumbles approximately every 1,500 years—which means a disaster is due any day now.
"It is just a matter of time before a destructive tsunami hits this region again," said Iain Stewart, an earthquake expert at the University of Plymouth in the United Kingdom not involved in the underwater survey.
The ample archaeological and historical evidence from the A.D. 551 earthquake indicate that it was truly a catastrophic event. The resulting tsunami damaged all major coastal cities between Tripoli and Tyr, and Tripoli was reported to have "drowned." (See a Lebanon map.)
Hitting the Jackpot
Earthquakes are common in Lebanon, but many of the faults remain hidden beneath the deep waters of the Mediterranean Sea.
Surveying the region is difficult because some of the continental shelf drops off very quickly in places, reaching water depths of around 4,921 feet (1,500 meters) only five miles (eight kilometers) from the shore.
Ata Elias of the National Center for Geophysical Research in Beirut, Lebanon, and his colleagues had a hunch that the fault responsible for the A.D. 551 earthquake would lie in this offshore region. So they did an underwater geophysical survey—and "hit the jackpot," Elias said.
By bouncing radio waves off the sea floor and studying the reflection patterns, Elias and his team were able to build a three-dimensional map showing all the lumps and bumps on the ocean bottom.
Running parallel to the Middle Eastern coast, they discovered a distinctive stepped ridge—the shape made by a "thrust" fault when one of Earth's tectonic plates shoves its way beneath another.
"We inferred that this thrust fault is the source of major earthquakes," Elias said.
The team was able to trace this fault along the coast for more than 62 miles (100 kilometers).
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