Huge Quake Lurks Off Lebanese Coast

Aug. 9, 2007 — The source of one of history's most catastrophic events, the tsunami-generating 551 A.D. Beirut-Tripoli earthquake, lies dangerously just four miles off Lebanon's coast, according to a new underwater survey by an international team of geophysicists.

Responsible for the build-up of the Mount Lebanon range that towers around 10,000 feet above sea level, the previously unknown submarine fault moves roughly every 1,500 years. That suggests a disaster similar to the earthquake and tsunami that on July 9, 551, destroyed most of the coastal cities of Phoenicia (modern-day Lebanon), could be due any day.

Historical accounts indicate that the ancient event was a true cataclysm, with the sea retreating up to 10,000 feet. Tripoli was reported to have "drowned," while Beirut took almost 1,300 years to recover.

"It was arguably one of the most devastating historical submarine earthquakes in the eastern Mediterranean," Ata Elias of the National Center for Geophysical Research in Beirut, Lebanon, and colleagues write in the current issue of the journal Geology.

To trace the origin of the disaster, Elias and colleagues used high-resolution sonar to map the contours of the sea floor between the Lebanese coastal towns of Enfeh and Damour.

"The images show details of spectacular submarine ruptures...that cut the smoothly sediment-mantled seafloor," the researchers wrote.

Running parallel to the coast offshore of Mount Lebanon, the relatively fresh seafloor seismic breaks indicate an active thrust fault is responsible for major earthquakes there.

The researchers found additional evidence after examining stretches of coast whose beaches rise like staircases out of the Mediterranean.

According to Elias and colleagues, those terraces are clear indicators of past quakes: each time the fault ruptured, the coastline rose about three feet.

To determine the occurrence of fault ruptures and the consequent earthquakes, the researchers dated mollusk fossils on the raised benches.

Indeed, the mollusks, which colonized the platforms when they were at sea level, died after coastline uplifts produced by the quakes.

Based on historical evidence and fossilized mollusks collected from a terrace dated to the 6th century A.D., the researchers estimated that the 551 disaster was caused by a rupture at least 62 miles long on the offshore Mount Lebanon thrust.

The rupture caused a magnitude 7.5 quake. Part of the seafloor collapsed by 5 to 10 feet, triggering a devastating tsunami.

The fossil dating also revealed that at least four earthquakes similar to the 551 event have occurred over the past 6,000 to 7,000 years, suggesting that the seismic behavior of the Mount Lebanon thrust is characterized by a series of clustered quakes — possibly four — each cluster separated by 1,500 to 1,750 years of relative calm.

If so, the quakes in 1837, 1918 and 1956 "might be forerunners of worse to come," the researchers concluded.

According to Rob Butler of University of Leeds's Institute of Geophysics and Tectonics, the researchers "make a convincing case about the source of the A.D. 551 earthquake and its link with the dramatic raised terrace along the coast."

But he is not convinced that that earthquakes of the last few decades might be harbingers of the next "big one."

"There is an unavoidable certainty that the [area] will be struck by a devastating earthquake. But it could be any time, perhaps within the next few years, perhaps a hundred years from now," he said.

"The bottom line is, we don't know the odds. In the case of Lebanon, people are literally betting their lives and houses on it," Butler added.

The U.S. Geological Survey

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