Sunday, July 29, 2007

Fresh fault

"In the 6th century AD, John of Ephesus wrote of a cataclysmic event that devastated coastal Lebanon: "The sea withdrew and retreated from the coastal cities of Phoenicia for a distance of nearly two miles.... A tremendous surge of the sea rushed up to return to its original depths." Tripoli was swamped. Beirut took almost 1,300 years to recover.

Now an international team of geophysicists has traced the origins of the July 9, 551, tsunami to an earthquake along a previously unknown fault roughly four miles off Lebanon's coast. The fault traces the coast for up to 90 miles. The data come from a research cruise in 2003 during which the scientists used high-resolution sonar to map the contours of the sea floor. The evidence shows up as relatively fresh fault scarps along the ocean bottom.

The team also examined stretches of coast whose beaches rise in stair-step fashion out of the Medi­terranean – indicators of past quakes that pushed up Lebanon's coast. They estimate that major quakes come in clusters, separated by 1,500 to 1,750 years of relative calm. If that's true, they say, the region is long overdue for a major seismic event. The results appear in the August issue of the journal Geology. "

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