In the ruins of a castle destroyed during the Crusades, researchers have for the first time found geologic evidence of a major earthquake that shook the Middle East almost 800 years ago. Excavations at an archaeological site in Israel have revealed a 2-meter kink in the walls of a fortress overlooking the Jordan River. The findings, reported in the current Geology, could help seismologists foretell future earthquake risk in the region.
Historical records describe several temblors on what geologists now call the Dead Sea Transform, a dangerous fault between the Sinai and Arabia tectonic plates. From accounts of damage throughout the eastern Mediterranean, modern geologists estimate that a magnitude 7.6 quake ruptured about 250 kilometers of the fault north and south of the Jordan Valley on 20 May 1202. Such an event today could devastate cities across Israel, Jordan, Lebanon, and Syria, but part of the fault has been ominously quiet for the past 8 centuries.
Researchers got their latest clues to the precise location of the 1202 temblor from the castle of Vadum Jacob, built by Latin crusaders in 1178 and sacked by Muslims 1 year later. Digs by archaeologist Ronnie Ellenblum of The Hebrew University in Jerusalem and his colleagues showed that the castle's ruins straddle one strand of the fault. The fortress walls, made of precisely aligned limestone blocks, had shifted 2.1 meters since 1179. Arrowheads and other battle remnants preserved in the soil at the base of the wall suggested that 1.6 meters of that offset dated to 1202, while the rest probably occurred in smaller quakes in 1759 and 1837.
However, a quake as big as the one in 1202 may strike again at any time, says co-author Thomas Rockwell, a geologist at San Diego State University. "There's every indication that the southern part of this fault will re-rupture," he says, because surveys of ground position across the fault reveal no "creep" that would release seismic stress.
Archaeological sites with their distinct linear features can yield more precise measures of earthquake motions than geologic trenches, says geologist David Schwartz of the U.S. Geological Survey in Menlo Park, California. "The Dead Sea Transform is capable of producing earthquakes like the San Andreas [Fault], but the big question is the frequency," Schwartz says. More discoveries of ancient ground motions at nearby sites may fill in the history of very large quakes in the region, he notes.