The earthquake of January 18, 749, is thought to be one of the strongest ever to hit the Middle East. Till recently, researchers knew about the quake only from historical sources. A Coptic priest from Alexandria reported that support beams in houses in Egypt had shifted; a Syrian priest wrote that a village in the region of Mount Tavor had moved a distance of four miles; while other sources spoke of huge tidal waves in the Mediterranean Sea, of Damascus shaking for a few days, and of smaller cities and towns being swallowed up in the earth.
The most detailed descriptions came from Jerusalem, where thousands were reported dead, where palaces and churches collapsed, and where the Al-Aqsa Mosque suffered serious damage.
The historical sources gave the geologists some idea of the intensity of the quake, and its epicenter, but no more than that. Findings during an archaeological dig in Tiberias a year ago, however, allowed geologists to analyze the quake using modern research techniques, as if it had occured just yesterday.
The findings of the research, conducted by Dr. Shmuel Marco of Tel Aviv University and Dr. Moshe Hartal of the Antiquities Authority, are now being published. The study stipulates the intensity of the quake and its epicenter; and the data is helping researchers predict the maximum strength of the next quake that will shake the region, as well as where it is likely to hit.
The rare findings were discovered last summer. During the course of a dig designed to facilitate the expansion of the Galei Kinneret Hotel, Hartal noticed a mysterious phenomenon: Alongside a layer of earth from the time of the Umayyad era (638-750), and at the same depth, the archaeologists found a layer of earth from the Ancient Roman era (37 B.C.E.-132).
"I encountered a situation for which I had no explanation - two layers of earth from hundreds of years apart lying side by side," says Hartal. "I was simply dumbfounded."
The mystery was solved only when geologists who arrived at the site determined that an earthquake of immense intensity had raised the Roman era layer of earth to the same level as the layer of earth from the Umayyad era. What Hartal had stumbled across was a rare geological find - an active fault line from 749 dividing two expanses of land that had moved during an earthquake.
Also found during the course of the dig was a circular-shaped well, some 10 meters wide, that had been split into two by the force of the earthquake. Hartal believes the wall was once part of Tiberias's Roman stadium.
The Syrian-African Rift, and particularly the section between the Arava in the south and the Hula Valley in the north, is one of the most earthquake-prone regions in the world because it constitutes a seam line between two plates of earth, one of which is moving faster than the other.
This movement creates immense frictional energy that builds up in the earth until it bursts forth in the form on an earthquake. During the quake, a segment of the Arabian plate (to the east of the Jordan River) measuring dozens of kilometers moves a distance of dozens of centimeters at once.
Today, geologists can calculate the amount of energy built up in the earth as a result of the movement of the plates. When one knows how much energy has already been released in earthquakes that occured in the past, it is possible to work out how much energy is likely to be released in the future.
The fault line found in Tiberias has taught the researchers that the earthquake of 749 would have measured 7-7.5 on the Richter Scale. During that quake, a segment of earth measuring hundreds of kilometers - from Tiberias in the north to Jericho in the south - moved northward an average distance of 1.5 meters at once.
And what about the future? Every few hundreds years, the stretch of land between Jericho and Tiberias suffered strong earthquakes, until the last one, in 1033.
"Almost a thousand years have passed since 1033; that's a long time," says Marco. "Therefore, we estimate that there is a high likelihood that at some time in the future in this region, particularly in the area of the Beit She'an Valley, there will be an especially strong earthquake."