While geologists have been collecting data on earthquakes occurring since the beginning of the 20th century, reliable records of earlier seismic activity are almost nonexistent. The discovery of two newly-uncovered faults in Israel, active long before modern scientists began to systematically record earthquakes, are the result of chance findings which have helped solve questions about the past, and could influence future planning.
by Daniella Ashkenazy
On a recent visit to an archeological site in the Negev, Professor Emanuel Mazor of the Weizmann Institute's Environmental Sciences and Energy Research Department heard about the work of Dr. Andre Korzhankov, a Russian geologist responsible for recording damage from earthquakes in Kirgistan. Korzhankov had defined some 22 tell-tale signs that earthquakes may "register" in damage to buildings: for example, the direction and degree to which cracks develop, how and to what distance debris is propelled, etc. He then calibrated these signs with data regarding the epicenter, direction and magnitude of the event as registered on monitoring devices at the time of the quake.
On hearing about Korzhankov's work, Mazor says that things suddenly "snapped in place". Archeological sites were, in essence, "seismic archives" for studying past earthquakes. Up until then, all that had interested archeologists in noting signs of earthquakes was whether a site had been razed by invaders or by natural forces.
Together with Dr. Alexander Becker, a structural geologist from the Ramon Science Center, Mazor and Korzhankov realized that by looking at "aftermath" patterns left by past quakes in archeological ruins, geologists could calculate the magnitude and pinpoint the epicenter of earthquakes which occurred much earlier than the 20th century - even those from 2,000 years ago.
Dr. Korzhankov's innovative approach was tested in a series of surveys of archeological sites in the Negev, such as the ancient Nabatean city of Avdat. During his many visits to Israel, evidence was found that the city of Avdat had been demolished by earthquakes not once, as previously thought, but twice: during the 4th century and again in the 7th century. The real surprise, however, was the epicenter of one of the quakes.
In the walls of a Byzantine structure in the ancient city of Shivta, the team came across a point where one 50x50 cm building stone at the base of the wall had been "shot" forward 74 cm, and others twisted and turned in place in a manner that indicated a strong, fast tremor caused by an earthquake. At another nearby site, the keystone in the arch of a Byzantine church in the ancient city of Mamshit "slipped" down in a split second when the arch was stretched open by the same devastating earthquake in the 7th century, causing the keystone to drop by several centimeters before it was "re-trapped" in a slightly different position when the arch "snapped" back in place. (See photo.)
After studying hundreds of tell-tale damage patterns such as these and their various degrees, it was concluded that unlike other quakes in the 4th, 8th and 20th centuries, the epicenter of the major earthquake in the 7th century was "closer to home", in the Negev Highlands, not in the Rift Valley - a major fault line that runs from the Sea of Galilee along the Jordan Valley to the Dead Sea and then down to Eilat, thought by geologists to be at fault for all previous earthquakes in the Negev. Until then, the Negev Highlands had been thought to be as solid and unmoving as the Rock of Gibraltar.
The new evidence also demonstrated that the Rift Valley and the Negev Highlands are not only two different systems, but they respond differently to pressure. The Rift Valley's tectonic pressure is relieved through numerous relatively small earthquakes, whereas the pressure that builds up in the Negev Highlands tends to be relieved in a single powerful snap, registered on the keystone among other signs.
A second unknown fault line, examined about the same time that Mazor and his team were surveying the Negev, was the focus of work done by a Haifa University underwater archeology team headed by Professor Avner Raban of the Recanati Center for Maritime Studies, together with Professor Kenneth Holum of the University of Maryland history department. The team had been exploring the ancient harbor of Caesarea, located about half way between Tel Aviv and Haifa. It was already known that most of the harbor, built by Herod between 22 and 10 BCE, had sunk into the seabed, yet geologists had associated this with other factors - perhaps the sinking of the harbor into sand over time. The archeologists sensed that the cause was far more sudden and dramatic.
During the most recent archeological season, the Raban-Holum team sought clear evidence of seismic activity, which they found under the northern jetty. When divers dug under the foundations of the jetty, they found the edge of a fault where the bedrock had broken off and slipped down three meters. This was discovered to be part of a submerged fault-line parallel to the coast, to date unknown to geologists. The fault, which bisects the basin of the harbor from north to south, is now known to have been responsible for at least two earthquakes - one at the end of the 1st century and one in the mid-6th century. The second earthquake toppled the two towers that stood at the entrance to the artificial harbor, and sunk the seabed to a depth of six meters.
The importance of the two discoveries goes beyond academics. While it cannot be predicted exactly when earthquakes will take place, it is agreed that where they have occurred in the past, there is a chance they may occur in the future. Consequently, Professor Mazor and Professor Raban both believe such information about past natural "movers and shakers" gathered from archeological evidence should be taken into account in future industrial and urban development plans.
If, for example, the coastline is less stable than was previously thought, the findings in Caesarea may have implications for off-shore construction, such as building artificial islands opposite Tel Aviv linked to the mainland by a causeway - already a subject of public debate. And if this sounds like an irrelevant and remote worry, Professor Raban adds that as a result of the recent earthquake in Turkey, streets along the beach in Sinachik - a coastal tourist spot - sunk six meters.