Every time the earth shakes and our screens are filled with scenes of destruction and casualties, we in Israel ask: when will an earthquake strike us, too? No one knows when it will happen, but there is a community of researchers in Israel dealing with the question of when such events took place in the past, and when they are liable to recur. The answers to this "academic" question can be useful in answering the practical question of how to minimize the risk. On the morning of May 20, 1202, a gigantic earthquake with a magnitude of 7.5 or more (the measure of the quantity of released energy at the earthquake epicenter) struck Lebanon, western Syria, and the Galilee. The shock waves spread out from the Beqaa Valley in Lebanon, the Hula, and Lake Kinneret (Sea of Galilee), where they smoothed out the plates of the earth's crust, the tectonic plates, which had lain undisturbed for hundreds of years. The Crusader castle at Tel Ateret on the banks of the Jordan River was torn along the fault line passing through it - a fault line that had been torn at least five times in the preceding 3,000 years.
In 1759, the plates were again smoothed by a pair of earthquakes, and in 1837, Lebanon was torn by an earthquake with a magnitude of 7. The cities of Safed, Tiberias, and Acre were destroyed again and again, usually together with other towns and villages. The episodic release of accumulated energy in the movement of the plates is unpreventable, and it continues to accumulate. According to GPS studies, the tectonic plates continue to move at a pace of 0.5 cm per year, but the upper part of the earth's crust in the area of the fault is flexible enough to accumulate energy for dozens and hundreds of years. South of the Jordan Rift, the period of quiet has continued since the 11th century. Here, too, however, energy continues to accumulate adjacent to the major population centers of Israel, Palestine, and Jordan. The Dead Sea area last saw a major earthquake on July 11, 1927, which caused severe damage in Jerusalem, Ramle, Nablus, and Amman. This event, however, was relatively modest (6.2 magnitude), and the quantity of energy released by this earthquake was many times smaller than that of a major event.
There is a wealth of data about the Dead Sea region: historic, archeological, and geological, which paints a picture of repeated destructive events. What will the readers of this article do with this information? Most of us will not be trapped under ruins or seriously injured, even if, the casualties are in the tens of thousands (such as in the reports of a few earthquakes in history). Take into account, however, that the hospitals and the rescue services will be busy with those not so lucky. It is therefore worthwhile preparing a store of bandages and non-prescription drugs now, because access to pharmacies will be limited. This is also true of supermarkets, electricity, and water. Every home should have several days' supply. What can the state do with this scientific information? The state enacts construction regulations and enforces them - slowly perhaps, but there is a great deal of progress in the matter. These regulations are based on models for the probability of earth acceleration above the critical value at a given location over decades and centuries.
The models are based on history, such a detecting small earthquakes and researching the mechanics of faults in the earth's surface and the spread of earthquake waves from the fault area. There is therefore room to hope that construction resources will be allocated optimally among regions and various buildings. It is the state's duty, however, to look beyond the probability span of dozens and hundreds of years. The establishment, construction, and design of critical facilities, including power stations, ports, ammonia containers, etc., should take into account the maximum possible ground acceleration, even if it takes place in an area where an earthquake takes place once in thousands of years. Geological studies in stalactite caves hint that an extreme earthquake occurs once every few thousand years beyond anything in recorded history, and the last word on this matter has not yet been spoken.
The author is from the Hebrew University of Jerusalem's Institute of Earth Sciences.
Published by Globes [online], Israel business news - www.globes-online.com - on April 27, 2015
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