“It is not a known area for large earthquakes,” says Douwe van Hinsbergen, a geologist specialized in earthquakes at Utrecht University, “and that may also be the treacherous thing.”
Earthquakes occur when pieces of the earth’s crust suddenly slide past each other along existing faults, suddenly releasing tension that has built up for years. In the Moroccan earthquake on Friday evening, this happened along one of the faults in the High Atlas Mountains.
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Major earthquakes in the area are rare because the tension builds up very slowly. Van Hinsbergen: “The Atlas is being pushed up because the Africa tectonic plate and the Europa tectonic plate are moving towards each other, but that is happening extremely slowly. In 40 million years they have moved about 35 kilometers. This means millimeters per year.”
Even by geological standards that is slow. The tectonic plates that push up the Himalayas are moving almost fifty times faster. At the fault in eastern Turkey, where large earthquakes occurred earlier this year, this amounts to 2 to 3 centimeters annually.
In addition, it is not only the Atlas Mountains that are absorbing the rising African-European tensions, but also mountain ranges in Spain, in the past even as far as the Pyrenees. “It is a very complex area,” says Van Hinsbergen. “In fact, the line between the two plates is quite difficult to draw.”
That slow movement, combined with a large area to distribute the stresses, means that stress increases very slowly along individual faults. Occasionally it discharges itself very locally in small earthquakes, which are regularly measured in this area. “But then it concerns shifts of centimeters, not meters as in this case,” says Van Hinsbergen.
Rare
Large earthquakes, such as this one with a magnitude of 7.2, are therefore rare from a purely statistical point of view. “In that sense, this earthquake is bad luck. In Marrakech, buildings around 500 years old have been damaged: this indicates that such heavy shocks are rare.”
The last major earthquake in this area, in 1960 in the city of Agadir, was much less strong, with a magnitude of 5.8, although it caused between 12,000 and 15,000 deaths, mainly because the epicenter was a densely populated city.
The largest shift in Friday’s earthquake occurred at a depth of 18 kilometers, relatively shallow. This makes the vibrations that reach the surface all the more powerful and therefore more harmful.
Because the tension is only discharged in one place along the fault, there is a risk of aftershocks, says Van Hinsbergen, if the tension is also discharged in other places due to the sudden change. This means that the danger has not yet passed.