Slow deformation of the earth’s crust, large fault, very long geological time… Several factors explain this phenomenon.
In Al Haouz, Ouarzazate, Marrakesh, Chichaoua and Azilal, the earth shook and reached an intensity of 6.8 on the Richter scale. On Friday evening, September 8, a powerful earthquake struck Al Haouz province in Morocco, a few dozen kilometers from the tourist city of Marrakesh. Four days later, the latest preliminary report listed 2,863 dead and 2,562 injured, according to the Moroccan Interior Ministry. The epicenter is at the height of the High Atlas massif, in the center of the country, a region with moderate seismic activity. “It is not the most active area in Morocco, being in the north, near the Rif Mountains and the Strait of Gibraltar,” explains Jérôme Van der Woerd, researcher at the Institute of Earth and Environment in Strasbourg, to franceinfo. So how can we explain such an earthquake occurring in this area?
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According to experts, the epicenter of the earthquake lies within a tectonic plate, which is defined as a fragment of the Earth’s solid outer shell. We are therefore talking about an intraplate earthquake, which is rarer and little known than the usual interplate earthquakes that occur at the peak of faults, i.e. fracture zones in the earth’s crust. The disaster “occurred on the northwest edge of the African Plate. Not all earthquakes are necessarily triggered at the perfect boundary between two plates,” explains Philippe Vernant, a teaching researcher at the University of Montpellier and a specialist in active tectonics. “In the case of Morocco, the fault separating the African from the Eurasian plates is quite diffuse. It’s not as pronounced as San Andreas.” [en Californie] For example”.
The epicenter of the earthquake that struck Morocco on Friday, September 8, was in Al Haouz province, north of Marrakesh. (LEA PRATI / HELOISE KROB)
In addition to this huge fault in the north of the country, Morocco is also crossed by other smaller faults, tens of kilometers long, in the Middle Atlas, High Atlas and Anti-Atlas massifs in the south of the country. “The geological base of the region consists of sedimentary rocks that are very fragile and malleable. It is subjected to considerable stress and deformation,” explains Jérôme Van der Woerd.
A movement of the High Atlas by one millimeter per year
In general, the Earth’s crust is constantly subjected to enormous tectonic pressures caused by plate movements. This can lead to deformation of rocks in the earth’s crust. For Morocco, the deformation is on the order of less than a millimeter per year and shifts the High Atlas into a vertical axis. The top layer of rock is pushed over the bottom layer, causing the mountain to grow. “This deformation is not rapid, but it is enough to cause small earthquakes at relatively regular intervals,” the scientist continues.
The deformations and stresses of the plates accumulate until they reach a critical value that the rock can no longer support. “The faults present in Al Haouz province are weak points that can rupture related to the accumulation of pressures and deformations,” adds Philippe Vernant. When it breaks, significant energy is released in the form of seismic waves. “In this earthquake, the rupture on either side of the epicenter is believed to be approximately 30 km wide.”
Errors of several dozen kilometers
Large faults are also required to trigger large earthquakes. “Although this area is not particularly active, there are structures under the Atlas massif in Al Haouz province that could withstand such an earthquake,” explains Jérôme Van der Woerd. On February 29, 1960, the earth had already shaken in Agadir, 230 km south of Friday’s earthquake (magnitude 5.7), killing more than 12,000 people. “The mechanism was exactly the same as the current earthquake. It’s just not the same fault that played a role,” describes the researcher.
If the seismic activity in the region was previously considered moderate, this can be explained by the scientists’ lack of perspective. The study of seismology is relatively new compared to the extremely long geological time scale. Seismologists rely on identifying recurring earthquakes to estimate seismic potential using available data and tools. However, there has been no recent earthquake of comparable intensity to this one, which experts say can give a false impression of stability. In reality, the region has significant faulting and signs of deformation and most likely experienced numerous earthquakes thousands of years ago. “This earthquake makes us wonder what is really happening beneath our feet,” concludes Jérôme Van der Woerd.