2023-09-09 18:30:03
In Marrakech, a resident makes his way through the rubble after the 6.8 magnitude earthquake which shook the region on the night of September 8 to 9, 2023. FADEL SENNA / AFP
A powerful earthquake struck Morocco on the night of Friday September 8 to Saturday September 9, causing more than a thousand deaths. According to the Moroccan National Center for Scientific and Technical Research (CNRST), the epicenter of the earthquake was located in the province of Al Haouz, southwest of the city of Marrakech. Robin Lacassin, geologist and tectonicist at the Paris Institute of Globe Physics and the CNRS, takes stock of the seismic vulnerability of the area.
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What do we know about the earthquake that struck the Marrakech region on the night of September 8 to 9?
It is a strong earthquake, from 6.8 to 6.9, according to the moment magnitude scale (Mw), which measures the energy released by the earthquake. [Cette échelle n’est pas limitée ; contrairement à celle des cyclones, elle est « ouverte ». Le plus gros séisme enregistré a eu lieu à Valdivia, au Chili, en 1960, avec une magnitude de 9,5.] This energy, linked to the sliding mechanism on the fault, when it ruptures, determines the size of the earthquake. The Richter magnitude scale, the first to be used by seismologists from 1935, is no longer used because it is imprecise and not suitable for measuring strong earthquakes.
Does this scale also qualify intensity?
No. The scale which measures the intensity is established by taking into account the damage, the feeling and it of course depends on the distance from the earthquake at which it is measured. This scale, formerly called the “Meralli scale”, is rated from I to XII. It is determined by damage assessments, the number and type of buildings and infrastructure destroyed. But we only have this information a few weeks later.
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Can we already assess the extent of the damage?
We have little information and it is still too early to take stock. What we see in the photographs are mainly old buildings. Often built with stones joined together with mortar, they are of course not earthquake resistant. But these photos mainly show old neighborhoods of Marrakech, the medina. In addition, it is a mountain earthquake, in a region where the localities are very vulnerable, because they are built in a traditional way.
In your opinion, should we count on future aftershocks?
As with any earthquake, there will be aftershocks, which may occur in the coming days or even weeks. They are generally of weaker magnitude than the main shock. But in the case of an earthquake of magnitude 6.8, these aftershocks could be of magnitude 5 or 6. Which, on already weakened constructions, can significantly increase the toll.
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