Seismic studies underestimated the danger of the Atlas: it rose and narrowed too much

If in 25 million years there are any of us left here, we would see that Morocco has collided with Andalusia and the Strait of Gibraltar has been closed. The African tectonic plate would continue to push the Eurasian plate and, if geologist Christopher Scotese is right, the Iberian Peninsula will begin to rotate clockwise, and in another 25 million years, northern Spain will collide with the French coast.

For now, what is certain is that the Eurasian and African plates have been pushing each other for millions of years, like a plasticine puzzle in which the pieces mix, or rise, or submerge, causing thousands of earthquakes a year.. The last one, this weekend in the Atlas mountain range, with the epicenter more than 700 kilometers from the collision of tectonic plates. “It is not predictable, but not impossible”, points out seismologist Juan Vicente Cantavella, from the National Geographic Institute (IGN). The Agadir earthquake, at the oceanic mouth of the mountain range, and even further away from the Plate collision, was 5.8 and in 1960 left 12,000 to 15,000 fatalities.

At 11:11 p.m. on September 8, the earth shook 60 kilometers southwest of Marrakech, and in Spain the IGN received more than 500 warnings, most of them from Andalusia, but also from Madrid and the islands of La Palma and El Hierro.

According to a study of the Atlas faults published in 2006 by researchers from the Tectonics Laboratory of the Pierre et Marie-Curie University, the Paul Cézanne University of Marseille, the Mohamed V University of Rabat and the Cergy Pontoise, the Atlas faults They had “the potential to generate earthquakes of between 6.1 and 6.4” at most on the Richter scale.. They fell short, making it harder to predict if, when, and of what magnitude there will be more.

Itahiza Domínguez, a seismologist at IGN, explains on social networks that seismic hazard maps have “a problem”: they are based on knowledge of the past, but if this is limited, or the recurrence times of earthquakes are long, it can be “underestimate” the danger. And this is believed to be what happened with the Moroccan earthquake, since the pre-existing map gave a lower probability in the hypocenter area compared to other points in the country, and in North Africa in general.. In the Iberian Peninsula, the most dangerous points are in the provinces of Granada and Murcia. In the first, a 6.7 earthquake was recorded in 1884 with a death toll of more than 1,000.. Lorca's in 2011 was 5.1 and caused 9 deaths and more than 300 injuries. “The focal mechanism does not match, and could be a fault in the southern Atlas. We are using five stations, but the closest to the earthquake is 150 kilometers away,” explains Cantavella.

Nineteen minutes after the first earthquake, a 5.1 aftershock occurred. And at 1:21 in the morning the third, 4.6. The next one arrived at 8:38, 4.2. The next one 22 minutes later, also 4.2. And the sixth at 12:19 noon, 4.1. The earth trembled on both the northern and southern fault lines of the Atlas mountain range, as if they were having a seismic conversation, which left fatalities wherever buildings had been built: Al Hauz, Taroudant or Chichaoua.

Scientists from the University of Granada (UGR), belonging to the Andalusian Institute of Earth Sciences and the Department of Geodynamics in collaboration with the University of Jaén and Moroccan universities, believe that there is a “high possibility of aftershocks of the earthquake occurring during months”, and that of course they will feel again in our country.

These seismic movements narrow and raise the mountain range at a rate of 0.1 millimeters per year, according to the same study by Franco-Moroccan universities.. More recent geodetic measurements obtained with GPS records of millimeter precision show that the Atlas is narrowing at a rate of one millimeter per year, which according to UGR scientists, contrasts with its low seismicity.. At the same time, 700 kilometers away, the Strait of Gibraltar narrows at a rate of between 4 and 25 millimeters per year, as a result of the same plate movement.

The geophysical and geological studies carried out by his team show that under the continental crust they discovered an anomalous, hot and not very dense mantle that supports the relief of this mountain range, and is related to the Quaternary volcanism of the region.. In a few months we will know if textbooks should correct the 4,167 meters of Toubkal, the highest peak in the mountain range.

According to the model developed by the US Geological Survey, the Moroccan earthquake was caused by displacement on a fault surface about 30 kilometers long (horizontally) by 20 kilometers wide (in the direction of dip of the fault). fault towards the interior of the Earth). It produced a 1.5 meter rupture, which began about 25 kilometers deep, and propagated radially, until it stopped about 10 kilometers from the Earth's surface.