The south of peninsular Spain and the north of Morocco look at each other across a geological border, which separates the Eurasian and African tectonic plates. There, those two continents slowly collide, causing the friction associated with earthquakes. However, on risk maps, the regions north of that rift are much darker than to the south, in Morocco. Because they are drawn based on recent seismicity data and the information is worse below the Strait of Gibraltar. And yet, the region hit by the 6.8 earthquake southwest of Marrakech rested on a known fault, in a mountain range, the Atlas, which rises precisely as a result of the thrust between the plates. There, the energy accumulated in that fault has generated a fracture about 25 kilometers long and 20 kilometers wide, in a landslide of up to 1.5 meters, as calculated by the US Geological Survey (USGS).
“The (risk) map is calculated probabilistically based on the earthquakes that have occurred. In the south of the peninsula Ibérica, we know of several earthquakes of significant magnitude in the last 200, 300 years, and also recent major earthquakes, such as the one in Lorca,” explains seismologist Itahiza Domínguez, from the National Geographic Institute (IGN). That is to say, there is a whole series of earthquakes that could have been used to make this calculation and, for this reason, regions such as Murcia or Granada appear more colored.
“In the north of Morocco there are also higher levels of danger, the problem is that in that area (where the earthquake occurred) there are hardly any high values, excluding Agadir, which is where there was another large earthquake of magnitude 5.8, but whose effects are limited only to a very specific area,” develops the expert. “The danger is calculated using the data we have, and if we do not have data, we cannot know that there is a danger,” he summarizes.
The earthquake that shook the Atlas on Friday night was of magnitude 6.8 – thirty times larger than that of Agadir, which killed more than 12,000 people – and affected a 100 kilometers radius. Since then, the IGN has recorded about forty aftershocks larger than magnitude 3, and two with magnitude greater than 4 (the last 4.2 at 9:00 a.m. in Morocco).
The data that seismologists had about this region (until this week) is that there is the Atlas, mountains that grow due to continental collision, and there are recognized geological faults in the area, but with low seismicity, because it is not continuous in geological terms. “That misleads a bit, it can make you think that it does not seem like an area where energy accumulates. And in this case it is an error, obviously, now we have realized it. It is something that has happened other times in other areas of the world, where perhaps a big earthquake was not expected, and in the end it happened,” adds Domínguez.
Slowly colliding plates
The African and Eurasian tectonic plates do not move against each other at high speed: that is why recurrence times are very long in this region, that is, the period that elapses until a fault accumulates enough energy to generate a great earthquake. Near Morocco, the Eurasian plate is moving south and east relative to the Nubian (African) plate, at a rate of only 4 millimeters per year. “It is not like in Japan or Chile, where every 50 years or 100 years there will be a big earthquake for sure, here it takes longer,” Domínguez clarifies. Perhaps the largest earthquake in Morocco occurred in 1624 near Fez, in north-eastern Morocco, but there are no direct records.
“In this region, the (plate) boundary is very complex, with multiple active deformation zones rather than a single well-defined fault,” plate tectonics expert Judith Hubbard explains in an article. “Because relative movements are slow, it is difficult to use tools like geodesy to define which faults are active and how fast they are actually moving. As a result, we still have a lot to learn about the danger posed by faults in this area,” summarizes this geologist from Cornell University.
This earthquake occurred in the mountains halfway between Agadir and Marrakech, about 500 kilometers south of the boundary between the African tectonic plate and the Eurasian plate, where there has not been an earthquake greater than magnitude 6.0 within a radius of 500 kilometers. around the current epicenter at least since 1900, when scientific seismic records began to be available.
That would explain why neither the citizens, nor the buildings, nor the authorities have been well prepared for a shock of these dimensions. The buildings were not earthquake resistant because they were not expected to be.
Brian Baptie, head of Seismology at the British Geological Survey, agrees with the diagnosis of the IGN expert: “Earthquakes are relatively uncommon in this region of Morocco, with the greatest seismic activity towards the northeast, closer to the plate boundary between Africa and Europe.” “However, earthquakes have occurred nearby in the past and earthquakes remain a major danger in this region of Morocco,” explains Baptie in statements to the scientific information service SMC.
“Earthquakes have occurred in that environment in the past and earthquakes continue to be a major hazard in this region of Morocco. In 1960, a 5.8 magnitude earthquake shook the city of Agadir, causing between 12,000 and 15,000 deaths, making it the deadliest earthquake in the history of Morocco,” continues Baptie.
Colin Taylor, emeritus professor of earthquake engineering at the University of Bristol, explains: “We know how to tackle this problem, but applying the knowledge is a huge long-term economic and political demand. “Morocco does not experience large earthquakes that often, perhaps one for every generation, so public awareness wanes and other, more immediate, life challenges capture their attention and drive political agendas.”
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