The 'unexpected' Moroccan earthquake: why seismologists did not expect a large earthquake in the Atlas

The Moroccan authorities raise the death toll to more than 2,500 from the earthquake that shook the central area of the country on Friday night. Meanwhile, assistance work in the affected areas continues, although for now the Moroccan Government has only accepted help from Spain, the United Kingdom, Qatar and the United Arab Emirates.

Friday's earthquake was the strongest recorded in Morocco in more than a century, with a magnitude of 6.8, followed by 25 aftershocks greater than magnitude 3. As a result of the collision of the African and Eurasian plates, the earthquake has been devastating. At about 20 kilometers, a relatively shallow depth according to experts, “strong” ground shaking was recorded around the epicenter of the earthquake.

A chain of 31 earthquakes

The earthquake had a length of about 30 kilometers, ten times less than the Turkey-Syria earthquake (last February), which was 300 kilometers.. In this way, the energy released to the subsoil is much lower and the earthquake is more localized.. “If an equivalent earthquake had occurred in Morocco, a large part of Marrakech would have been destroyed,” says seismologist Florent Brenguier, from the Institute of Earth Sciences at the University of Grenoble .

According to data from the National Seismic Network of the Spanish National Geological Institute, between 11:11 p.m. on Friday, local time, and 2:58 p.m. on Saturday, 31 earthquakes with a magnitude between 3.4 and 6.8 occurred.. The first, of 6.8 and the second, of magnitude 5.1 just 20 minutes later, were the strongest. These two were followed by a series of aftershocks that moved towards the north of the country and with their epicenter approaching Marrakech.

The precedents

Although such powerful tremors are rare, the current one is not the deadliest that Morocco has suffered.. Just over 60 years ago, the country was shaken by a 5.8 magnitude earthquake that killed more than 12,000 people on its western coast, where the city of Agadir, southwest of Marrakech, collapsed.

In February 2004, what until now was the largest earthquake in Morocco occurred.. It was in Al Hoceima and the magnitude reached 6.4. However, the number of fatalities barely exceeded 600.

What caused the earthquake

From what geologists and seismologists have known so far, the earthquake occurred when a reverse fault occurred – in which the edge of the rock on one side of a fault slides under the other – between the Moroccan and Iberian microplates, which are part of the larger African plate. During the earthquake, the edge facing the mountains slid over the other, pushing the slope upward, a consequence of tension built up between the African and Eurasian plates over time.

According to Paula Marques Figueiredo, a geologist at the University of North Carolina, the reverse tectonic faults were located north of the Atlas Mountains and tilted toward it at one point.. “Faults cannot withstand stress to a certain extent and, from time to time [thousands of years], an earthquake occurs as a mechanism to release the built-up stress,” he told Al Jazeera.

Earthquake zone, but further north

Earthquakes in this part of the world are not frequent, but they are not unexpected. We have already seen that in 1960 there was one of magnitude 5.8 that caused the city of Agadir to collapse.. According to the United States Geological Survey (USGS), an earthquake of magnitude greater than six had not been recorded within a 500 kilometer radius since records began in 1900.

Since that date, at least nine magnitude 5 earthquakes have occurred within a 500-kilometer radius around this latest earthquake.. “There is no doubt that n earthquakes occur in that region,” says Hadi Ghasemi of Geoscience Australia.. However, earthquakes had never reached the magnitude of the current one. “An earthquake could be expected there, but not so strong, at most magnitude 5,” seismologist from the National Geological Institute, Carlos González, told the Efe agency.

An earthquake could be expected there, but not as strong, at most magnitude 5.”

In Morocco, seismic movements are much more frequent and of greater intensity in the north, near the Mediterranean. “It is an area that we know can produce earthquakes of this magnitude,” comments Mark Quigley, professor of Seismic Sciences at the University of Melbourne, to the Australian ABC News.

A surprising force

That's what another seismologist is referring to.. “It is important to remember that all of Morocco, and the entire Mediterranean region in general, is susceptible to large earthquakes. However, the majority of earthquakes are concentrated in the north of the country, where the African and European tectonic plates converge, especially around the Strait of Gibraltar,” explained seismologist Brenguier on France 24.

But all students of the phenomenon are surprised by the strength of this occasion. Yes, in this part of Morocco there are important and very old faults, “but it is not common for an earthquake of such magnitude to occur in an area that is not located on a plate boundary,” says Brenguier.. That is why, on the other hand, an earthquake like the one in February in Turkey was to be expected, because it occurred on the boundary of two large plates, those of Anatolia and Arabia.

“It is unusual for an earthquake of such magnitude to occur in an area that is not located on a plate boundary.”

Lahcen Mhanni, from the National Institute of Geophysics, agreed with this.. “In general, mountainous regions do not produce earthquakes of this magnitude,” he told Moroccan news channel 2M TV.

The possibility of another earthquake

Looking to the immediate future, caution. The initial shock of one earthquake can trigger another (no longer an aftershock). “For this reason, it is not impossible that we will see another large earthquake, but not necessarily on this same fault. Could happen further north or south. This usually occurs in the hours or days following an earthquake, but can also happen weeks or even months later,” says seismologist Florent Brenguier.

“It is not impossible that we will see another large earthquake, but not necessarily on this same fault.”

“The index decreases over time. That does not mean that the strongest aftershock cannot occur five or ten days later. We don't know, but the frequency decreases over time,” seismologist Remy Bossu told Al Jazeera.

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