Just a natural disaster that can happen. This is how earth scientist Rob Govers (Utrecht University) describes the earthquake in Morocco. It’s all about bad luck, chance, probability. A disaster, however, that surprises earthquake experts like Govers. “We call this an intraplate earthquake, an earthquake that is not located in a known zone between Earth’s plates, but takes place on an Earth’s plate itself. Those are rare. But they happen. The big surprise is that it is so heavy.”
The affected area is on the African plate, which is moving entirely towards Spain, at about 3 to 4 millimeters per year. And just like moving a tablecloth, it can sometimes happen that a fold appears somewhere down the line.
That’s what happened last night 75 kilometers southeast of Marrakech. At a depth of 18.5 kilometers, a piece of rock broke loose over an area of about 30 kilometers and slid over another piece of rock. Only about 30 centimeters, geologists calculate. Enough to send a huge shock wave through the Earth’s crust.
That shock wave hit hard, especially in Marrakech. Govers: “That city is located in a basin, unlike Agadir, for example, which lies on solid rock. The ground under Marrakech is softer. This causes more shaking.” A bit like waves giving more deflection to a rope than to an iron bar.
What also plays a role: the architectural style is “particularly bad,” says Govers. “Not surprising in an area where these types of earthquakes rarely occur. I think it partly explains the large number of victims.”
The Moroccan Ministry of the Interior has now counted 820 deaths, it announced at the end of the morning. However, that number is increasing rapidly. Hundreds of people have also been injured.
Govers does not expect many aftershocks. “The experience with this type of earthquake is that it is finished in one go. A tremor, and then it is quiet. That could also be the case here, although you can never be completely sure.”
Old fault lines
Earth scientists have mainly focused on less stable areas, such as the transition zone between the African and European tectonic plates. Partly because of this, information about the currently affected area is limited. There are some old fault lines beneath the Atlas Mountains, but they have not been active for tens of millions of years, says Govers.
Nevertheless, earthquakes around Marrakech are not completely impossible. This is what Govers points out a Moroccan study who found indications in the subsurface for an earthquake with magnitude 4 to 5 that must have occurred sometime between 1900 and 2007. The earthquake apparently caused so little damage that it went unnoticed.
Govers found one study in the professional literature that does indicate that an earthquake could occur nearby – although it would be further to the east and would result in a much less powerful quake. “It has certainly been very quiet over the last fifty to a hundred years.”
