Here’s what we know about what caused the Turkey earthquake

The area of Turkey and Syria that has been hardest hit by Monday’s 7.8-magnitude earthquake and its aftershocks is known for having big quakes, but it had been decades since one this large last hit.

More than 5,00 people had died across Turkey and Syria.

Here’s a look at what happened, geologically-speaking, and why it has caused so much damage.

Earthquakes are common in Turkey and Syria

The Arabian Peninsula is part of a tectonic plate that is making its way north into the Eurasian Plate, and the entire nation of Turkey is getting squeezed aside.

“Arabia has slowly been moving north and has been colliding with Turkey, and Turkey is moving out of the way to the west,” says Michael Steckler of Columbia University’s Lamont-Doherty Earth Observatory.

That tectonic shift has been behind earthquakes for millennia in the area, including one that flattened the Syrian city of Aleppo in 1138. More recent quakes, such as the 1999 one that struck the city of İzmit, have killed many thousands.

Monday’s quake is believed to be the most powerful that Turkey has seen in more than 80 years.

This particular region was overdue for a big one

Most of the largest earthquakes in the past hundred years have been along the North Anatolian Fault.

But stress has been building along another major fault: the East Anatolian Fault. That fault has seen some big earthquakes in the past, says Patricia Martínez-Garzón, a seismologist at GFZ Potsdam, a research center in Germany. But more recently, there hasn’t been as much activity.

“It was unusually quiet in the last century,” she says.

Some researchers had begun to suspect the fault was due for a major quake, according to Fatih Bulut, with the Kandilli Observatory and Earthquake Research Institute at Boğaziçi University in Istanbul. His research group and others had run computer models showing that this fault could have a magnitude 7.4 or greater earthquake.

“This is not a surprise for us,” Bulut tells NPR.

But that doesn’t mean that seismologists could say exactly when a big one would hit, according to Ian Main, a seismologist at the University of Edinburgh in the United Kingdom. The time between big quakes on a fault can vary quite a bit in unpredictable ways, he says. “They’re not like buses, they don’t come along on a timetable.”

And not all the shaking has occurred on this one fault. The initial quake spilled over onto the Dead Sea Transform, another fault region where the Arabian, Anatolian and African plates converge. And a second, magnitude 7.5 quake took place hours later on a nearby fault that had been mapped but isn’t part of the East Anatolian Fault.

“It’s a pretty busy and complicated area with multiple fault systems,” Steckler says.

This was a “strike-slip” earthquake

This earthquake occurred because “two pieces of the Earth are sliding horizontally past each other,” Steckler says. It’s the same kind of quake that occurs along the San Andreas fault in California.

In this case, the Arabian Plate is sliding past the Anatolian Plate.

That sliding motion also meant the shaking was spread out for many kilometers along the fault, says Bulut. The affected area “is quite large,” he says. “Ten cities were structurally affected in Turkey.”

Turkey has seismic codes to try to prevent buildings from collapsing, but Bulut says because this region has escaped a major quake for decades, it’s possible that some older buildings are vulnerable. “Sometimes there are very old things, built before the rules existed,” he says.

Steckler says he suspects that even some newer buildings may not have been up to code. “I know, certainly in Istanbul, there’s a lot of illegal construction that goes on,” he says.

More aftershocks are likely

The U.S. Geological Survey has already recorded more than a hundred aftershocks in the region, and experts expect they will continue for some time.

“That whole area, all the pieces of the Earth will slowly adjust and break and rupture and come to a new equilibrium,” Steckler says.