Why the ground shakes so often in Iran

The epicenter of  the earthquake on November 12, 2017 was in the middle of the Zagros Mountains in the western Iranian province of Kermanshah, near the Iraqi town of Halabja. It reached a magnitude of 7.3 on the Richter scale.

Shaped by forces of nature

The mountain range reaches from the border with Turkey all the way down to the strait of Hormus. It grew because the Arabian Plate submerges under the Iranian Plate at a speed of 3.5 centimeters per year.

The Iranian Plate is also considered a "micro plate," because it is actually a part of the larger Eurasian plate, albeit somewhat separated.

Between the Eurasian and Arab Plates it is practically squeezed - like in a vice. That's why the mountain range keeps rising.

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The Zagros emerged along with the main high mountain ranges of the northern hemisphere - the Pyrenees, the Alps, the Caucasus and Himalayas. Also, plate tectonics at the same time resulted in islands such as Sumatra and Java.

This process started about 100 million years ago, reaching a highpoint in speed of growth around 20 million years ago. Today, the mountains continue to grow, albeit much slower.

First big quake in Kermanshah

No comparable earthquakes have been recorded in the immediate area of the current epicenter in Kermanshah province. While there are examples of historical quakes in the larger region of Iran and Iraq, none have occurred in that specific part of the mountain range.

More often, the earth shook in the northern regions of Iran - close to the Caucasus. That's were an earth quake near Tabriz in 2012 reached a magnitude of 6.4.

Also, along the Caspian Sea, earthquakes are more common. Near the cities of Rudbar and Rasht, an earthquake with a magnitude of 7.4 took more than 40,000 human lives in 1990.

It was one of the heaviest quakes Iran has ever experienced. The epicenter was roughly 400 kilometers north of the current quake. In the border region with Turkmenistan, the border is also shaking often, as was the case repeatedly near town of Shar-e Qumis.

On the Iraqi side, several severe earthquakes have been recorded, including one in 871, in the town of Wasit with about 20.000 victims, and another in 1007 in Baghdad. Both towns are about 250 kilometers away from the latest quake, and neither is located in the mountains.

Vast endangered area

The area in which earthquakes can take place in the Zagros mountain range is huge. In 2003, an earthquake with magnitude 6.6 devastated the city of Bam - more than 800 kilometers south of the latest quake. In 2006,  there was a tremor of magnitude 5.8 directly at the strait of Hormus, and in 2010 another one far in the south - in the province of Kerman.

"What is special about Iran is that the seismic belt in the collision zone extends to a very vast area," seismologist Torsten Dahm from the German Research Center for Geosciences, GFZ Potsdam, told Deutsche Welle.

The area where earthquakes can strike is more than 1600 kilometers long and 400 kilometers wide. "It is not one single sharp plate boundary, but there are numerous fracture zones," Dahm explains. "That's what makes Iran different from the subduction zones."

Subduction zones are plate boundaries where huge oceanic plates submerge under the continental plates, deep under the earth's mantle - like around the Pacific Ring of Fire.

That's where earth quakes are often even stronger - sometimes with magnitudes exceeding 9.0 on the Richter scale. But that does not mean, however, that the seismic danger for people living in Iran is any less grave. After all, there is an important difference between these geologic formations: "At the Iranian collision zone, the tectonic plates push into each other in more shallow layers. Therefore, there are earthquakes in small depth of only 15 to 30 Kilometers," Dahm says.

In the Pacific, however, quakes can emerge in depths of several hundred kilometers. "And, for example, an earthquake in a small depth will cause considerably more damage than another earthquake of the same magnitude in a greater depth," the seismology professor says.

The bottom line for Iran remains; the seismic danger is high throughout the country.