This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
2017's deadliest earthquake (so far) struck Iran at 9:18pm local time on November 12th. As of this writing, 530 people have died in Iran, with a further 10 confirmed dead in Iraq. Almost 8,000 Iranians and over 400 Iraqis are injured, and almost a hundred thousand people have been left homeless. Those numbers will undoubtedly climb. Victims desperately need aid, and you can find many ways to help them here.
When Plates Collide
Unlike the Mexico and Chile quakes we've discussed recently, the Iran-Iraq Earthquake wasn't caused by subduction. Here in the Zagros Mountains, two continental plates are colliding. When masses of continental crust meet, neither is eager to dive beneath the other: there's generally nowhere to go but up. These mountains along the border of Iran and Iraq, marching through southeastern Turkey and terminating near the Strait of Hormuz, testify to profound changes in the earth's crust. What used to be a basin full of sedimentary rocks has been buckled and thrust skyward into the largest mountain range in the region. You can get a sense of the scale from this photo taken from the International Space Station. The photo below shows you these mountains up close: steep flanks rising almost vertically into the sky, a classic fold-and-thrust belt.
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Zagros Mountains. Credit: Farid Atar (CC BY 3.0)
The Zagros Mountains have been growing for the last twenty million or so years. They are made from earthquakes: they are born from a maze of thrust, normal, and reverse faults. Sedimentary rocks, crumpling as the Arabian plate moves inexorably north against the Eurasian plate at a rate of about 26 millimeters (one inch) per year. This may not sound like much, but it's actually fairly vigorous by geologic standards. And it's not all smooth, constant motion. The stresses that build up are released, sometimes catastrophically, in earthquakes that cause sudden devastation to the people who make these mountains home.
A Regional Event
Diagrams and pinpoints on maps make us think of earthquakes as discrete entities, happening on a slender line in a constrained place. It's hard to envision what they actually are, but let's try to grasp the scale of the thing.
The Iran-Iraq earthquake was a magnitude 7.3 event. That's a phenomenal release of energy - it would take 1,344,028 tons of TNT to match it. The USGS states that quakes of this type and magnitude are about 65 kilometers long by 25 kilometers wide (40 miles by 15.5 miles for us heathens still on the imperial units). That's the actual earthquake, mind you: the area shaken by it is enormous. This one was felt all the way to the Mediterranean.
Shake map of the Iran-Iraq quake. Credit: USGS
This is considered a shallow earthquake: it happened about 25 kilometers down. And it's an interesting type of faulting: an oblique-thrust, which is something of a hybrid between a strike-slip and a thrust fault. It went a bit vertical, and a bit sideways, all at once.
Why So Destructive?
Iran, like many countries, doesn't have the regulations and enforcement it needs to earthquake-proof its buildings. Homes, commercial buildings, and infrastructure are often poorly-constructed with materials that aren't designed to resist the terrific power of sudden earth movements. And in the Kurdish mountains where the quake struck, many homes are built of mud brick: a cheap, easy, adequate material that works very well up until the hard shaking starts.
Because of the mountainous terrain, the earthquake and its aftershocks have set off landslides, secondary disasters that make getting rescue and aid to affected areas more difficult.
Will there be larger earthquakes?
It's possible, but they're not likely to be too much bigger. The type of faults in this area of Iran may only be capable of events up to this magnitude. That is probably cold comfort to the people suffering there, but if you compare and contrast subduction zone quakes like Mexico's Chiapas earthquake (M8.2) with this one, you can see the staggering difference in energy: Chiapas was over twenty-two times as strong.
This isn't the last we'll hear of earthquakes from Iran. Most of the country is prone to them, and the capitol city, Tehran, is in such a dangerous area that there's been frequent talk of moving the capitol somewhere safer. Keep your compassion flowing. The citizens of Iran need all they can get.
*Edited to correct the TNT tonnage.