Beyond magnitude: A shallow earthquake hammered Turkey

The earthquake in Turkey that killed more than 3,100 people and set off a series of aftershocks ruptured on a shallow fault line just over 11 miles beneath the Earth’s surface, making it one of the most consequential and damaging earthquakes in recent history.

Earthquakes can originate at various depths beneath the Earth’s surface — even hundreds of miles deep. Consequences on the surface can depend on how close the shaking is. Shallower earthquakes can be more destructive.

“Turkey is extremely earthquake-prone, but this is probably the largest earthquake in Turkey in several hundred years,” said Harold Tobin, the director of the Pacific Northwest Seismic Network and a professor in the University of Washington Department of Earth and Space Sciences. “It’s among the largest continental earthquakes.”

The magnitude-7.8 earthquake, which struck along the East Anatolian Fault zone, was followed by several aftershocks, including a magnitude-7.5 earthquake recorded at an even shallower depth. That aftershock, which was uncommonly powerful for an aftershock, most likely ruptured on a nearby, branching fault line within the fault system.

Turkey experienced a 7.8-magnitude earthquake in 1939. The high-powered aftershock is part of what sets Monday’s earthquake apart.

Earthquakes stronger than magnitude 7.0 are not uncommon worldwide — the U.S. Geological Survey has 347 of them in its records since 2000, according to an NBC News analysis of its records — but focusing on magnitude alone can be misleading. Unlike this tragic event, few of the previous quakes ripped on land in well-populated areas and at such shallow depths beneath the surface.

Magnitude can also be a confusing measure, because it uses a logarithmic scale: A 7.8-magnitude earthquake releases nearly 16 times more energy than one measured at 7.0, according to a U.S. Geological Survey tool.

What Turkey and neighboring Syria have experienced is one of the most damaging scenarios conceivable, highlighting the risk for earthquake-prone regions, particularly if buildings aren’t constructed or updated to modern seismic standards.

Men search for survivors in the debris in Adana, Turkey, on Feb. 6, 2023. (Khalil Hamra / AP)
Men search for survivors in the debris in Adana, Turkey, on Feb. 6, 2023. (Khalil Hamra / AP)

“Ten major cities were affected by shaking,” Tobin said. “The scale is remarkable.”

The location of the earthquakes wasn’t a surprise. They ruptured near what seismologists call a “triple-junction” — where the African, Arabian and Anatolian tectonic plates meet. The East Anatolian Fault is a known, mapped fault system.

The East Anatolian, like the San Andreas Fault in California, is a strike-slip fault. The earthquake was the result of stress — and then a slip — as tectonic plates rubbed against one another laterally.

Unlike other types of earthquakes, such as those produced by subduction zones, strike-slip faults are known to produce shallow earthquakes that cause shaking relatively close to the Earth’s surface.

Tobin said it was what he considers a “long” earthquake, meaning energy traveled for a great distance along the fault line.

“The length of the fault and the size of the slip is what generates the very large shaking, which causes such damage,” Tobin said.

In this case, the shaking most likely destabilized another fault line that branches off within the East Anatolian Fault system, touching off a 7.5-magnitude earthquake.

The affected areas in Turkey are especially vulnerable, because many buildings were constructed with unreinforced masonry or brick and concrete that is brittle and unable to withstand strong and prolonged shaking, according to the USGS.

Tobin said early videos from Turkey showed collapsed buildings next to other buildings that appeared to be largely intact, a sign that those that weren’t constructed to modern seismic standards were at great risk, although shaking can vary over short distances.

“This region unfortunately had a great deal of risk of substandard structures for earthquakes, and that’s what we’re seeing play out right now,” Tobin said.

Dozens of aftershocks have been recorded already, and they could be a danger for some time as the network of faults in the area absorbs new changes to the stress in the Earth’s crust.

CORRECTION (Feb. 6, 2023, 7:01 p.m. ET): A previous version of this article misstated the name of the U.S. agency that tracks earthquakes. It is the U.S. Geological Survey, not the U.S. Geological Society.

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