“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.