Early Monday morning, a magnitude 7.8 earthquake ripped through Turkey and Syria, followed nine hours later by a 7.5 aftershock. The death toll stands at over 3,800, and rescuers have only just begun to comb through the collapsed buildings. 

Aftershocks will continue to shake the area as local faults adjust to such a huge initial tremor, and scientists say that process could continue for not just days, but months or even years. There’s even a chance—albeit a small one—of an aftershock bigger than the original quake. 

“The aftershock risk is greatest, essentially, right after the mainshock, but there will be noticeable aftershocks to this earthquake for years afterwards,” says David Oglesby, a geophysicist at the University of California, Riverside. “Right now, I can forecast for you that there will be many more aftershocks of magnitude 5, probably 6 or so, in this area. That’s an easy call to make, because historically speaking, statistically speaking, that’s almost guaranteed.”

That will turn a humanitarian crisis in Turkey and Syria into something even more awful. “We can’t say to people: OK, it’s good, you’re done. That was terrible, and it’s over now. Because that’s just not how the Earth works,” says earthquake geologist Wendy Bohon. “It just really sucks to know that these people are going to have to continue to feel shaking from earthquakes for a long time, after they’ve been so traumatized and gone through such a devastating experience.”

Earthquakes are products of plate tectonics: Plates are great masses of rock that move independently in the Earth’s crust, but contact each other along faults. “Eventually, the stress and strain is going to overcome the friction that is holding the rocks together, and those rocks are going to break in an earthquake,” says Bohon. “When the rocks break, they release energy in the form of waves, and those waves are what we feel as shaking.”

The mainshock Monday morning struck along some 125 miles of the East Anatolian Fault, a well-known fault line in southern Turkey. Specifically, this was a “strike-slip” earthquake, meaning stress built up between two masses of rock moving horizontally until the fault ruptured. It was also very shallow underground, meaning it created more intense shaking at the surface. (The San Andreas Fault in California is also a strike-slip fault—that was the one that virtually destroyed San Francisco in 1906.)

Generally speaking, the larger the mainshock, the larger the aftershocks, which tend to decrease in frequency and severity as time goes on. As you can see in this map, aftershocks of various intensities have been swarming along the original quake’s fault line, as well as at a different but connected fault line to the north, where the magnitude 7.5 aftershock seems to have hit. “This is a really complicated system of faults, since the crust is really crushed there,” says Alice Gabriel, a seismologist at the Scripps Institution of Oceanography.

That complexity means that what happens in one fault doesn’t stay there. It may have been that the stress that led to the 7.5 quake had been building for some time, and the jolt from the mainshock unleashed it. “It sort of advanced its clock a bit, so that it had the large earthquake that it would have eventually anyway, probably a bit sooner,” says Austin Elliott, an earthquake geologist at the United States Geological Survey. Such aftershocks are “just simply other earthquakes—there’s nothing that makes them distinct. It’s just that an earthquake so large changes the stress in the Earth’s crust so substantially that it increases the rate of all other earthquakes locally.”

Days or weeks later, monster aftershocks can follow. In Nepal in 2015, for instance, a magnitude 7.8 earthquake was followed 17 days later by a 7.3. The chance of an earthquake producing an aftershock stronger than itself is only about 5 percent, Elliott says—but it’s been well-documented: In July 2019, the magnitude 6.4 earthquake in Ridgecrest, California was followed two days later by a 7.1. (In these cases, the terminology gets extra confusing: At first, the 6.4 earthquake was the mainshock. But when the 7.1 hit, it took over the label of mainshock, and the 6.4 became the foreshock.)

Think of this seismic activity as the geological equivalent of an old house settling after a storm. “It’ll creak for a few days afterwards, as it settles into its new position,” says Bohon. Similarly, both the specific fault where the earthquake happened, as well as separate faults in the vicinity, adjust for days and months afterward. The stronger the storm, the more creaking—and the stronger the mainshock, the stronger the aftershocks. “The earthquake has changed the local stress regime in the crust in that whole area,” says Bohon. “So it will make earthquakes more likely in some places, because it’s added some stress into the system. And it’ll make earthquakes less likely in other areas, because it’s kind of relieved a little bit of the stress.”

For rescue efforts in Turkey and Syria, the likelihood of continued shaking poses real problems. “If the buildings have collapsed in the street, how are you going to get your ambulances through?” asks Oglesby. “How are you going to get your rescuers through? How are you going to get your food through, even to the people who are sheltering in place?” 

“The placement of it is almost a worst-case scenario,” Oglesby continues. “This area, especially in northern Syria, is an area of a tremendous humanitarian crisis already before the earthquake. Here you have some of the most vulnerable people on the planet.”