10,000 tremors

Thousands of earthquakes struck southern Turkey in the weeks after a 7.8 magnitude earthquake on Feb. 6, killing more than 50,000 people in Turkey and northwest Syria.

These additional quakes, called aftershocks, are common after a large earthquake; many aftershocks can be small relative to the main quake, but some have the potential to be severe and destructive, as was the case in Turkey.

A powerful 6.4 magnitude aftershock struck near Antakya city in southern Turkey on Feb. 20, two weeks after the main quake, killing at least six. Another 5.6 magnitude quake struck near Malatya on Feb. 27, killing one person, injuring 110 and causing 29 buildings to collapse.

More than 570 aftershocks were recorded within 24 hours of the main quake on Feb. 6, and more than 10,000 were recorded in the three weeks afterward. The frequency and intensity of the aftershocks has diminished since then, but the temblors continue to be a threat.

“There’s a short decrease in overall number at the beginning, but there’s a long tail,” said Susan Hough, a seismologist at the United States Geological Survey (USGS).

The Turkey quake also triggered a magnitude 7.5 earthquake that caused a separate rupture in the Earth’s surface, which in turn caused thousands of aftershocks.

Domino effect

Seismologists define aftershocks as temblors triggered by a large earthquake, close in time and location. Because many of these aftershocks are themselves large, they start their own chain of additional aftershocks.

“You can think of every earthquake as a parent event that has a probability of triggering daughter events, mostly smaller but occasionally large,” Hough said.

More than 230 aftershocks occurred near the fault that ruptured during the main quake before the 7.5 magnitude temblor struck. Many aftershocks then occurred close to the second rupture as well.

“Every earthquake or aftershock has a probability of triggering subsequent events, either smaller or larger,” said Hough. “But on average the largest aftershock is about 1 unit smaller than the main shock.”

It can be difficult to associate all aftershocks with their parent events because of how many there are - in this case, thousands. However, the pattern can be clearly visualised around Antakya, where the magnitude 6.4 aftershock on Feb. 20 caused dozens of smaller quakes.

Larger earthquakes generally cause aftershocks in a wider area, according to Harsh Gupta, former director of India’s National Geophysical Research Institute.

Aftershocks that followed the two major quakes on Feb. 6 were spread across hundreds of kilometres along and beyond the ruptured faults, including around Antakya. But aftershocks that followed 6.4 magnitude quake on Feb. 20 were more clustered around that temblor’s epicentre - another indication they were connected to that quake and not the earlier ones.

Deadly aftershocks

Aftershocks, even if smaller in magnitude, pose a concern in regions devastated by an earthquake, disrupting rescue and relief work as well as causing further damage.

Buildings that withstood the 7.8 earthquake might have been too weak to survive a strong aftershock, Gupta said. “Repeated jolting, repeated movement can make a difference,” he said.

According to a Reuters analysis, more than 11 million people live in areas that witnessed strong shaking – registering at least six on the Modified Mercalli Intensity scale – from one or more of the earthquakes or aftershocks. About 5.3 million people experienced strong shaking from two earthquakes, and 230,000 experienced it from three earthquakes.

Aftershocks can pose a threat to buildings already damaged, further endangering anyone trapped inside or working to rescue victims.

Aftershocks hampered rescue operations in Turkey’s Kahramanmaras province, the Russian Emergencies Ministry said in the days following the quake, according to Russian news agency TASS. “The rescuers have to deal with huge piles of debris, ruined buildings and wobbly structures amid ongoing aftershocks,” the TASS report said.

Aftershock anxiety

Aftershocks, regardless of their potential to cause damage, also affect the population psychologically, Hough said.

“One issue with perception of shaking is that even a large earthquake might generate relatively weak shaking at first, which continues to build over time and can become quite strong; so if you feel a small-magnitude earthquake, you don’t immediately know if it’s small or the start of a larger earthquake,” she said.

Huvva Tuncay, who was living in a tent in the center of Antakya when the Feb. 20 earthquake hit, said: “I cannot sleep at night. Is the same thing going to happen, are we going to experience another earthquake? We are very scared."

The chart below shows more than 150 aftershocks that occurred within 30 kilometres of Antakya in the 24 hours after the 6.4 magnitude aftershock.

Indian doctors who established a field hospital in Turkey have said they were treating increasing numbers of patients suffering from post-traumatic stress disorder and panic attacks after the quake.

Aftershocks hamper psychological recovery, said Ayse Bilge Selcuk, a professor of psychology in Turkey.

“If these aftershocks did not happen, we would expect psychological symptoms to get weaker over the following weeks and months,” Selcuk said. “But the aftershocks trigger their anxieties and fears repeatedly, so the natural healing process is in a way impeded.”

She said building resilience and teaching people how to prepare for aftershocks can help them cope with the psychological impact of the tremors.

Lasting danger

Aftershocks can occur for weeks, months or in some cases years after a major earthquake, Hough said, even as the frequency and intensity generally decrease over time.

Harold Tobin, director of the Pacific Northwest Seismic Network and professor at the University of Washington, said in a statement that more large earthquakes in Turkey could not be ruled out. “Adjacent segments of the faults could still have built-up strain to be released,” he said.

The USGS forecasts a 90% probability that the area already affected by the main shock will see moderate to large aftershocks of between magnitude 5 and 6 until March 10. A scenario with about a 10% probability would see one or more additional aftershocks larger than magnitude 7. The least likely scenario, with about a 1% probability, would see an aftershock of magnitude 7.8 or larger.

The frequency and intensity of aftershocks varies with factors such as the intensity and depth of the main shock, and plate tectonics. The chart below shows aftershocks of magnitude 4 and above that originated within 250km of the main shock from some of the deadliest earthquakes since 2010. Foreshocks, small earthquakes that occur before a larger one, preceded some of these large quakes.

The actual number of aftershocks in some of these cases could have been many more than what is recorded because of sparse seismometer coverage in certain areas, especially in the sea, said Margarita Segou, a seismologist with the British Geological Survey.

Earthquakes that originate deeper underground tend to see fewer aftershocks, she said.

“Distribution of aftershocks varies from location to location, and on factors such as plate tectonics and on the kind of faults that are involved in earthquakes,” Gupta said. “Thumb rule is the larger the magnitude of the earthquake, the longer the duration of aftershocks.”

Dawn Lehman, professor of civil and environmental engineering at the University of Washington, said aftershocks and their effects were crucial to consider - especially in terms of protecting victims from building collapses after the main danger appeared to have passed.

“Clearly we have to think about the magnitude of aftershocks and simple mechanisms to reinforce brittle structures,” Lehman wrote.