A major magnitude-7.8 earthquake, followed by another strong quake, has devastated wide swathes of Türkiye and Syria, killing more than 6,000 people.
Seismologists say it is likely to be the deadliest this decade, with a rupture measuring more than 100 kilometres between the Anatolian and Arabian plates.
But what type of earthquake caused the destruction and why was it so devastating?
Where did the earthquake originate?
The epicentre was about 26km east of the Turkish city of Nurdağı, at a depth of about 18km on the East Anatolian Fault. The quake radiated towards the north-east, bringing devastation to central Türkiye and Syria.
In the first 11 hours after the initial quake, the region felt 13 significant aftershocks with a magnitude of at least 5.0, Alex Hatem, a US Geological Survey (USGS) research geologist says.
Another strong quake — magnitude 7.5 — hit Türkiye nine hours after the main jolt. Scientists agree that the two quakes were related.
What type of quake was it?
Researchers say the earthquake was a strike-slip quake, where two tectonic plates slide past each other horizontally.
The Earth is divided up into different pieces, “kind of like a jigsaw puzzle,” Eric Sandvol, a seismologist at the University of Missouri, says.
Those pieces meet at fault lines, where the plates usually grind against each other slowly. But once enough tension builds up, they can snap past each other quickly, releasing a large amount of energy.
In this case, one plate moved west while the other moved east — jerking past each other to create the quake, Dr Hatem says.
Are earthquakes common in this area?
The quake occurred in a seismically active area known as the East Anatolian fault zone, which has produced damaging earthquakes in the past.
"Almost all of Türkiye is really seismically active," Professor Sandvol said. "This is not something new to the country."
Only three earthquakes have registered above 6.0 on the Richter Scale since 1970 in the area, according to the US Geological Survey. But in 1822, a 7.0-magnitude quake hit the region, killing an estimated 20,000 people.
Türkiye was struck by a major earthquake in January 2020 — a magnitude-6.7 event that caused significant damage in the eastern part of the country.
In 1999, a 7.4-magnitude quake struck near Istanbul and killed an estimated 18,000 people.
How bad was it?
On average, there are fewer than 20 earthquakes over 7.0 magnitude in any year, making Monday's event severe.
Compared with the 6.2-magnitude earthquake that hit central Italy in 2016 and killed some 300 people, the Türkiye-Syria earthquake released 250 times as much energy, according to Joanna Faure Walker, head of the University College London Institute for Risk and Disaster Reduction.
Only two of the deadliest earthquakes from 2013 to 2022 were of the same magnitude as Monday's quake.
Why was this one so devastating?
The earthquake was powerful — especially for a quake that hit on land. Typically, very strong earthquakes occur underwater, Margarita Segou, a seismologist with the British Geological Survey, says.
On top of that, the quake hit near heavily populated areas. The epicentre was near Gaziantep, a major city and provincial capital in Türkiye.
Kishor Jaiswal, a USGS structural engineer, says the affected regions are home to vulnerable buildings.
While new buildings in cities like Istanbul were designed with modern earthquake standards in mind, this area of southern Türkiye has many older high-rise buildings, Mr Jaiswal says.
Rapid construction in Syria — plus years of war — may have also left structures vulnerable, researchers say.
Officials reported thousands of buildings collapsed in the wake of the earthquake. They included "pancake" collapses, where upper floors of a building fall straight down onto the lower floors — a sign that the buildings couldn't absorb the shaking, Mr Jaiswal says.
Rescue efforts have been hampered by freezing temperatures and traffic jams from residents trying to leave quake-stricken areas.
What kind of aftershocks can be expected?
Only 11 minutes after the initial quake, the region was hit by a magnitude-6.7 aftershock. A magnitude-7.5 quake came hours later, followed by another spasm in the afternoon, measuring 6.0 on the Richter Scale.
Earthquakes of similar magnitudes in populated areas have killed thousands of people. Nepal's magnitude-7.8 earthquake in 2015 claimed nearly 9,000 lives.
"More aftershocks are certainly expected, given the size of the main shock," Dr Hatem, from the USGS, says.
"We expect aftershocks to continue in the coming days, weeks and months."
What might the final death toll be?
Roger Musson, an honorary research associate at the British Geological Survey says the rising death toll could be significant.
"It will be in the thousands, and could be in the tens of thousands," Mr Musson says.
Cold winter weather, he says, means that people trapped under rubble have less chance of survival.
Catherine Smallwood, the WHO's senior emergency officer for Europe, says there is continued potential for further collapses.
"We always see the same thing with earthquakes, unfortunately, which is that the initial reports of the numbers of people who have died or who have been injured will increase quite significantly in the week that follows," she says.
ABC/wires
