Ankara: Turkey and Syria can't catch a break. Since Monday's devastating quake, aftershocks have ripped through the region. Altogether some 5,000 people have been announced dead so far. The number is expected to rise because countless people are still trapped below the rubble — and because continued aftershocks will cause already-unstable buildings to collapse.
What is an aftershock?
Large earthquakes are almost always followed by small earthquakes called aftershocks. They are generally strongest in the 48 hours following the main quake and can last weeks and even years in some cases.
Typically, the magnitude of aftershocks starts around one degree less than the initial event — so, for example, if the quake has a magnitude of 7, seismologists might expect an aftershock of 6.
"That's the average occurrence but it sometimes happens that it isn't like that at all," seismologist Roger Musson, an honorary research associate with the British Geological Survey, told DW. "Sometimes you get an aftershock that's actually larger than the main shock. So as a seismologist, you always have to be prepared to be surprised by what the Earth throws at you."
In the Turkey-Syria border region, the aftershocks have been almost as strong as the initial quake.
An earthquake is considered an aftershock and not an individual quake when it occurs between 1 and 2 fault lines away from a preceding earthquake. Generally, aftershocks are the result of the Earth's tectonic plates trying to shift back into place along a fault line.
More than 100 aftershock earthquakes have occurred since the initial earthquake in Turkey and Syria.
According to Musson, aftershocks may not occur if the initial earthquake is very small. But in the case of larger earthquakes, they are a given.
Why was this aftershock so much bigger than normal?
The 7.5-magnitude aftershock that occurred after Monday's 7.8-magnitude earthquake is significantly stronger than typical aftershocks.
Musson, who also does research at the University of Edinburgh in the UK, said that is because rather than occurring along a single fault, the initial earthquake can be better described as having occurred in the East Anatolian fault zone. This is like a large fault line with several minor fault lines congregating around it, he said.
The main quake on Monday triggered what Musson called a "swarm" of earthquakes happening along new faults coming out of the main fault rather than the typical aftershock movement, which takes place along the same fault as the initial earthquake.
When do aftershocks occur? And for how long?
This depends on the size of the initial earthquake. Although the more intense aftershocks will probably stop around two days after the initial quake, according to Musson, it is possible for the aftershocks to gradually decay, but not completely cease until a year after the main quake.
Other research suggests that in some areas, like the New Madrid fault in the US state of Missouri, small aftershocks can continue for centuries after the initial shock.
How high is the risk of more big aftershocks in Turkey and Syria?
We will have a better idea of the answer to this in a day or two. The activation of the fault where the 7.5 aftershock occurred could in turn produce aftershocks of its own, which means we are not in the clear yet. Even when they are smaller in magnitude, aftershocks can still be devastating. A small tremor may be enough to lead an already half-demolished building to collapse, causing major destruction.
"An aftershock will often do a disproportionate amount of damage relative to its size, just because the buildings are in a weakened state," said Musson. "This is one reason why, after major earthquakes, civil defense authorities will go around and mark buildings which are in a damaged state and therefore unsafe, warning people not to get back into them."