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Explained: How two major fault lines make Turkey prone to devastating earthquakes

Turkey sits on the East Anatolian and North Anatolian fault lines

turkey earthquake The epicentre of the first quake was at Gaziantep in southern Turkey | Reuters

A powerful earthquake of magnitude 7.8 on the Ritcher scale hit Turkey and Syria early Monday morning, crumbling buildings and killing thousands. Two more tremors of magnitude 7.5 and 6 followed within 24 hours, bringing devastation. Over 4,300 people have died as of Tuesday morning and the United Nations estimates the toll to rise eight-fold.

The epicentre of the first quake was at Gaziantep, a city of more than two million inhabitants in southern Turkey. In addition to this, millions of Syrian refugees live outside the city of Gaziantep. A New York Times report stated that Turkey hosts the largest number of Syrian refugees in the world, 3.6 million, according to the United Nations High Commissioner for Refugees, which runs one of its largest operations from Gaziantep.

Why were the earthquakes so severe

The earth's outermost layer comprises of massive slabs called tectonic plates. Boundaries between tectonic plates are made up of a system of faults—cracks caused by stress as the plates move, triggering earthquakes. Turkey sits on two major fault lines—East Anatolian and North Anatolian—positioning it in one of the world's most active earthquake zones.

Though there have been massive earthquakes on the North Anatolian fault zone, the jolts on Monday were in the East Anatolian fault zone.

The first quake on Monday was similar to the one that killed 30,000 people in Turkey in December 1939. During the 20th century, the East Anatolian Fault yielded little major seismic activity, Reuters reported. "If we were going simply by (major) earthquakes that were recorded by seismometers, it would look more or less blank," Roger Musson, an honorary research associate at the British Geological Survey, told Reuters.

The East Anatolian is strike-slip fault. Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. Experts say that the seismic activity is spreading to neighbouring faults, and aftershocks can be expected.

Chris Elders, professor at Australia’s Curtin University, told Al Jazeera that the depth of the initial earthquake at about 18km also contributed to making it particularly devastating. The quake at a shallow depth was felt on the surface with much greater intensity. Strike-slip faults, like the East Anatolian, are known to create shallow earthquakes. 

Despite being quake-prone, the buildings in Turkey were not designed to withstand the calamity. Though legislation was approved in 1999 to enforce mandatory design checks, very few buildings were constructed according to earthquake-resistant design codes.

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