Dhe catastrophic earthquake in the past week in the Turkish-Syrian border area, with a magnitude of 7.8, was not only the most severe earthquake in Turkey for more than eight decades. It also challenges some common assumptions of geophysicists about the tectonic situation in the Levant and about the origin and propagation of earthquake fractures. It is widely believed that the February 6 earthquake and most of the aftershocks occurred at the East Anatolian Fault.
The Anatolian crustal plate scrapes along the Arabian Plate on its way west along this almost 700-kilometer-long line running from southwest to northeast through southern Turkey. Geoscientists are largely agreed that the East Anatolian Fault terminates to the northeast in the Karliova Basin of eastern Turkey. Because there it meets the north-west running North Anatolian Fault.
On the other hand, opinions differ as to how far the East Anatolian Line extends to the southwest. Up to the vicinity of the cities of Kahramanmaraş and Türkoğlu, which were severely damaged by the recent earthquake, the course is largely straight. From there, according to some geophysicists, the East Anatolian Fault continues in the same direction into the Mediterranean Sea in the Bay of Iskenderun, where it meets another fault, namely the Cyprian Arc. Other researchers say the fault ends at Kahramanmaraş, where it meets the northern reaches of the Dead Sea Fault, which runs south through Syria, Lebanon and Israel to the Red Sea.
A reason speaking for the second hypothesis is the fact that the area around Kahramanmaraş and Türkoğlu is considered one of the so-called triple points of the earth. This term comes from the theory of plate tectonics and describes those points on the earth’s surface where three crustal plates meet. One of the best-studied triple points is in northern California, where the North American, Pacific, and Juan de Fuca Plates meet. Its location is known up to a few kilometers. It is located on Cape Mendocino, the westernmost tip of California.
Acceleration sensors and GPS measure the epicenter
In contrast, the location of the meeting point of the three crustal plates active in the Levant – the African, the Arabian and the Anatolian plates – is somewhat diffuse. It is somewhere between Kahramanmaraş and the city of Gaziantep, which was also badly damaged by the earthquake. In the technical literature, however, it is always described as the triple point of Kahramanmaraş or shortened as the Maraş triple point. The epicenter of the severe earthquake was exactly in the area where the triple point is also suspected.
The controversy over the southwest end of the East Anatolian Fault is compounded by measurements of the rate of plate motion along this line using GPS receivers. According to a description last year by a Turkish research group from the Technical University in Istanbul, the velocity in the north-east of the fault was about ten millimeters per year. Towards the southwest, however, it steadily decreased and in the area of Kahramanmaraş was only less than four millimeters per year. Sezim Güvercin and his colleagues concluded that the tectonic stresses in this area had accumulated to such an extent that a severe earthquake could occur there in the foreseeable future. This is exactly what happened on Monday February 6th at 4:17 am local time.
The tremors on the earth’s surface caused by the quake were exceptionally strong. According to the accelerometer in a measuring network operated by the Turkish Government Agency for Disaster Management (AFAD), the acting forces increased by up to 1.4 times the gravity of the earth, called “1.4 g” for short. The values in the focus area were consistently more than 0.6 g. This also explains the extremely severe degree of destruction, because the first damage to buildings usually occurs at 0.2 g.