Why Have So Many Earthquakes Hit Turkey and Syria?
Earthquakes in Syria and Turkey are common, but the magnitude 7.8 that shook the region on 6 February at 4:17am local time is clearly impressive. To find earthquakes this strong on this particular fault, we would have to go back to the year 1114.
Ten minutes after the strongest earthquake, an aftershock of magnitude 6.7 struck near the epicentre. “Aftershocks” are earthquakes that occur after every major earthquake, and their statistical behaviour is well known. At the time of writing, others continue to affect an area stretching over 350 kilometres from eastern Turkey to the Syrian border.
|The Geology Behind the Deadly Earthquakes in Turkey and Syria|
More surprisingly and dramatically, a second earthquake of magnitude 7.5 struck at 1:24pm local time, further north. This earthquake was not an aftershock: according to the first data processed live by the major international seismological agencies, it would have occurred on an east-west fault crossing-cutting the main rupture trace.
Seismologists consider Turkey a tectonically active area, where three tectonic plates—the Anatolia, Arabia, and Africa plates—touch and interact with each other. The two major fault lines surrounding it, the North Anatolian Fault and the East Anatolian Fault—which has a slip rate of between 6 and 10 millimeters per year—are gradually squeezing the country westward toward the Mediterranean Sea. Yet, many buildings in the region are not built to withstand large earthquakes, according to the US Geological Survey (USGS), making the destruction worse.
The Anatolian Plate is a continental tectonic plate that is separated from the Eurasian plate and the Arabian plate by the North Anatolian Fault and the East Anatolian Fault respectively. Most of the country of Turkey is located on the Anatolian plate. Most significant earthquakes in the region have historically occurred along the northern fault, such as the 1939 Erzincan earthquake. The devastating 2023 Turkey–Syria earthquake occurred along the active East Anatolian fault at a strike slip fault where the Arabian plate is sliding past the Anatolian plate horizontally.
The Anatolian transform fault system is "probably the most active in the world".The East Anatolian Fault, a left lateral transform fault, forms a boundary with the Arabian Plate. To the south and southwest is a convergent boundary with the African Plate. This convergence manifests in compressive features within the oceanic crust beneath the Mediterranean as well as within the continental crust of Anatolia itself, and also by what are generally considered to be subduction zones along the Hellenic and Cyprus arcs. The northern edge is a transform boundary with the Eurasian Plate, forming the North Anatolian Fault Zone (NAFZ).
Research indicates that the Anatolian Plate is rotating counterclockwise as it is being pushed west by the Arabian Plate, impeded from any northerly movement by the Eurasian Plate. In some references, the Anatolian Plate is referred to as a "block" of continental crust still coupled to the Eurasian Plate. But studies of the North Anatolian Fault indicate that Anatolia is de-coupled from the Eurasian Plate. It is now being squeezed by the Arabian Plate from the east and forced toward the west as the Eurasian Plate to its north is blocking motion in that direction. The African Plate is subducting beneath the Anatolian Plate along the Cyprus and Hellenic Arcs offshore in the Mediterranean Sea.
Earthquakes may trigger other earthquakes
An earthquake happens when a fault – a fracture in the first kilometres of the Earth’s crust – slips rapidly, within seconds, abruptly releasing the energy that has been growing over tens to hundreds of years by the slow motion of tectonic plates. When this happens, the released energy leads to shaking of the ground: the earthquake.
Some earthquakes are linked to each other: when the fault breaks, earthquakes release part of the energy and reorganise part of it in the Earths’ crust, which can trigger new earthquakes. For instance, one can consider the series of earthquakes of magnitude greater than 7 that have cascaded from east to west for about 800 kilometres over the course of the 20th century along the North Anatolian fault. Each earthquake brought the nearby fault segment of the North Anatolian fault closer to a rupture.
The notable point is that the entire length of the North Anatolian Fault ruptured between 1939 and 1999. The last unbroken segment runs across the Sea of Marmara, very close to Istanbul, between the epicenters of the 1999 Izmit and 1912 Ganos earthquakes.
If a fault section is already well loaded (close to rupture), when a big earthquake hits nearby, then a second earthquake might happen. Otherwise, it will be necessary to wait for the motion of tectonic plates to bring the remaining energy necessary to trigger an earthquake. This is called “static triggering”.
When giant earthquakes trigger other earthquakes… at a distance
There is also a type of triggering known as “dynamic”. In some cases, the energy excess resulting from a large earthquake is not large enough to explain the occurrence of certain earthquakes, especially if they are located far away from the epicentre of the main shock.
For example, following the 1992 Landers and 1999 Hector Mine earthquakes in California, earthquake swarms were observed several hundreds of kilometres from the epicentre. It has been shown that these earthquakes occurred exactly during the passage of the strongest seismic waves emitted by these two earthquakes. At such distances, this shaking, the seismic waves, cannot be felt by humans, but apparently, seismic faults do.
Similar observations have been made in the laboratory to show that during the passage of these seismic waves, the material that makes up the core of the fault weakens, causing a sudden sliding, i.e., an earthquake.
This kind of behaviour comes from the physics of granular media, which behave like fluids while being shaken. Shaking a pile of sand quickly will cause it to flatten under its own weight, whereas without the shaking, it would stand still.
So shaking a fault quickly can cause it to slip, producing earthquakes. It has also been observed that these seismic waves can trigger slow slippage at colossal distances. The seismic waves emitted by the Maule earthquake, a magnitude 8.9 earthquake that struck Chile in 2010, caused a slow earthquake along the Mexico subduction, about 7,000 kilometres from the epicentre.