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Turkey has been getting hammered by a series of powerful earthquakes since the early hours of Monday, with the first one, a quake of magnitude 7.8, being described as the strongest the country has experienced in over a century.
Within a span of 12 hours, at least 41 more earthquakes of magnitude 4 or more have been recorded in the same area — southeastern Turkey close to the borders with Syria — according to information on the United States Geological Survey (USGS) website. One of these later earthquakes was as large as the first one, measuring 7.5 in magnitude.
Both Turkey and Syria have been badly affected by the tremors, with news agencies reporting the death of at least 1,700 people by Monday evening. Over 1,000 of these casualties have been reported from Turkey.
As is expected in big earthquakes, aftershocks are likely to continue for the next few days, even weeks.
Turkey and Syria lie in a seismically active region
The region where the earthquake has struck lies along a well known seismic fault line called the Anatolia tectonic block that runs through northern, central, and eastern Turkey.
It is a seismically active zone — though not as active as, say, the Himalayan region which is one of the most dangerous regions in the world from the perspective of earthquakes.
Large earthquakes, of magnitude 5 or higher, have not been very frequent in recent years. According to USGS, only three earthquakes of magnitude 6 or more have happened in the region since 1970. The last major quake in this area came in January 2020.
The seismicity in this region is a result of interactions between the African, Eurasian, and Arabian plates. The Arabian plate is known to be pushing northward, which results in a slight westward movement for the Anatolian plate, where Turkey is located.
The USGS said Monday’s earthquake happened around the near-vertical fault line on the eastern Anatolian block, close to the Syrian border.
“The mechanism and location of the earthquake are consistent with the earthquake having occurred on either the East Anatolia fault zone or the Dead Sea transform fault zone. The East Anatolia fault accommodates the westward extrusion of Turkey in the Aegean Sea, while the Dead Sea Transform accommodates the northward motion of the Arabian peninsula relative to the Africa and Eurasia plates,” the USGS said in its event summary.
Do shallow earthquakes cause greater damage?
Monday’s earthquakes emerged from relatively shallow depths which made them devastating. The first earthquake, of magnitude 7.8, originated 17.9 km below the Earth’s surface. All the subsequent ones, including the one of 7.5 magnitude, emerged from even closer to the surface.
Shallow earthquakes are generally more devastating because they carry greater energy when they emerge on the surface.
Deeper earthquakes lose much of their energy by the time they come to the surface. The deeper quakes spread farther though — the seismic waves move conically upwards to the surface — even as they lose energy while travelling greater distances, and hence cause less damage.
The earthquake in Nepal two weeks ago, whose tremors were felt in many parts of northern India, for example, originated about 25 km below the Earth’s surface. It did not cause large scale damage, though one woman was reported to have been killed by a stone that fell from a hill because of the tremors.
But the Nepal earthquake also had a low magnitude of 5.8 — magnitude is the other indicator of how destructive an earthquake will be.
Magnitude is a measure of how big the waves are, while the strength refers to the energy it carries. Magnitude is measured on a logarithmic scale, which means the seismic waves produced by a magnitude 6 earthquake have 10 times higher amplitude than the ones produced by a magnitude 5 earthquake. The energy differential is even higher, 32 times for every change of 1 in magnitude.
This means that the 7.8 magnitude earthquake in Turkey on Monday was 100 times bigger — produced 100 times bigger waves — than the 5.8 earthquake in Nepal, and 1,024 (32 x 32) times more powerful. In general, every change of 0.1 in magnitude results in about 1.4 times change in energy.
Why earthquakes remain unpredictable?
Earthquakes continue to remain the most common natural hazard that cannot be predicted. As such, no early warning systems can be developed.
Theoretically, it is possible to offer a lead time of a few seconds between the time of the origin of the earthquake and the time it reaches the Earth’s surface. Seismic waves travel significantly slower than the speed of light — between 5 and 13 km per second. So if the earthquake is detected as soon as it is triggered, information about it can be related a few seconds ahead of it reaching the ground.
Such systems are already in use in some locations to issue alerts about earthquakes. However, these are not predictions. The alerts are issued post-event.
Attempts to find reliable predictors to earthquakes have not been fruitful so far. Scientists have been able to map the areas that are earthquake prone, and are likely to generate earthquakes in future, but there is no way to predict when.
For example, scientists say the Himalayan region has so much accumulated stress beneath the surface that it could result in multiple 7 or 8 magnitude earthquakes. But it cannot be predicted when that would happen.
Between one and three earthquakes of magnitude 8 or above are recorded every year on average, while 10-15 earthquakes of magnitude between 7 and 8 occur.
AREA AFFECTED BY EARTHQUAKES
SHAKE INTENSITY around Nurdagi in southern Turkey’s Gaziantep province, the epicentre of the 7.8 magnitude earthquake that struck at 4.17 am local time on Monday (map on left). The elongated patch bound by the dark orange line is the area of most severe shaking; the outer green lines mark areas of progressively lighter shaking.
A SECOND EARTHQUAKE measuring 7.5 struck around mid-day, with its epicentre at Ekinozu in Kahramanmaras province, about 80 km to the north of Gaziantep. There are another 40-odd smaller aftershocks as well.
TURKEY, A HOTBED OF SEISMIC ACTIVITY
IN THE EASTERN MEDITERRANEAN REGION comprising Turkey, Syria and Jordan, tectonics are dominated by complex interactions between the African, Arabian, and Eurasian tectonic plates, and the Anatolian tectonic block.
DOMINANT STRUCTURES here are (i) Red Sea Rift, the spreading centre between the African and Arabian plates; (ii) Dead Sea Transform, a major strike-slip fault that also accommodates Africa-Arabia relative motions; (iii) North Anatolia Fault, a right-lateral strike-slip structure in northern Turkey accommodating much of the translational motion of the Anatolia block westwards with respect to Eurasia and Africa; (iv) Cyprian Arc, a convergent boundary between the Africa plate and the Anatolia block.
