The 6.3-magnitude tremor hit Turkey’s southern Hatay province, near the Syrian border, on the evening of February 20, killing at least 6 people and injuring hundreds more.

The aftershock was not unexpected following the large earthquake. Aftershocks occur close by to where an original earthquake took place, and are due to the fault readjusting itself after the sudden slip.

David Rothery, a professor of Planetary Geosciences at The Open University, told Newsweek: “Aftershocks are inevitable. [This aftershock] was unusually big, but not exceptional.”

1 of 1 Photos in Gallery ©KONSTANTINOS TSAKALIDIS / Contributor Damage Turkey earthquake

The U.S. Geological Survey (USGS) initially recorded the aftershock as being magnitude 6.4, but later lowered that score.

It was the largest of 90 aftershocks that occurred yesterday. The second largest was 5.8. Aftershocks usually continue for a few weeks after the initial earthquake.

The first 7.8-magnitude earthquake hit the Turkish city of Gaziantep at 4.17 a.m. local time on February 6. It was felt by people living all across the wider area, including neighboring countries like Syria, Israel and Lebanon. The death toll continued rising for days afterwards, as rescue efforts continued pulling people from rubble and fallen buildings. Over 46,000 people have now died.

A map showing where the latest Turkey earthquake struck

“After a magnitude 6 like yesterday, I would expect magnitude 5 and magnitude 4 aftershocks for a few days, hopefully dwindling quickly,” Rothery said.

Aftershocks are usually smaller than the original quake. Ones that are a magnitude 5 or larger are quite substantial and can cause damage to buildings and endanger human life.

Why is Turkey having so many earthquakes?

Turkey is one of the most seismically active countries in the world, being located between two major tectonic plates.

These particular earthquakes are being caused by the East Anatolian Fault. The faultline extends along the southeast of Turkey from a junction on the North Anatolian Fault, which is larger and extends to the north.

The East Anatolian Fault had only caused a handful of earthquakes previously.

The last notable earthquake that occurred in this region happened on January 24, 2020, in Elazığ, at a magnitude of 6.7. This earthquake killed 41 people and injured over 1,600.

“There is a major fault there where two tectonic plates slide past each other. The motion goes in fits and starts, because of resistance to motion along the fault,” Rothery said. “Once the accumulated stress is sufficient, things give way—that was the initial quake—and then nearby bits of fault begin to fail too.”

“In the long term, it is no more active than many other plate boundaries.”

Although earthquakes in southern Turkey are not usually common, scientists believe that the 7.8 earthquake could potentially have been building for thousands of years.

Scientists say that three previous earthquakes in particular—which occurred in the East Anatolian region in 1513, 1872 and 1893—may have built up the stresses on the fault line for years.

In a 2013 study by Duman and Emre, published by Geological Society London Special Publications, scientists highlighted segments of the fault they believed had ruptured in the past during these three previous earthquakes.

Ruptures occur when strong earthquakes move the fault deep within the earth, causing it to break through the surface. If Duman and Emre’s study is correct, it shows that the recent 7.8 earthquake included all the segments of the fault that ruptured in 1513, 1872, and 1893. In theory, this 7.8 earthquake could have been building up following these earthquakes.

Caroline M. Eakin, a fellow and senior lecturer at the Research School of Earth Sciences at Australian National University, previously told Newsweek: “One thing we can consider however is the history of large earthquakes previously along the East Anatolian Fault. In basic terms, sections of the fault that last ruptured a long time ago, have had time to build up stress and are in theory most due to rupture again.

“However the intervals between large magnitude earthquakes are not regular, it could be 100 years for 1 earthquake cycle and then 1000 years for the next. It seems the last time this section of the East Anatolian Fault had an M7+ earthquake was 130 years ago.”