Powerful aftershock or new tremblor? New Türkiye quake raises questions

A 6.4 magnitude earthquake followed by a 5.8 magnitude aftershock hit Hatay province on February 20 two weeks after the twin earthquakes left tens of thousands dead and affected millions across southern Türkiye and northern Syria.

The Turkish disaster management agency, AFAD, confirmed on February 21 that the latest earthquake caused at least six new deaths and injured 294 people. The earthquake's epicentre was found to be in Hatay's Defne district. It was followed by 90 aftershocks. 

The latest tremblor triggered questions about whether it was a fresh earthquake or a giant aftershock linked to the twin quakes of February 6. The scientific community is divided on this matter.

“I believe it is an aftershock. Because the fault starting from Kahramanmaras, which we call the Amanos segment, was broken into Antakya,” said Asen Sabuncu to TRT World, who is an earthquake scientist working on the ground with a team of other scientists.

The afflicted areas have already suffered thousands of tremors in the last two weeks. 

According to Sabuncu, Monday's tremor had a powerful acceleration rate as previous ones because of years of high energy accumulation in the earthquake zone. 

Monday's earthquake occurred in a significantly shallower part, and as a result of this boundary, it ruptured with what is known as an oblique character, said Sabuncu. 

Due to this oblique structure, some geoscientists might have thought of it as a new fault, but this type of geometry change can be observed at fault boundaries all the time, Sabuncu added.

Such geometry can also be witnessed in fault segments in North Anatolian Fault in the Sea of Marmara because transform faults require this type of geometry, he said.

The typical behavior of aftershocks is usually explained with two principles. The first, known as Omori's Law, predicts that the majority of shocks will occur right after an earthquake and gradually decrease in frequency over time. The other is known as Bath's Law, which indicates that on average, the biggest aftershock is around 1.2 magnitudes smaller than the primary earthquake.

"Twin quakes and aftershocks"

Therefore, when a stronger seismic event occurs after an earthquake, the earlier event is automatically classified as a foreshock. The largest one is always described as the mainshock, and the rest is either a foreshock or an aftershock.

According to a report published by USGS National Earthquake Information Center, the 6th February 7.7-magnitude earthquake was triggered by two faults moving laterally against each other or a strike-slip. In this case, a "left-lateral" strike-shift occurred, as both masses of the earth's crust were moving to the left relative to the other.

Nearly nine hours after the first 7.7 magnitude earthquake struck Kahramanmaras' Pazarcik district at 04:17 on Monday in Türkiye, a second 7.6 magnitude earthquake occurred. Normally this could easily be considered an aftershock (although of unusual intensity). However, experts said that this situation in Türkiye is a more complex one.

Earthquake expert Judith Hubbard said in a statement on the subject, “If we look at the earthquake map of the region and note both the 7.7 magnitude earthquake and the next 7.6 magnitude earthquake, we see two different sets of activity: This is a 7.6 magnitude earthquake on the same fault. This means that there is no aftershock, but rather a secondary mainshock on a different fault triggered by the first 7.7 magnitude earthquake to the south.”

Therefore, the recent 6.4 magnitude quake that struck Hatay province also raised confusion among people on whether it was a third mainshock triggered by previous ones. However, many scientists who shared their comments on social media believe that it was a powerful aftershock. 

Jascha Polet, a seismologist stated that “although according to Omori’s law, the number of aftershocks is expected to decrease with time since the main shock, even aftershocks relatively late in the sequence may be large and damaging.”

Another geologist, Robin Lacassin also highlighted the possibility of powerful aftershocks that can happen weeks and months after the main event. 

According to Sabuncu, the low strength of the ground in the region and the dominance of quaternary alluvium on the foundation of the structures can alter the acceleration values and the sensor values of the earthquake. Therefore, an earthquake that does not have a long duration; causes high oscillations in some regions.

The 6.4 is a fault that rises to the surface and breaks the surface. If it creates a surface fracture another earthquake of this magnitude can be expected in the region, further emphasized by Sabuncu. 

“The length of the fault there is quite continuous towards the sea. We could see similar earthquakes a few more times.”