How a 6.3 magnitude quake caused another of same intensity

The first shallow focus earthquake of 6.3 magnitude, which occurred near the far western terminus of the Hindu Kush Mountain range in Afghanistan, was followed by another 6.3 magnitude quake 30 minutes later

October 14, 2023 09:00 pm | Updated October 18, 2023 08:47 am IST

In what can be termed as unusual, a shallow focus (14 km depth) earthquake of 6.3 magnitude struck about 40 kms northwest of Herat in Afghanistan at around 11:00 am local time on October 7 (Saturday). The earthquake occurred as the result of thrust faulting near the far western terminus of the Hindu Kush Mountain range. Just 30 minutes after the first quake, Herat was struck by another shallow earthquake (about 13.5 km depth) of the same intensity — 6.3 magnitude. According to the U.S. Geological Survey (USGS): “[The first] earthquake was followed by a second earthquake [with 6.3 magnitude] that occurred approximately 30 minutes later. Both earthquakes occurred on east-west striking fault planes that dip to either the north or south. The earthquakes occurred within the Eurasia plate in an intracontinental mountain belt”. There were about a half-a-dozen aftershocks following the earthquakes.

Four days later, on October 11 (Wednesday), Herat witnessed yet another shallow earthquake at around 5.10 am local time with the focus of the quake being just 9 km below the surface. It had the same intensity (6.3 magnitude) as the previous ones on October 7.

All three earthquakes have been thrust faults, otherwise known as reverse faults. Thrust faults form due to horizontal compressive stresses and so cause shortening of the crust. Here one block or wall (the hanging wall) moves up relative to the other (called the footwall).

These three quakes struck a little over a year after another shallow earthquake at 10 km depth struck near Khōst, Afghanistan on June 22, 2022 at around 1:25 am local time.

Cause of second quake

Earthquakes are generally followed by aftershocks of relatively lesser magnitude. Why was the second earthquake on October 7 that struck within 30 minutes called a fresh quake and not an aftershock? “To call a quake an aftershock, the magnitude has to be lesser than the magnitude of the main event [quake],” explains Dr. R. K. Chadha, former senior scientist at the Hyderabad-based National Geophysical Research Institute (NGRI) and currently a Raja Ramanna Fellow at NGRI. “A quake of 6.3 magnitude was followed by another of the same magnitude. This can happen when a fault at one place ruptures resulting in an earthquake that releases the stress. The release of stress in one fault results in the loading of stress at another fault. The loading of stress can result in another earthquake which can be of similar magnitude or even higher magnitude. But the magnitude will not be smaller than the first quake.”

In the case of the October 7 earthquakes, the two quakes occurred in the same fault. Generally, in Afghanistan, the fault can be very long and wide. “In subduction zones and in the Himalayas where there is interaction between two continental plates, the fault lengths can be very large and also very wide. That is the reason why an earthquake in the fault can trigger another in the same fault,” he says. The second earthquake occurred quite close to the first one (about 20 km distance) in the same fault.

“When I see the three earthquakes in two parallel faults, there could be an interplay of these faults,” he says. “All three earthquakes are quite close by, and if you see the locations of all the three quakes, there is a systematic migration pattern visible. In this case, there is a migration of quakes from west to east.”

Two faults

“In any thrust fault environment in a subduction zone, there will be a series of parallel thrust faults. In this case the faults are aligned in an east-west direction. The two faults on October 7 and the one on October 11 have been on two faults trending east-west,” Dr. Chadha adds. The precise location of the first quake epicentre on October 7 was 34.610 degree north and 61.924 degree east, while the October 11 earthquake was 34.557 degree north 62.045 degree east.

Earthquakes are quite common in Afghanistan due to active interactions between three tectonic plates — the Arabia, Eurasia, and India plates. According to the USGS, earthquakes in western and central Afghanistan are “primarily influenced by the northward movement of the Arabia plate relative to the Eurasia plate.”

According to USGS, beneath the Pamir-Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. “Shallower crustal earthquakes in the Pamir-Hindu Mountains occur primarily along the Main Pamir Thrust and other active Quaternary faults, which accommodate much of the region’s crustal shortening. The western and eastern margins of the Main Pamir Thrust display a combination of thrust and strike-slip mechanisms,” notes USGS.

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