In what will be termed as uncommon, a shallow focus (14 km depth) earthquake of 6.3 magnitude struck about 40 kms northwest of Herat in Afghanistan at round 11:00 am native time on October 7 (Saturday). The earthquake occurred because the consequence of thrust faulting close to the far western terminus of the Hindu Kush Mountain vary. Just half-hour after the primary 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 had been about a half-a-dozen aftershocks following the earthquakes.
Four days later, on October 11 (Wednesday), Herat witnessed but another shallow earthquake at round 5.10 am native time with the main target of the quake being simply 9 km under the floor. It had the same intensity (6.3 magnitude) because the earlier ones on October 7.
All three earthquakes have been thrust faults, in any other case referred to as reverse faults. Thrust faults kind because of horizontal compressive stresses and so trigger shortening of the crust. Here one block or wall (the hanging wall) strikes up relative to the opposite (known as the footwall).
These three quakes struck a little over a yr after another shallow earthquake at 10 km depth struck close to Khōst, Afghanistan on June 22, 2022 at round 1:25 am native time.
Cause of second quake
Earthquakes are typically adopted by aftershocks of comparatively lesser magnitude. Why was the second earthquake on October 7 that struck inside half-hour known as a recent quake and never 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. Okay. Chadha, former senior scientist on the Hyderabad-based National Geophysical Research Institute (NGRI) and presently 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 2 quakes occurred within the same fault. Generally, in Afghanistan, the fault will be very lengthy and large. “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 fairly near the primary one (about 20 km distance) within 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 provides. The exact location of the primary quake epicentre on October 7 was 34.610 diploma north and 61.924 diploma east, whereas the October 11 earthquake was 34.557 diploma north 62.045 diploma east.
Earthquakes are fairly widespread in Afghanistan because of energetic 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 happen to depths as nice as 200 km as a consequence 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.
