Source: The Hindu (Page No II)
Relevance: GS-I Earthquake
Prelims Take away: Earthquake
Mains Take away: Causes & Implication of earthquake
Why In News:
A series of earthquakes in Afghanistan, specifically near Herat.
Earthquake in Afghanistan
- The earthquake had a shallow focus, with a depth of approximately 14 kilometers.
- Shallow-focus earthquakes are often more destructive than deep-focus ones because their energy is closer to the surface.
- What's particularly noteworthy is that just 30 minutes after the initial earthquake, another earthquake struck Herat. This second earthquake was also shallow, occurring at a depth of about 13.5 kilometers, and had the same intensity as the first one, both registering a magnitude of 6.3.
Seismic Event of the region
- Fault Plane Orientation: The first and second earthquakes that occurred approximately 30 minutes apart both occurred on east-west-striking fault planes. These fault planes can dip either to the north or south. This information suggests the direction and orientation of the geological fault responsible for the seismic activity.
- Geological Setting: The earthquakes took place within the Eurasia plate, and the region is described as an intracontinental mountain belt. This indicates that these seismic events occurred within the broader context of tectonic interactions within the Eurasia plate, likely as a result of the ongoing geological processes in this mountainous region.
- Aftershocks: Following the main earthquakes, there were about half a dozen aftershocks. Aftershocks are common occurrences following significant seismic events and are usually smaller in magnitude than the main earthquakes.
- Third Earthquake: Four days later, on October 11, another shallow earthquake with the same intensity of 6.3 magnitude occurred in Herat. This event suggests ongoing seismic activity in the region and may be related to the earlier earthquakes.
- Thrust Faults: All three of these earthquakes were identified as thrust faults, specifically reverse faults. Thrust faults result from horizontal compressive stresses in the Earth's crust, leading to the shortening of the crust. In this type of fault, one block or wall (the hanging wall) moves up relative to the other (the footwall).
- Timing of Events: The passage notes that these three earthquakes occurred a little over a year after another shallow earthquake with a 10 km depth struck near Khōst, Afghanistan, on June 22, 2022. This timeline indicates a relatively short interval between significant seismic events in the region.
Why the second earthquake on October 7 was considered a "fresh quake" and not an aftershock is significant for understanding seismic events:
- Magnitude Criterion: Dr. Chadha explains that to classify an earthquake as an aftershock, its magnitude must be lesser than the magnitude of the main event or the first earthquake. In this case, both the first and second earthquakes had the same magnitude of 6.3. Since the magnitude was not smaller, it did not meet the typical criterion for an aftershock.
- Stress Release and Loading: The explanation highlights the complex nature of earthquakes. When a fault ruptures and releases stress through an earthquake, it can lead to a redistribution of stress in the surrounding geological environment. This redistribution of stress can result in further earthquakes. In this case, the second earthquake occurred due to the loading of stress at another fault, even though it had the same magnitude as the first earthquake.
- Fault Characteristics: The passage also emphasizes that in regions like Afghanistan, faults can be extensive in length and width. This is particularly relevant in areas with complex geological settings, such as subduction zones and mountain ranges like the Himalayas. In such regions, fault systems can be interconnected and widespread, making it more likely for one earthquake to trigger another within the same fault system.
- Geographical Proximity: The second earthquake occurred quite close to the location of the first earthquake, about 20 kilometers away, and on the same fault. This geographical proximity further supports the idea that the two seismic events were interconnected and part of the same fault's activity.