How a 6.3 magnitude quake caused another of same intensity
- October 15, 2023
- Posted by: OptimizeIAS Team
- Category: DPN Topics
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How a 6.3 magnitude quake caused another of same intensity
Subject: Geography
Section: Physical geography
Chronology of earthquake events in Afghanistan:
- October 7: two earthquakes (within 30 minutes) of shallow focus (14 km depth) and 6.3 magnitude struck herat province.
- October 11: Another shallow focus quake (13.5 km) of the same intensity (6.3 magnitude) struck the same province.
Cause of these earthquakes:
- Locational analysis of all the three quakes shows that there is a systematic migration pattern. In this case, there is a migration of quakes from west to east.
- The earthquakes occurred within the Eurasia plate in an intracontinental mountain belt.
- 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).
- The thrust faulting occurred near the far western terminus of the Hindu Kush Mountain range.
Why is the second earthquake not called an aftershock?
- To call a quake an aftershock, the magnitude has to be lesser than the magnitude of the main event [quake].
- 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 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.
Foreshocks, aftershocks – what’s the difference?
- “Foreshock” and “aftershock” are relative terms.
- Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.
- Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or “mainshock.”
- They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time.
- Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes.
Earthquakes in Afghanistan:
- Earthquakes are quite common in Afghanistan due to active interactions between three tectonic plates — the Arabia, Eurasia, and India plates.
- Earthquakes in western and central Afghanistan are primarily influenced by the northward movement of the Arabia plate relative to the Eurasia plate.
- 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.
- Shallow 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.
Source: TH