All four quakes were in the same fault system

All four quakes were in the same fault system

Subject :Geography

Section: Physical geography

Context:

• In a short span of about a week, a region about 40 km from Herat, Afghanistan was struck by four shallow focus earthquakes of 6.3 magnitude.

Details:

• All four 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.
• They did not occur in the exact same spot; rather, they ruptured different portions of the same fault along its length.
• It is rare for an earthquake to rupture the entire length of the fault that the earthquake occurred on, so it requires multiple earthquakes, spread out over some unknown amount of time, to fully rupture a geologic fault.
• Because these two earthquakes [on October 7] and the two subsequent earthquakes [on October 11 and October 15] are all approximately the same magnitude, we would call them ‘multiplets’ rather than mainshocks, foreshocks, or aftershocks.
• Probable cause of ‘multiplets’:
  • The release of stress in one fault [in Herat] can result 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.
• Consequence:
  • Each earthquake causes both uplift and subsidence, with the primary deformation being uplift. The earthquake sequence has led to an accumulation of uplift along the fault that is rupturing.
  • Since all the four earthquakes occurred due to thrust faulting, where one block moves up relative to the other, the area where the earthquakes had occurred would experience upliftment.

Faults

A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.

Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion. Faults which move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults.

Based on Movement:

1. Normal Fault: In a normal fault, the hanging wall moves downward relative to the footwall. This type of fault is associated with extensional tectonic forces, typically found at divergent plate boundaries.
2. Reverse Fault (Thrust Fault):In a reverse fault, the hanging wall moves upward relative to the footwall. Reverse faults are associated with compressional tectonic forces and are commonly found at convergent plate boundaries.
3. Strike-Slip Fault: In a strike-slip fault, the movement is primarily horizontal, with minimal vertical displacement. The rocks on either side of the fault slide past each other horizontally. Examples include the San Andreas Fault in California and the North Anatolian Fault in Turkey.

Based on Geological Setting:

1. Plate Boundary Faults: These faults are located at the boundaries of tectonic plates and play a significant role in plate tectonics. Examples include the San Andreas Fault (a transform fault) at the boundary between the Pacific and North American plates and the Himalayan Thrust Fault at the convergent boundary of the Indian and Eurasian plates.
2. Intraplate Faults: Intraplate faults occur within the interior of tectonic plates, away from plate boundaries. They are less common but can still generate significant seismic activity. An example is the New Madrid Seismic Zone in the central United States.
3. Importance of Studying Faults: Understanding faults and their characteristics is vital for various geological and societal reasons:
4. Earthquake Hazard Assessment: Faults are often associated with seismic activity. Monitoring and studying faults help in assessing earthquake hazards. Knowledge of fault location, slip rates, and past seismic events can inform earthquake preparedness and building construction practices in earthquake-prone regions.
5. Resource Exploration: Faults can act as conduits for the movement of fluids, such as oil, gas, and groundwater. They can trap and concentrate valuable mineral resources. Geologists study faults to locate and exploit these resources effectively.
6. Plate Tectonics: Faults are essential components of plate boundaries, which are central to the theory of plate tectonics. Understanding the behavior of faults helps scientists comprehend the movement of tectonic plates, which, in turn, explains the creation of mountain ranges, ocean basins, and continental drift.
7. Geological History: Faults provide a record of the Earth’s geological history. By examining the rocks and structures associated with faults, geologists can reconstruct past tectonic events, changes in stress regimes, and the evolution of landscapes.
8. Environmental and Engineering Considerations: Knowledge of fault locations is critical for infrastructure planning and environmental protection. Avoiding building structures on or near active fault lines can reduce the risk of damage during earthquakes and other ground movements.

In conclusion, faults are integral to the field of geology and have far-reaching implications for understanding the Earth’s dynamics, natural hazards, and resource distribution. Studying faults is essential for both scientific advancement and practical applications in areas like earthquake mitigation and resource exploration.

Source: TH