Earthquake Seismology and Seismic Waves: UPSC Geography

Earthquake Seismology and Seismic Waves

Introduction & Conceptual Foundation

An earthquake is a sudden shaking of the Earth's surface caused by the rapid release of accumulated energy in the lithosphere. This energy is released along a fault plane and propagates in all directions as seismic waves.

• Focus (Hypocenter): The point within the Earth's crust or upper mantle where the initial rupture starts and seismic energy is released.
• Epicenter: The point on the Earth's surface directly vertical to the focus. It experiences the seismic wave arrivals first and typically suffers the highest intensity of shaking.

Seismology is the scientific study of these seismic waves, their propagation, and the physical characteristics of the structures they pass through.

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Causes of Earthquakes

Earthquakes are triggered by both natural (endogenic) and human-induced (anthropogenic) activities:

                            Causes of Earthquakes
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         |                                                           |
   Natural Causes                                            Anthropogenic Causes
  1. Tectonic Movements & Faulting                           1. Reservoir-Induced Seismicity (RIS)
  2. Elastic Rebound Theory                                  2. Deep Mining & Rock Collapses
  3. Volcanic Magma Intrusions                               3. Underground Nuclear Explosions
  4. Isostatic Adjustments (Glacial melt)                    4. Fluid Injection (Fracking)
  5. Mass Movements (Landslides/Avalanches)

Natural Causes

1. Tectonic Movement: The movement of lithospheric plates along convergent, divergent, or transform boundaries accumulates frictional stress. When this stress exceeds the shear strength of the rocks, a sudden slip occurs along a fault plane, releasing seismic energy.
2. Elastic Rebound Theory: Formulated by H.F. Reid, this theory states that rocks under tectonic stress deform elastically, storing strain energy. Once the stress exceeds the rock's elastic limit, the rock fractures along a fault. The accumulated strain is released as seismic waves, and the fractured blocks snap back (rebound) to a relatively unstrained shape.
3. Volcanic Activity: The movement of high-pressure magma fracturing conduit rocks beneath active volcanoes triggers volcanic tremors (e.g., near Mount Etna, Kilauea).
4. Isostatic Adjustment: The vertical rising or sinking of the crust to maintain gravitational equilibrium. For instance, the melting of massive continental ice sheets due to global warming relieves pressure on the landmass (e.g., Greenland), causing the crust to rebound upward, triggering earthquakes.
5. Landslides and Avalanches: Rapid downslope movements of rock or ice in steep terrains (e.g., the Himalayas) generate localized seismic vibrations.

Anthropogenic Causes

1. **Reservoir-Ind