An earthquake refers to the vibration or shaking of the Earth’s surface, resulting from the release of energy within the Earth’s interior.
Key Terms Associated with Earthquakes
- Hypocenter/Focus: The point beneath the Earth’s surface where the earthquake originates.
- Epicenter: The point directly above the focus, located on the Earth’s surface.
- Shallow Earthquakes: Earthquakes with focal depths less than 70 km.
- Intermediate Earthquakes: Earthquakes with focal depths between 70 and 300 km.
- Deep Earthquakes: Earthquakes with foci at depths ranging from 300 to 700 km.
- Aftershocks: Smaller earthquakes that follow a larger, shallow-focus earthquake.
- Foreshocks: Minor earthquakes that occur before a significant seismic event.
- Earthquake Swarms: A cluster of small earthquakes without a prominent mainshock.
Measuring Earthquakes
| Richter Scale | Mercalli Scale |
|---|---|
| Measures earthquake magnitude. | Measures earthquake intensity. |
| Developed by Charles F. Richter in 1935. | Developed by Giuseppi Mercalli in 1902. |
| Magnitude ranges from 0 to 10. | Intensity ranges from I to XII. |
Previous Year Questions
- Which of the following statements regarding earthquakes are correct? (2001)
- 1. The intensity of earthquakes is measured on the Mercalli Scale.
- 2. The magnitude of an earthquake indicates the energy released.
- 3. Magnitude is determined by measuring the amplitude of seismic waves.
- 4. On the Richter Scale, each whole number represents a hundredfold increase in energy release.
Answer: (a) 1, 2, and 3
Causes of Earthquakes
| Natural Causes | Human-Induced Causes |
|---|---|
| Volcanic Activity: Example – The 1985 Nevado Del Ruiz eruption in Colombia caused earthquakes. | Mining and Drilling: Activities such as underground mining, drilling, blasting, and dynamite use can induce seismic activity. |
| Faulting: Example – The 1975 Guatemala earthquake resulted from movement along the Montagu Fault. | Reservoir-Induced Seismicity: Example – The 1967 Koyna earthquake in Maharashtra occurred due to the Koyna reservoir. |
| Plate Tectonics: Shallow earthquakes at divergent boundaries, deep and high-magnitude earthquakes at convergent boundaries, and earthquakes at transform boundaries (e.g., San Andreas Fault in California). | Nuclear Tests: Underground nuclear explosions can trigger earthquakes. |
Global Distribution of Earthquakes
| Zone | Extent | Cause |
|---|---|---|
| Circum-Pacific Zone | Encircles the Pacific Ocean. | Subduction of the Pacific Plate beneath other plates. |
| Mid-Atlantic Belt | Along mid-oceanic ridges in the Atlantic Ocean. | Seafloor spreading. |
| Mid-Continental Belt | Extends through the Mediterranean, Himalayas, and Myanmar. | Plate convergence. |
| Intra-Plate Earthquakes | Occur within plate interiors (e.g., Latur, India, 1993). | Stresses along ancient fault lines or rift zones. |
Earthquake-Prone Zones in India
| Zone | Regions |
|---|---|
| Zone V | Kashmir, Central Himalayas, Bihar, Northeast, Rann of Kutch, Andaman & Nicobar Islands. |
| Zone IV | Parts of Jammu & Kashmir, Himachal Pradesh, Delhi, Uttar Pradesh, Bihar, Gujarat, and Rajasthan. |
| Zone III | Kerala, Goa, Lakshadweep, parts of Punjab, Rajasthan, Tamil Nadu, and Odisha. |
| Zone II | Remaining areas of India. |
Effects of Earthquakes
- Landslides: Displacement of rocks and soil.
- Ground Deformation: Changes in the Earth’s surface.
- Tsunamis: Example – The 2004 Indian Ocean tsunami triggered by the Sumatran-Andaman earthquake.
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