Model Answer
0 min readIntroduction
Earthquakes, among the most devastating of natural hazards, pose a significant threat to life and property globally. Understanding the mechanisms behind earthquake occurrence and accurately assessing their potential impact is crucial for effective disaster mitigation. The assessment of earthquakes relies on two primary scales: intensity, which measures the effects on people and structures, and magnitude, which quantifies the energy released at the source. India, situated in a complex tectonic environment, is particularly vulnerable to seismic activity, necessitating a thorough understanding of its seismic zones and associated risks. This answer will discuss the commonly used scales, global distribution, and the seismic zoning of India.
Intensity and Magnitude Scales
Both intensity and magnitude scales are used to characterize earthquakes, but they measure different aspects of the event.
Intensity Scales
- Modified Mercalli Intensity Scale (MMI): This is a qualitative scale that assesses the effects of an earthquake on people, buildings, and the natural environment. It ranges from I (not felt) to XII (catastrophic damage). Intensity varies with distance from the epicenter and local geological conditions.
- MSK-64 (Medvedev-Sponheuer-Karnik 1964): This is another intensity scale, widely used in Europe and India, similar to MMI but with more detailed descriptions of damage levels.
Magnitude Scales
- Richter Scale (ML): Developed by Charles Richter in 1935, this scale measures the amplitude of seismic waves recorded on seismographs. It is logarithmic, meaning that each whole number increase represents a tenfold increase in amplitude and approximately a 32-fold increase in energy released. It's most accurate for local earthquakes.
- Moment Magnitude Scale (Mw): This is the most widely used scale today, especially for larger earthquakes. It is based on the seismic moment, which is related to the physical size of the fault rupture and the amount of slip. It provides a more accurate estimate of earthquake size than the Richter scale, particularly for distant and large events.
Key Difference: Intensity measures the *effects* of an earthquake, while magnitude measures the *energy released*. An earthquake has one magnitude but can have varying intensities at different locations.
Global Distribution of Earthquakes
The global distribution of earthquakes is closely linked to plate tectonics. Approximately 90% of all earthquakes occur along plate boundaries.
- Circum-Pacific Belt (Ring of Fire): This is the most seismically active zone, encompassing the coasts of North and South America, East Asia, and Oceania. It's characterized by subduction zones where one tectonic plate slides beneath another.
- Alpine-Himalayan Belt: This belt extends from the Mediterranean region through Turkey, Iran, and the Himalayas. It's formed by the collision of the Eurasian and Indian plates.
- Mid-Atlantic Ridge: This is a divergent plate boundary where new crust is created, resulting in frequent, but generally less powerful, earthquakes.
- Intraplate Earthquakes: While most earthquakes occur at plate boundaries, some occur within the plates themselves, often associated with ancient fault lines or stress buildup.
Seismic Zones of India
India is divided into four seismic zones based on the probability of earthquakes, as per the National Earthquake Hazard Map of India (released by the Bureau of Indian Standards - BIS). The map is based on historical earthquake data and geological features.
| Seismic Zone | Intensity | Area Covered | Earthquake Probability |
|---|---|---|---|
| Zone V | Very High | Kashmir, Western and Central Himalayas, North-Eastern India, Andaman & Nicobar Islands | Highest; frequent and intense earthquakes |
| Zone IV | High | Seismic gaps in the Himalayas, states in the Indo-Gangetic Plain (Delhi, Haryana, Punjab), parts of Gujarat, Maharashtra, and coastal Andhra Pradesh | High; significant earthquake potential |
| Zone III | Moderate | Coastal regions of India, including Kerala, Goa, Maharashtra, and parts of Tamil Nadu, Karnataka, and Gujarat | Moderate; occasional earthquakes |
| Zone II | Low | Remaining parts of India | Lowest; infrequent and minor earthquakes |
Recent Developments: The BIS is currently working on revising the seismic hazard map to incorporate new data and improve accuracy. The National Disaster Management Authority (NDMA) plays a crucial role in coordinating earthquake preparedness and response efforts.
Conclusion
Understanding the nuances of seismic scales, the global distribution of earthquakes, and the specific seismic zones within India is paramount for effective disaster risk reduction. The Indian subcontinent’s complex tectonic setting demands continuous monitoring, robust building codes, and comprehensive preparedness measures. Investing in earthquake-resistant infrastructure, public awareness campaigns, and early warning systems are crucial steps towards mitigating the devastating impacts of future earthquakes. The ongoing revision of the seismic hazard map by BIS is a positive step towards enhancing our understanding and preparedness.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.