It has recently come to light that out of 35 seismometers and accelerographs installed across earthquake-prone areas in Maharashtra, specifically seismic zones III and IV, 20 were found to be non-functioning. A seismometer is an instrument used to measure and record earthquakes, while an accelerograph records the intensity of earthquakes.
Seismic zones III and IV in India are classified as moderate and strong earthquake-prone zones respectively. Seismic waves produced by an earthquake, some of which can be detected by these instruments, are categorized as P waves or S waves.
Decoding Seismic Waves: P-Waves and S-Waves
P-waves or primary waves are the first waves that are picked up on a seismograph during an earthquake event. These waves are longitudinal, meaning they vibrate in the same direction as their line of travel. Sound waves and waves in a stretched spring are other examples of longitudinal waves.
S-waves, on the other hand, are secondary waves that reach detectors after the P-waves. These are transverse waves which vibrate at right angles to their direction of travel. Light waves and water waves are other types of transverse waves.
While both P-waves and S-waves can be detected near the earthquake’s epicentre, only P-waves can be detected on the other side of the Earth. This is because P-waves can travel through both solids and liquids due to their longitudinal nature, whereas S-waves can only pass through solids.
Measuring Earthquakes: The Richter and Mercalli Scales
The severity of earthquake events is commonly measured using either the magnitude or intensity of the shock. The Richter scale measures the magnitude of an earthquake, correlating to the release of energy during the event. The scale ranges from 0-10, with each unit representing a tenfold increase in amplitude.
Conversely, the intensity of an earthquake is measured using the Mercalli scale which ranges from 1-12. This scale uses observed effects and structural damage to estimate intensity.
| Richter Scale | Mercalli Scale |
|---|---|
| 0-10, measures energy released | 1-12, measures observable damage |
Earthquake Prone Zones in India
An earthquake occurs due to the release of accumulated stress within the Earth’s lithospheric or crustal plates. In India, over 59% of the land area is threatened by moderate to severe earthquakes.
The Bureau of Indian Standards has divided the country into four seismic zones – Zone II, III, IV, V, based on past seismic activity. The areas that were considered relatively safe from damaging earthquakes were those away from the Himalayas and other inter-plate boundaries. However, following the occurrence of the Killari earthquake in Maharashtra in 1993, the seismic zoning map has been revised and Zone I was merged with Zone II and thus does not appear on the map anymore.
Zone V is the most seismically active region, while Zone II is the least active. These zones are divided based on their Modified Mercalli (MM) intensity, which measures the impact of earthquakes.
Distribution of Seismic Zones in India
Zone II is a low-intensity zone covering about 40.93% area of the country, consisting majorly of the peninsular region and the Karnataka Plateau.
Zone III, covering about 30.79% area of India is a moderate intensity zone and consists of Kerala, Goa, remaining parts of Uttar Pradesh, Gujarat and West Bengal, among others.
Zone IV, the severe intensity zone covers about 17.49% area of the nation including certain parts of Jammu and Kashmir, Himachal Pradesh, and the National Capital Territory of Delhi amongst others.
Zone V, the most seismically active region, a very severe intensity zone covers about 10.79% of the country’s area including entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Islands.
