An earthquake is a vibration of the surface of the earth which is caused by elasticity or isostatic adjustment of the rocks under the Earth's surface. It can also be due to anthropogenic (human beings) or natural activities. That is, when the earth's plates are collided, the earthquake comes. When the outer layer of the earth seems to be stirred suddenly, due to which energy arises. This energy generates seismic waves on the surface of the previous, due to which the land starts moving and the earthquake comes.
Do you know that before earthquake waves hit an area, the amount of radon gas in the atmosphere of that area increases? The rise of radon gas indicates that this area is going to be vulnerable to an earthquake. Various studies have been done on this and it has been concluded that high concentration of radon gas in soil or groundwater can be a sign of an impending earthquake.
The point where seismic waves are generated is referred as the 'Focus' of the earthquake and is present below the surface of the Earth. While, the place that is perpendicular to the focus, on the surface of the earth where earthquake tremors are felt for the first time is called 'Epicentre'. The energy dissipated from the focus is known as 'Elastic Energy'.
What can be the causes of earthquake?
Earthquakes can arise either due to natural phenomena or anthropogenic causes. It has been observed that earthquakes often occur due to geological faults.
What are Seismic Waves?
The waves which are produced during an earthquake are called seismic waves. They are categorised into 3 types:
1. Primary or Longitudinal waves (P-Waves)
These are longitudinal waves which are analogous to sound waves.
2. Secondary or Transverse Waves (S-Waves)
These are transverse waves analogous to light waves.
3. Surface or Long Period Waves (L-Waves)
These are produced when P-Wave collides with the surface.
What is a Seismograph?
The instrument which is sensitive to seismic waves and helps in measuring the intensity of earthquake is called seismograph. There are various scales which are used to measure the magnitude or intensity of earthquakes. These are Mercalli scale, Rossi-Forel scale and Richter scale.
Also, we must let you know that the lines which joins the regions of equal seismic intensity are called isosseismal lines and the lines which joins the regions which experience the tremors of the earthquake at the same time are known as homoseismal lines.
List of Seismic (Earthquake) Zones in India
Based on the past seismic history, the BIS or Bureau of Indian Standards has divided the country into four seismic zones namely Zone-II, Zone-III, Zone-IV and Zone-V. Among these four seismic zones, Zone-V is the most seismically active zone while Zone-II being the least.
Areas that fall under the Seismic Zones in India
Zone-V includes entire Northeast India, parts of Ladakh, parts of Jammu and Kashmir, Uttarakhand, Himachal Pradesh and Rann of Kutch in Gujarat, parts of North Bihar and Andaman and Nicobar Islands.
Zone-IV includes the rest parts of Himachal Pradesh, Ladakh and Jammu and Kashmir, the Union Territory of Delhi, Sikkim, northern parts of Bihar, Uttar Pradesh and West Bengal, parts of Gujarat and small parts of Maharashtra along the west coast and Rajasthan.
Zone-III includes Goa, Kerala, Lakshadweep Islands, remaining portions of Uttar Pradesh, Gujarat and West Bengal, parts of Punjab, Madhya Pradesh, Rajasthan, Bihar, Maharashtra, Chhattisgarh, Jharkhand, Andhra Pradesh, Odisha, Tamil Nadu and Karnataka.
Zone-II includes the remaining portions of the country.
The seismic zone map of India helps in identifying the least, moderate and most dangerous or earthquake-prone regions in India. In addition, such maps are used to check the level of seismicity in a particular area before the construction of multi-storey building. In the long run, it also helps in saving lives.
So now you must have understood the entire concept of earthquake and the areas under the seismic zones in India.
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