EARTHQUAKES IN INDIA

India falls quite prominently on the global seismic belt which runs in an east-west direction and is called the 'Alpine-Himalayan belt'. The main seismic zone lies along the 'main boundary fault' which runs from the Hindukush mountains in the west to Sadiya (Assam) in the east and bends southwards while passing through the Andaman and Nicobar Islands on its way to the Indonesian archipelago.

The unique geographical location of the Indian subcon­tinent bordered by the Himalayan belt (which is part of the Tethys geosyncline) between the Gondwanaland and Laurasian plate is held to be responsible for the unstable geological nature of the Indian subcontinent in general. According to the scientists of the Wadia Institute of Hima­layan Geology, the frequent earthquakes in the Himalayas can be attributed to the northward movement of the Indian landmass for the last 80 million years. Such northward plate movement builds up stress and releases itself as energy from the earth's interiors. The recent instances of earthquakes at Jabalpur and Killary suggest that the frequency of earthquake has shifted from quake-prone areas to the so-called stable landmasses. Although some scientists at­tribute such earthquakes to reservoirs, others believe the root cause of earthquakes to lie in the unique geological structure of the country.
Geologically, the country is formed of several sequences of rock units, which in a vast region of peninsular India display the most ancient Archaean rocks. Geologists are still not clear whether the Indian subcontinent is made up of smaller plates merged together or there is a single massive block dissected by faults, joints or lineations. Since the Archaean rock strata dates back to 2.5 billion years, it forms the base rock over which other rock layers exist. Since the base rock is not visible, it is not subject to study and analysis regarding its physical and chemical status. Due to continued denudation, the Himalayas are also rising to maintain equilibrium. On the other hand, sedi­ments regularly accumulated in the Bay of Bengal and the Arabian Sea put an enormous load on the oceanfloor. This phenomenon, in turn, is believed to exert pressure on the mainland too. The oceanic ridges and other complex structural features of the Indian Ocean may also have influenced the neighbouring landmass in a significant manner.
Based on seismic data and different geological and geophysical parameters, India is divided into five seismic zones. Of the five seismic zones, zone five is the most active region and zone one shows least seismic activity.
The entire north-eastern region falls in zone five. In fact, in the last 100 years, as many as five major earth­quakes with a magnitude of 7.0 occurred in this region­Assam in July 1918, July 1930 and October 1943, Arunachal Pradesh-China border in August 1950 and Manipur­Myanmar border in August 1988.

The high level of seismicity in this zone results from the fact that this belt is the line along which the Indian plate (or pristine Gondwanaland) meets the Eurasian plate in accordance with the terms of the theory of plate tectonics. This being a convergent edge, the Indian plate is thrusting underneath the Eurasian plate at a speed of 5 cm per year. This movement gives rise to a tremendous stress which keeps accumulating in the rocks (just as a compressed spring conserves stress) and is released from time to time in the form of earthquakes. Since a lot of energy is released through these seismic tremors, the earthquakes are asso­
ciated with kuch devastation and loss of life and property.

Thus, a relatively young Himalayan region which has not yet attained stability has witnessed many mqjor earth­quakes in the recent past.
Besides th~ north-east, zone five includes parts of Jammu and Kashmir, Himachal Pradesh, the hills of Uttaranchal, Rann of Kutch (which includes Bhuj) in Gujarat, northern Bihar and the Andaman and Nicobar islands.

One of the reasons for this regiQn being prone to earthquake is the presence of the young-fold Himalayan mountains here which have frequent tectonic movements.
Zone four which is the next most active region of seismic activity covers Sikkiin, Delhi, remaining parts of Jammu and Kashmir, Himachal Pradesh, Bihar, the rest of Uttaranchal, the northern parts of Uttar Pradesh and West Bengal, parts of Gujarat and small portions of Maharashtra near' the west coast.

In the last ten years, Uttar Pradesh (areas now in Uttaranchal), Maharashtra and Madhya Pradesh have had a number of severe earthquakes. These include the dev­astating Uttarkashi (now in Uttaranchal) earthquake of 6.6 magnitude in October 1991, Latur-Osmanabad (Maharashtra) quake in September 1993, Jabalpur (Madhya Pradesh) in May 1997 and Chamoli (Utta,'anchal) in March 1999-all of a magnitude of over 6.
Zone three comprises Kerala, Goa, Lakshadweep, re­maining parts of Uttar Pradesh and West Bengal, parts of Punjab, Rajasthan, Maharashtra, Madhya Pradesh, Orissa, Andhra Pradesh and Karnataka.

The remaining states with lesser known activity fall in zones one and two.
The Seismic Zones map ot' India in circulation needs
revision, say experts; more and more areas, hitherto not on the map, are proving susceptible to earthquakes of greater magnitude than attributed to them at present.