The earthquake zone of India falls within the Trans-Asiatic earthquake belt which traverses the earth from east to west. The prominent seismic belt with which the Himalayan zone is connected runs from the mid-Atlantic Coast to the extreme end of the Malayan Arc.
It may be pointed out that this great belt is characterised by being the zone of mountain building tectonic earth movements of the Tertiary age. The Himalayan belt is still unstable and has several thrust planes.
Apart from displacements taking place along the Himalayan boundary faults or thrusts, there is the presence of a strained zone of crumpled and fractured rocks beneath the Indo- Gangetic alluvium. This zone is the source of earthquakes.
It may be pointed out that the Himalayan zone is one of maximum intensity regarding the magnitude of earthquake shocks. This is due to the fact that it is in the subduction zone of the Indian and Asiatic plates. In this zone the process of mountain building is still continuing.
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The convergence of the Asiatic and Indian plates results in the subduction of the Indian plate causing the formation of a large number of folding and faulting.
As we are aware, the young folded mountain of the Himalayas is slowly rising. This is the main reason for the occurrence of earthquakes in the great northern plain of India.
The northwestern and the northeastern part of India offer more favourable conditions for the seismic shocks because there are sharp changes in strikes, and rocks experience greater stress. The Indo-Gangetic plain is more vulnerable to earthquakes that originate in the Himalayan belt.
Since the rocks below the plain are undergoing constant changes, there is greater frequency of earthquakes. The Assam earthquake of 1918 and Bihar earthquake of 1934 had their foci in this trough.
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The earthquake zone extends from Suleman and Kirther shear zones in the west to the Burmese arc in the east. The Indian and the Asiatic plate movements are responsible for causing earthquakes in the northern and northeastern parts of India.
Indian Peninsula is a stable block and is supposed to be free from earthquakes. But there are occasional earthquakes which occur along its margins. Koyna earthquake of 1960 and the Latur earthquake of 1993 are typical examples of unexpected earthquakes which occurred in the Peninsular India.
The Kutch region of Gujarat experienced the most violent earthquake on January 26, 2001 at 8.46 A.M. Its magnitude was recorded 8.1 on the Richter scale. The epicentre of this earthquake was 23°3′ north and 69°8′ east, 20 km north-east of Bhuj.
Geologists and the seismologists are of the opinion that the Bhuj earthquake is a reawakening of the so-called Allah- Bund fault which came into existence in 1819. This earthquake was caused, according to the Seismologists, due to the movement of the Indian Plate towards the north.
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It may be pointed out that this plate is moving in the direction of northeast at the rate of about 5 cm per year against the Eurasian plate along the Himalaya Mountain.
The sea floor spreading of the Indian Ocean was another contributory factor in the causation of the Bhuj earthquake. The presences of two subterranean faults located in the Kutch region have been the main cause of the frequent occurrence of earthquakes here.
One of the most disastrous earthquakes in India occurred in Assam in 1897, which played havoc in an area 3, 90,000 sq. km surrounding the town of Shillong.
The earth’s surface in the plain received such violent seismic shocks in less than one minute that gaping fissures were formed in the ground from which water gushed out, the mountain slopes experienced huge landslides, and all means of transport and communications were disrupted.
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The Kangra earthquake of 1905 was even more destructive as it killed about 20,000 people and left many more injured. Its shock was experienced in the major part of the country north of the Tapti Valley.
The most characteristic features of this seismic event were that hundreds of aftershocks of moderate intensity were felt for several months. Besides, there were changes in the levels of a few places on the surface.
The Bihar earthquake of 1934 was also very violent. In a very short time of only a few minutes the towns of Monghyr and Bhatgaon (Nepal) were turned into ruins. The impact of the earthquake was felt over long distances.
For example, the buildings in towns as far apart as Kathmandu, Patna, and Darjeeling were badly affected. The earthquake shocks were felt as far away as Varanasi and Allahabad. On either side of the river Ganga numerous fissures opened from which water and sand were thrown up.
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The death toll rose to about 12000. All the 74 violent earthquakes in the country during the last 200 years had their epicenters in the Himalayan mountain belt or the northern plains lying at the foot of the Himalayas.