The geological placement of India was, for a long time, one of the puzzles of geology. It is now believed that the explanation might lie in the drifting of continental masses over the surface of the world after the break up of the ancient continent of Gondwanaland. One of these masses , peninsular India , would, according to this theory, have collided with the southern shore of Asian landmass, causing the uplifting of the Himalayan mountain ranges. The erosive action of rivers flowing down from the Himalayas would then, in subsequent eras have resulted in the creation of the Indo-gangetic plain, which is one of the world’s longest stretches of alluvium and also one of the most densely populated areas in the world.

The geological history of India provided her with the present soil and rock structures, natural resources, vegetation etc. The geological formations which started millions of years ago are continuing even now. The country has been divided into following main Geological Divisions or Systems :

The Archaean System :They are the first found rocks in the earth’s crust and are devoid of any life form. Forms about two-third of the Peninsular surface, and also occurs in several localities and is recognized there under various names-Bellary gneiss, Hosur Gneiss, Arcot Gneiss, Cuddapah Gneiss etc. The third important group of the Archaeans is the charnockites[1] which occur widely in Tamil Nadu-forming the Niligiri, Palini and Shevaroy hills. The bulk of the higher ranges of the Himalayas forming the central zone is formed of crystalline and metamorphic rocks like granites, granulites, gneisses, phyllites and schists.

The Dharwar System[2]: The area included in this system is the Dharwar - Bellary - Mysore belt of Karnataka; in the Aravallis between Jaipur and Palanpur, Rewa, Satana, Balaghat and Jabalpur district of Madhya Pradesh, Nagpur district  of Maharashtra, Ranchi, Hazaribagh and Gaya districts of Bihar. Sundargarh and Keonjhar districts of Orissa, and Ladakh and Zaskar Ranges of Jammu and Kashmir. The rocks in this system are of both igneous and sedimentary origin and are often metalliferous containing ores of iron, manganese, copper, lead and gold.

The Cuddapah System:  The Cuddapah System is composed of a number of parallel-series of ancient sedimentary strata of great thickness. They rest with a great unconformity on the Dharwars and Archaean gneisses and schists and underline the Vindhyan System of MadhyaPradesh. Anunconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentarygeologic record. The most extensive occurrence of the Cuddapahs is found in the Cuddapah district of Andhra Pradesh, The Chattisgarh, Singhbhum district of Jharkhand, Kalahandi and Keonjhar district of Orissa.

The Vindhyan System: Mainly composed of undisturbed sand-stones, shales and limestones reaching a thickness up to 4,000 metres, the Vindhyan System occupies about the lakh square km, stretching from Sasaram and Rohtas in western Bihar to Chittorgarh in Rajasthan. The Bundel Khand gneisses mark a gap in this belt while a large area of these rocks is covered by the Deccan Trap.

The Deccan Trap:  A large part of the Peninsula was affected by a great volcanic outburst, resulting in the eruptions of a thick series of lava and associated pyroclastic materials. The eruptions proceeded from fissures and cracks intermittently over a long period and, as a result, several thousand-meter-thick lavas spread over an area of about 10 lakh square km covering fully the pre-existing topography. This basaltic rock of volcanic origin is called the Deccan Trap. The Deccan Trap today, after denudation over a long period, is cut into many valleys and other depressions and occupies about 5 lakh square km covering large parts of Kutch, Saurashtra, almost the whole of Maharashtra, the Malwa Plateau and northern fringe of Karnataka.

The Tertiary System:  The Tertiary age is the most important period in India’s geological history as it was during this period that the Himalayas came into existence. Great mountain-building movements during the period led to the folding of the thick massive sediments deposited in the Tethys geosyncline since the Carboniferous age.

The central axis of the ancient sedimentary and crystalline rocks was ridged up during this period. It was followed by a movement of greater intensity about the mid-Miocene period. The most important phase came during the post-pliocene times when the Himachal and the Siwaliks were folded. There are evidences to prove that the elevatory movement has been continuing even during the recent times.

Other important geological divisions are the Upper carboniferous and Permian Systems, occurring in Western Himalayas from Kashmir to Kumayun; the Eocene System, found in Jammu and Kashmir, Himachal Pradesh, Assam, Rajasthan and Gujarat and the Siwalik System found along with the foothill zone of the Himalayas.

Earthquake and causes in India

The earthquake itself is caused by the sudden release of compressed energy which over decades had been slowly stored in the rocks of the region. As they reached their breaking point, these rock masses snapped or slipped along some weak zone or an earlier scar.

The compression in this case is provided by the continued northward motion of the Indian landmass pushing against the already buckled and thickened Tibetan plateau, as it is dragged at its base by a gigantic convection current rising from deep below the crust.

The first result of this compression before the formation of the Himalayas 40 million years ago, was to buckle and thicken the Tibetan plateau. Later, when this process reached its limit, continued compression caused the advancing edge of the Indian landmass to buckle and evolve into fold mountains. Thus were raised the Himalayas, metre by metre every few centuries, into the highest mountains on earth - a testimony to the immutable power of gigantic forces constantly at work pushing India ever closer to Tibet.

But, as rocks are strong and even their fractured surfaces tend to stick by friction or get locked by asperities when pushed, the continued compression does not result into a steady, energy release but it manifests itself in a cycle of sticking and slipping as a hard spring would exhibit when steadily compressed slowly shrinking for a while and then snapping as the resulting strain exceeds its strength.

Likewise, the continuing northward convergence of India towards Tibet takes place in spurts, as accumulating strains reach the minimum breaking strength of rocks. A major earthquake then relieves most of the accumulated strain, and the Himalayan range lurches forward by a few metres in a sudden leap.

Or, looked at it another way, the Indian continent thrusts beneath the stacked piles of its sliced headlands that form the Himalayan mountains by an equal amount thereby moving the two continents closer by a few metres.After a major earthquake, the region would be relatively quiet for a few decades or centuries whilst being pressed slowly and steadily to the next breaking point. The cycle will continue as long as the underlying conveyer belt of convection current continues to move.  Meanwhile, this intervening period of relative quiescence would not be wholly free from earthquakes.Many small ones, most of them too feeble to be felt, would perhaps occur weekly or even daily and a few strong ones such as the recent Nepal earthquake[3], as the accumulated strain budget approaches the breaking limit. Small and moderate earthquakes however relieve little energy to make any substantial difference to the accumulated strain budget particularly when close to the breaking point. These can only be relieved by a major earthquake which, if past experience is any guide, could have a magnitude of 8.5 for the Himalayan region.

Past History

Four major earthquakes, all of the magnitude greater than 8, have between themselves ruptured about half of the 2,400 km long Himalayan boundary in recorded seismic history, which in our country is unfortunately limited to a mere 500 to 600 years. All these four earthquakes, each one of which on an average caused about 250 km of the compression front to suddenly slip by about 8 metres, have occurred in the last 100 years.

A back-of-the envelope calculation will show that this would set a maximum rate at which strain accumulates in the Himalayas, i.e. 8 metres every 600 years, or about 1.3 cm a year.

The October 20, 1991 Kumaon (Uttarkashi) earthquake occurred (9.8 degrees No, 79.8 degrees E) in the central Himalayan seismic gap lying between the rupture zones of the Great Kangra (1905) and Bihar (1934) earthquakes. But since the energy released by it is only about a thousandth of what a fully ripe seismic zone in the Himalayas may be capable of releasing, its precise significance to future earthquake activity in the region is not quite clear at this stage, although it may be tempting to suggest that this fifth moderate earthquake of the region within the past 25 years may already mark the onset of precursory phase preparatory to a major earthquake of magnitude greater than 8.

On January 26, 2001, the Rann of Kutchh in the westernmost corner of India was rocked by a violent earthquake. 13,805 people were killed  and 1,67,000 were injured . Tremors from the shock were experienced across the Indian Sub-continent, as far east as Shillong , in north-east India. This earthquake was the largest to strike India in 50 years. This event is officially called the “Bhuj earthquake”, but unofficially it is also known as the “Republic Day Earthquake” or the “Gujarat Earthquake”. The magnitude of the earthquake was estimated at M7.6 by teleseismic studies . This event sent ripples in the scientific community not just because of its size and location but also as it lacked a surface rupture, a feature that accompanies earthquakes of this size.

The epicentre of this earthquake was 13 kilometres NW of Bhachau (Gujarat), India,

Slippage is believed to have occurred on the south dipping North Wagad reverse fault in the Kutch aulacogen or failed rift .

A surprising feature of this earthquake was the lack of a primary surface rupture, that usually accompanies events of such large magnitude. This implies that the earthquake was blind[4]. Much of the ground deformation was concentrated near the eastern edge of the rupture, north and north-east of Bhachau. Features of this nature are commonly observed after thrust-faulting earthquakes.

Seismic Zones

The issue of seismic hazard in India has been addressed by scientists as early (2) as 1956 when a 3 zone; Severe, Moderate, Minor hazard map of India was produced). This map was based on a broad concept of earthquake distribution and geotectonic. The severe hazard zones are roughly confined to plate boundary regions i.e the Himalayan Frontal Arc in the north, the Chaman fault region in the north west and the Indo-Burma border region in the north east. The lower hazard zone is confined to Indian shield region in the south and the moderate hazard zone confined to the transitional zone in between the two.

Since then, many versions of the seismic zoning map of India have been brought out. The Bureau of Indian Standards which is the official agency for publishing seismic hazard maps and codes in India, produced a six zone map in 1962, a seven zone map in 1966, a five zone map in 1984 and a four zone map in 2000 which is currently valid. Presently a five zone map is being used with Zone-V most vulnerable. Zones I and II have been merged into a single Zone-II.