Here is your paragraph on the causes of earthquakes in India!
Although earthquakes occur due to several causes such as volcanic eruption, faulting, hydrostatic pressure etc. the plate tectonic theory is the most convincing and widely accepted.
The term ‘plate tectonics’ was first used by Terzo Wilson of the University of Toronto in 1965 and the theory of plate tectonics was first published by W.J. Morgan of the Princeton University in 1967.
A lithospheric plate is a broad segment of lithosphere (including the rigid upper mantle, plus oceanic and continent crust that floats on the underlying asthenosphere. This theory assumes that the lithospheric plates, huge rafts are about 110 cm thick, which drift over the semi-molten underneath.
When two plates approaching each other meet, their movement is restricted, causing a building of stress along the line of contact. When a breaking point is reached, the stresses are released through the fracturing of the rocks, thus causing earthquakes.
A majority of the earthquakes occur along the plate boundaries. Nearly 95 per cent of the earthquake activities in the world occur at the plate boundaries. The earthquakes generated in this way are most damaging.
1. Indian Plate:
Study of Indian plate is very important for us because the whole of India lies on this plate and the entire seismic activity in India and neighbouring countries is associated with the Indian plate. This plate extends from the Himalayas beyond the Andamans and Java, Sumatra to Australia where it touches the Pacific plate.
It is moving in a north-northeast direction at an average velocity of about 5 cm per year. It encounters the adjacent plates in Myanmar-Andaman Sumatra region in the east, the Himalayan foothills in the north and Suleman ranges of Pakistan in the west. Great earthquakes in Shillong (1897), Kangra (1905), Bihar-Nepal (1934) and Assam (1950) have occurred on the northern boundary of the Indian plate which is underthrusting the Eurasian plate.
2. Intraplate Tectonics:
Seismicity of the Indian Plate assumed more importance after the occurrence of earthquakes in the Peninsula. The major earthquakes in the peninsular India are those of Koyra (1967), Bhadrachelam (1969), Broach (1970), Bay of Bengal (1972, 1973), Latur (1993) and Jabalpur (1997).
Plate tectonics does not offer a satisfactory explanation for earthquakes occurring within the plate itself. Prior to this, the great Rann of Kachchh earthquake (1819) located far away from the boundary of the Indian Plate could also not be accounted for by the plate tectonics model.
This quake reduced Bhuj town into ruins, killing about 2000 persons in Bhuj town alone. Seismologists believe that this quake was associated with perceptible fault. The resulting fault scarp brought into existence in the Rann of Kachchh is known as ‘Allah Bund’.
The scarp, about 3 km high is located about 8 km north of Sindri town and can be traced in east-west direction for about 26 km. The southern pojtion of the terrain south of this fault was thrown down, giving the scarp an appearance of a bund (embankment) when seen from south.
The local people used to consider this scarp as an act of God and hence the name ‘Allah Bund’. The recurrence of similar great Bhuj earthquake (2001) only after a gap of 182 years has raised many questions related to intraplate seismicity.
The intraplate stress is due to the fact, that not every part of the plate is similar and these are weak zones and faults. This is known as stable continental region earthquake. This type of earthquake is rare. Latur (1993) belonged to this category. Detailed studies reveal that Bhuj too was of this type. Further research is required to answer such intricate questions.
3. Reservoir Induced Seismicity (RIS):
The Koyna earthquake (1967) could not be explained on the basis of intraplate tectonic with any degree of satisfaction as a result of which some geologists and seismologists turned to the theory of reservoir induced seismicity in a big way; This theory assumes that the load of the huge volume of water impounded can trigger a quake by destabilising a fault and therefore the early studies were mainly directed to correlating local seismic events to fluctuations of water levels in the reservoirs.
At a later stage this theory was modified to include a speculation that the water may step down underneath to act like a lubricant along a fault plane and reduce the shear strength of rocks under stress and thus facilitate movement along the fault which otherwise is in a state of equilibrium. This line of thought deserves more credibility because of the observation that the fluid injections through bore-holes in the oilfields to increase yield causes earthquakes.
However, the RIS theory is still controversial and does not find wide acceptability. There are many large dams free from water induced seismicity even after the reservoirs were full. The dams at Mangla in Pakistan and at Bhakra, Pong, Pandoh and Ramganga have not induced seismicity even though they are located well within an active seismic belt.