Fears of quakes in South India 'exaggerated'

By K.C.Vikas Kumar

MANGALORE, JAN. 31. Eversince the series of quakes rocked several cities along the West Coast in mid-December 2000, to culminate in the major quake of Bhuj on Republic Day this year, the nation has been a mute witness to the havoc that the fury of nature could wreck on it. While there have been serious concerns, following the incident, over the possibility of further quakes rocking several parts of South India, including the West Coast, a few in the scientific fraternity feel that the concerns are in fact misplaced.

Speaking to The Hindu, the Chairman of the Department of Marine Geology, Mangalore University, Dr K.R.Subrahmanya, said that while it was not possible to rule out the occurrences of quakes along the belt that connected Shoranur, Coimbatore and Madras, it was possible to rule out any such activity along the West Coast.

He also ruled out the possibilities of Bangalore expecting a major quake at any given point. The region had been experiencing several low intensity quakes for the past several thousand years and there had not been a build-up of any kind that could possibly cause a greater quake of the magnitude of 5 and above on the Richter scale.

Explaining the phenomenon of quakes, Dr Subrahmanya said that the spreading of the sea floor caused by the breaking of the Indo- Australian Plate had caused the Indian part of the plate to move in the northerly direction at a rate of five cm per year. However, the presence of the Chinese plate in the north had been opposing this northerly movement and causing a strain in the region surrounding the 13 degree north latitude, but the gradual dissipation of most of the pent up energy in small bursts made the area relatively safe, he inferred.

The evidence for such a strain could be observed by the convexing of the shorelines on either sides towards the sea and the unusual behaviour of rivers that run on either side of the line. While the rivers on the northern end of the line drained into the sea in the northerly direction, those in the southern part had been showing a drain pattern in the southerly direction for the past several million years (pic 1). The tide guage data for Mangalore and Chennai also indicated a relative fall in sea level.

The line, which was also called the Mulki-Pulicat Lake Axis, would be the centre of a series of low intensity tremors. Dr Subrahmanya said it was hard to say if the recent quake that rocked the city of Bangalore was even of the quake itself or that it was a ``foreshock''. It was common for a major quake to be preceeded by a small intensity ``foreshock'' and followed by a mild aftershock.

The theory of plate tectonics stated that continents and oceans were carried on huge plates that floated on the Earth's semi- molten magma. For the past 50 million years the continental plate of India had been drifting in the north-easterly direction and slowly slamming into the southern belly of Southern Asia and pushing up the Tibetan Plateau and the Himalayan mountains. This gradual northerly movement of the huge plate had caused a distortion of the Asian continent.

A team that recently conducted studies using sound waves to probe the oceanic rock in the region believed to be the new plate, unearthed the presence of systematically alligned cracks or faults in the oceanic lithosphere created when the earlier plate buckled and crashed, thus proving that the giant Australian plate was rotating and pushing against the Indian plate in a northerly counter clockwise direction.

Dr Subrahmanya maintained that it was usual for the oceanic plates to show the ``subduction effect'' wherein the older plates sunk into the magma and newer plates were formed elsewhere. Such constant activity along the surface were a major cause for the shifting of continents and the phenomenon of high strain regions and associated quakes.

The Saurashtra region (pic 2), which lay just beneath Bhuj, the epicentre of the January 26 quake, was a confluence of major faults and had been known as a seismically active zone. In fact, the region had experienced a major quake in 1819 and an excavation by a team of geologists had unearthed a 80 feet-100 feet wide geographic formation, 100 km to the north of Bhuj, today called the ``Allah Bund'' showing multiple liquefaction features induced by earthquakes. The formation also showed the presence of an older feature probably generated 800 years-1000 years ago strengthening the fact that strong quakes of a similar magnitude had rocked the place earlier.

Dr Subrahmanya said that the aftershocks that were continuing in the region would subside with time and that the region would return back to peaceful existence once more.

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