Shaken South

The movement of the Indian plate was the basic cause of the earthquake that struck some parts of Tamil Nadu, Andhra Pradesh and Karnataka on September 25.

ON September 25 an earthquake struck Chennai and other parts of Tamil Nadu, including the composite districts of Thanjavur, North Arcot, South Arcot and Chengalpattu, the Union Territory of Pondicherry, Nellore and Chittoor districts in Andhra Pradesh, and parts of Karnataka including its capital Bangalore. There was no major damage anywhere.

According to A.K. Bhatnagar, Deputy Director-General, Meteorology, Regional Meteorological Centre, India Meteorology Department, based in Chennai, the earthquake, which measured 5.6 on the Richter scale, occurred at 8-27 p.m. It had its epicentre in the Bay of Bengal, 50 km east of Pondicherry, located at a latitude of 11.8 N and a longitude of 80.4 E.

D.C. Mishra, Director, National Institute of Geophysical Research, Hyderabad, said the earthquake was of moderate intensity. He said it occurred at the point of contact between the deep sea and the continental shelf in the Bay of Bengal. Such zones are vulnerable to earthquakes because they are usually faulted. Faults are weaknesses in the earth's crust.

Mishra said the earthquake did not whip up tidal waves because its focus was deep in the sea and its magnitude was too low for tidal waves to occur.

Dr. L.S. Suryanarayanan, Director-in-charge, Geological Survey of India (GSI), Tamil Nadu, Kerala and Pondicherry, said the focus of the earthquake was 12 km below the sea surface.

The basic cause of the earthquake was the Indian plate's movement. The Himalayas form the northern margin of this plate. According to Suryanarayanan, the reason for the movement is the heat generation inside the earth. This heat difference led to the movement of the plate several million years ago. When movements take place not only at intra-plate but inter-plate junctions, collisions occur and this leads to earthquakes.

Suryanarayanan said there were alternating hill and valley formations deep below the Cauvery delta area, which formed the basement. These were created by tectonic disturbances millions of years ago. There were structural weaknesses in the planes of these hills and valleys. "On account of these weak planes, differential movements do take place in the basement at the time of the plate's movement," he said.

Cracks had developed in the Indian plate about 80 million years ago. The entire Indian plate, along with the cracks, started moving north about 50 million years ago. The Himalayas were thrown up by this movement. About 25 to 30 million years ago, the Indian plate started hitting against the mass of the Eurasian plate, which is situated in the north.

Suryanarayanan said, "In this process of jerks which it (the Indian plate) is undergoing, earthquakes occur in weak planes, along faults and at places where major stresses are developing. The release of the strain leads to earthquakes, and further displacement follows."

THERE have been tremendous advances in the understanding of the earth and its processes in the last 50 years. Geologists had tried to understand the structure of the earth by studying earthquakes, Suryanarayanan said.

The GSI, Chennai, has sent two teams to make macroseismic surveys to study the effect of the September 25 earthquake on people, the ground, and structures such as buildings and bridges in Chennai, Pondicherry, Cuddalore, Chidambaram, Vellore, Tiruchi, Dharmapuri and other places.

Suryanarayanan said the "felt-effect" of the Bhuj earthquake was high in Pondicherry, Bhuvanagiri, Cuddalore, Chidambaram and other places. (Tremors were felt in Chennai also on that day.) The felt-effect of that earthquake was high in Pondicherry because it was absorbed, not passed on, he said. The absorption was because of the loose packing of sediments.

Asked how an earthquake had jolted Tamil Nadu, hitherto considered to be a seismically safe zone, Suryanarayanan explained that the rim of the Indian plate, which had borne the brunt of collisions and a high number of earthquakes, had totally broken. The plate proper, which was away from the margin, was considered to be less prone to earthquakes.

Now peninsular India might have to be categorised as Zone III instead of Zone II. The GSI's seismic zonation map made in the 1960s has undergone several revisions. A draft revision is being done now.

Following "predictions" that there could be after-shocks, rumours filled the air about the imminence of another earthquake. But Bhatnagar asserted that there was no scientific method anywhere in the world to predict earthquakes in respect of location, time and magnitude. In the case of moderate earthquakes, only aftershocks of mild intensity could occur and these might not cause any major damage, he said. He clarified that no forecast had been issued in this regard by the IMD. "Predictions circulated by any individual are not to be trusted," Bhatnagar said.

Officials of the Nuclear Power Corporation of India Limited (NPC) said the two reactors of the Madras Atomic Power Station (MAPS) at Kalpakkam, about 55 km from Chennai, continued to operate safely at their rated capacity (170 MWe each). "There was no disturbance to the nuclear and conventional systems, and power flow to the southern grid is being maintained as usual," said K. Hariharan, Station Director, MAPS. He added: "The Indian nuclear reactors have adequate safety design features built in to take care of seismic effects." He pointed out that the nuclear power stations at Kakrapar (Gujarat) and Tarapur (Maharashtra) were not affected in any way by the January 2001 earthquake.