The Hindu : National : Absolute gravity station set up at Port Blair

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Monday, Apr 03, 2006

National

National

Absolute gravity station set up at Port Blair

Y. Mallikarjun

HYDERABAD: The National Geophysical Research Institute (NGRI) has established the first absolute gravity station at Port Blair in the Andaman and Nicobar Islands. This is to determine the extent of the Indian plate's subduction in the light of seismic findings that it had gone down by almost a metre following the Sumatra earthquake of December 2004.

Studies carried out with the help of the Global Positioning System (GPS) and broadband seismometers deployed in the region indicated that some areas had gone down by a metre. "We want to confirm it by absolute gravimeter, which can record very precisely even a three millimetre elevation or subsidence," Bijendra Singh, who led a team of NGRI scientists to the islands, told The Hindu.

Mr. Singh said the gravimeter was set up in the Science and Technology Department of the Andaman administration and preliminary measurements were carried out for a week from February 10 to 16. The evaluation would be repeated every six months for the next five years as part of a broader initiative, including GPS and broadband seismometers. A network of GPS measurements was also being established in the seismic-prone region.

Explaining the importance of gravimeter, he said it would provide foolproof indication of the rate of change on "whether stress is building up normally or abnormally." Absolute gravity measurements could provide vertical deformation associated with tectonic disturbances, as a change in gravity results from both vertical motions and mass changes below and above the surface of the earth.

The sensitive instrument, in conjunction with GPS measurements, could discriminate between vertical motion and mass changes. Since the Andaman and Nicobar region was part of an active tectonic zone due to subduction of the Indian plate beneath the Burmese plate, appreciable vertical deformation was predicted in the region. Therefore, establishing the gravity station and repeating measurements would enable precise determination of the quantum of deformity in the earth's crust. The gravimeter was being used in the country for the first time, Mr. Singh said.

NGRI Director V.P. Dimri, along with Mr. Singh, met the Chief Secretary and other senior officials of the Andaman administration and apprised them of the studies undertaken for a better understanding of the earthquake processes in the region.

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