Saturday, August 18, 2007

2004 earthquake shifts southern Indian cities

The Hindu, Aug. 15, 2007

Y. Mallikarjun

Horizontal movement towards Nicobar

Major tectonic shift occurred at a fast pace

Land mass might take long to return to earlier position

HYDERABAD: The Andaman and Nicobar belt moved horizontally by 3 metres to 6 metres, Chennai by 2 cm, Bangalore by 1.5 cm and Hyderabad by 11 mm following the undersea Sumatra-Andaman earthquake in 2004.

In the normal course such a tectonic shift would have taken hundreds of years to occur but it happened in less than 10 minutes during the earthquake. The impact caused by the 9.2 magnitude temblor could be gauged by the fact that the Indian plate was moving at the rate of 4 cm a year with respect to the Burmese plate.

Scientists from the National Geophysical Research Institute (NGRI) who carried out GPS-based studies in the Andaman and Nicobar islands before and after the earthquake, told The Hindu on Monday that horizontal movement was noticed t owards the Nicobar side.

While a 3-m movement was found in the middle of Andamans, it was 6 m between Car Nicobar and Great Nicobar.

The entire island also subsided by 1 m to 2 m vertically. Interestingly, it began to rise again but at a slow speed, and 30 per cent of the land had ‘re-emerged,’ said NGRI Director V.P. Dimri and seismologist V.K. Gahalaut.

Explaining that the uplift of the subsided land mass was occurring in a non-linear manner, they said it might take up to a couple of hundred years for it to return to the pre-2004 position.

Dr. Gahalaut said the boundary between the Indian plate and the Burmese plate, in the sub-duction zone, is about 150 km west of the Andamans. The overriding plate (Burmese) moved by 3 m to 6 m during the earthquake along the 1,500-km faultline extending from North Andaman to West Sumatra.

He explained that before the earthquake, the Indian and Burmese plates were moving together as they were locked and there was no relative movement between the two.

After the earthquake, they got disengaged and the Burmese plate was moving in a southwest direction with respect to the Indian plate at a rate which is faster than the normal plate motion but less than the speed which occurred during the massive temblor.

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