The Hindu Business Line : Survey finds upheavals, subsidence in Andamans

PRELIMINARY observations by the Survey of India (SOI) of the tsunami-hit Andaman and Nicobar region suggest that while some islands have had upheavals, others experienced subsidence.

The movements of the islands have not been uniform either in magnitude or direction, according to a top official of the Department of Science and Technology, Government of India, who did not want to be named.

Meanwhile, the islands continued to be rattled by after-tremors almost a month after the fateful morning. The National Earthquake Information Centre of the US Geological Survey (USGS) said a strong quake of magnitude 6.3 on the Richter scale was recorded in the Nicobar region at 9.46 a.m. on Monday in the latest in the series. The epicentre was traced to 120 km southwest of Misha, Nicobar Islands.

As if on cue, Sulawesi in Indonesia, further to the east, also came under the influence of a quake of the same magnitude at 4.10 a.m. (local time) the same morning.

On Sunday, the USGS had reported another quake of lesser magnitude (5.5) along the mid-Indian ridge, the epicentre being located to 2,730 km south southwest of Sri Lanka.

The Survey of India has 12 ground control points (GCPs) in the Andaman and Nicobar region that determine its topography, all of which have been affected due to the devastating impact of the earthquake and the tsunami that followed it on December 26 last.

Fresh survey: The prevailing scenario in the region, the official said, will necessitate a fresh survey to quantify the shift of the GCPs. Tidal heights in Port Blair have since increased by about 2.6 metres. At the same time, the extent of subsidence has been estimated to be in the region of 60 cm.

The net result is that there is an overall increase of two metres in the water level at the Port Blair tide gauge.

These are preliminary observations to be confirmed by more extensive ground surveys, the official told Business Line.

Earlier, the Surveyor-General of India, Mr P. Nag, had said in New Delhi on return from a tour of the affected areas that the islands might have moved by about 1.25 metres towards the southeast and been twisted by several degrees after the tsunami battered the region as well as the southern parts of the mainland.

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