NEW DELHI, Jan. 3 -- The tsunami triggered by a massive undersea earthquake Dec. 26 not only washed away thousands of people living on India's coastlines, it also may have permanently altered India's geography.
India's chief surveyor said Monday that some islands in the Andaman and Nicobar archipelago, a group of about 550 islands stretching between Burma and Indonesia, may have shifted when the Earth shook.
The surveyor, Prithvish Nag, said a team of geodetic surveyors has been sent to the islands "to study how the contours may have changed or islands may have [been] shifted by the impact of the earthquake."
"This is a huge and time-consuming exercise -- to see the shift, the tilt and the tectonic changes in the area," Nag said.
Officials said it was important to assess the changes because of their potential impact on ocean shipping and because a possible northward shift in tectonic plates could affect a quake-prone region in northeastern India.
Military officials confirmed that India's southernmost point in the Indian Ocean, a coral reef in the Nicobars called Indira Point, had vanished beneath the water. The island, about 86 miles from Sumatra, was hit by the full wrath of the tsunami.
"Indira Point has visually disappeared; there is a water column over it," Commandant Salil Mehta said last week in Port Blair, the capital of the Andaman and Nicobar Islands. "Only time will tell if the island will reemerge or if it is gone forever. There is a shifting of landmass, and a survey has to be carried out, because the contour levels have changed in many places. It is vital for shipping."
The surveyors will operate from about 20 control points throughout the islands, using GPS systems to study latitude, longitude and height over a 24-hour period at each point and then compare the results to past surveys.
"Our mapping may have to be revised based on the data we collect," Nag said. "It is not merely changing of the contours. Such was the impact that the islands may have shifted, too."
After the undersea quake, Indian geologists were also concerned about the possibility that the tectonic plate known as the India Plate may have shifted north, affecting the seismic vulnerability of the region.
"The Indian Plate has been moving toward north at 3 to 5 centimeters a year, building tremendous stress in the Himalayas," said Gautam Dasgupta, a geologist at the Geological Survey of India. "When the Sumatra earthquake occurred, we wanted to see if it had pushed it further north toward our earthquake-prone state of Assam."
The Earth's subsurface is composed of tectonic plates that move slightly over time; abrupt shifts in those plates can cause earthquakes. According to the U.S. Geological Survey, the Dec. 26 quake occurred when the Burma Microplate suddenly moved above the adjoining India Plate.
The USGS said the quake caused slippage of about 700 miles of sea bottom along the boundary line between the two plates, probably by about 50 feet, and lifted the sea floor by several feet. It did not indicate whether the India Plate had moved northward, however.
The Andaman and Nicobar Islands have had more than 80 moderate tremors since the first quake, but Indian officials rejected suggestions that the aftershocks were moving northward.