PORT BLAIR, India (Reuters) - A strong earthquake with a magnitude of 7.5 struck in the ocean near India's Nicobar Islands on Sunday, sparking some tsunami warnings, the U.S. Geological Survey and local officials said.
There were no initial reports of casualties or damage, although people ran from their homes in fear on Nicobar, witnesses said.
The quake was originally registered with a magnitude of 7.7 but that figure was later revised down slightly to 7.5, the USGS said in a statement.
The Pacific Tsunami Warning Center in Hawaii initially issued a regional tsunami watch that was put in effect for all areas of the Indian Ocean, including India, Indonesia, Sri Lanka, Myanmar, Thailand and Malaysia.
The warning was later revised down to cover India only and was then canceled for all areas.
"Sea level readings indicate that a significant tsunami was not generated," it said on its website (www.weather.gov/ptwc/).
The epicenter of the quake was 97 miles west of Misha on the Nicobar Islands at a depth of 21.7 miles, USGS said, slightly revising earlier figures.
Indian meteorological authorities said there was no tsunami threat to the Indian mainland.
"There has been no significant changes in the water level and we have not issued any tsunami alert for the region," said Srinivas Kumar, an official from the Tsunami Center in India's southern city of Hyderabad.
People ran from their houses in Port Blair, capital of the Andaman and Nicobar islands, immediately after the quake struck, officials and witnesses said.
"There was a total blackout and the whole town was plunged into darkness for a while as we ran out of our houses fearing for our lives," said Robin Biswas, a witness in Port Blair.
Authorities said no casualties had been reported. "We are however checking for damage in remote areas," said a duty officer from the local disaster management department in Port Blair who refused to be named.
Sri Lanka also issued a separate tsunami warning for its coastal areas but later withdrew the notice after watching for possible evacuations of people in low-lying areas.
In Indonesia, which bore the brunt of a devastating tsunami in 2004, officials in Aceh in northwest Sumatra issued a tsunami warning after the original USGS quake alert.
"It is estimated that it could take an hour for a tsunami to arrive. If there is no sign we will lift the warning," Rahmat Triyono, an official at the meteorological quake agency's quake center, told Reuters.
In December 2004, a magnitude 9.15 quake off the coast of Sumatra triggered an Indian Ocean tsunami that killed around 226,000 people in Indonesia, Sri Lanka, India, Thailand and nine other countries.
Additional reporting by John O'Callaghan in WASHINGTON, Bappa Majumdar in NEW DELHI, Ranga Sirilal in COLOMBO, Sara Webb in JAKARTA; Editing by Paul Tait