India and Sri Lanka issued tsumani alerts after a strong earthquake of 7.5 magnitude struck near India's Nicobar Islands and northwest of the tip of Indonesia's Sumatra island.
The quake struck at 1.26am on Sunday local time. Its epicentre was 155km west of Misha on the Nicobar Islands at a depth of 34km, the USGS said.
There were no initial reports of casualties or damage, although people ran from their homes in fear on Nicobar, witnesses said.
The Pacific Tsunami Warning Center, based in Hawaii, initially issued a tsunami watch for the entire Indian Ocean region.
The agency later downgraded the warning to India only, before cancelling the alert altogether.
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.