A second round of tremors was felt in India in cities like Kolkata and Chennai at 4.25 pm, but there was no tsunami threat from these aftershocks. India's Nicobar Islands, closest to the epicentre of the quake, is still on alert.
The original earthquake struck Indonesia at 2.08 pm, and first reports said it measured 8.9 on the Richter scale. It spurred warnings of a possible tsunami. The quake was later downgraded to 8.6 magnitude.
A little over two hours after the first earthquake, Indonesia reported an aftershock that hovered around 8.2 on the Richter scale.
Officials in the Andaman and Nicobar Islands are prepped for emergency processes. People have been asked to move away from the coast and the government evacuated some people from South Andaman as a precaution.
The Indian Air Force has kept ready for relief ops; six teams of 40 people each from the National Disaster Relief Force (NDRF) have been positioned at Hindon near Delhi, and at Chennai and two C-130 J heavy-lift aircrafts of the Indian Air Force are leaving for Port Blair with NDRF teams and 10 tonnes of relief material.
Before the strong aftershocks, the Home Secretary RK Singh said, "As of now there are no signs of a tsunami." But he said the union government was in constant touch with officials in Andhra Pradesh, Tamil Nadu and Odisha.
The minutes after the first quake saw emergency drills in Tamil Nadu and Andhra Pradesh. On the beaches at Chennai, policemen used megaphones to ask people to leave, with little effect. Mobile phone networks were jammed in the city till 4 pm. In Andhra Pradesh, fishermen were asked to return from sea. In the Andaman and Nicobar Islands, civilians were asked to move to higher ground. In Kolkata, metro trains were stopped at 2.40 pm for nearly an hour; as a precaution, passengers were evacuated.
The quake was felt in Singapore, Thailand and India. High-rise apartments and offices on Malaysia's west coast shook for at least a minute.
In India, tremors were reported in Mumbai, Kolkata, Bangalore and the southern part of Chennai. The tremors lasted for a few seconds. In Bhubaneshwar, people were seen running out of their homes and offices. No damage has been reported so far.
NDTV viewer AG Kalidass emailed from Bangalore to say, "We were feeling our building shaking." Another viewer Shyam Agarwal writes, "I am from Kolkata. Just now a few minutes back heavy stroke of earthquake has been observed here."
Indonesia's disaster management agency said power was down in Aceh province and people were gathering on high ground as sirens warned of the danger.
"The electricity is down, there are traffic jams to access higher ground. Sirens and Koran recitals from mosques are everywhere," said Sutopo, spokesman for the agency.
The U.S. Geological Survey said the powerful quake was centred 20 miles (33 kilometres) beneath the ocean floor around 308 miles (495 kilometres) from Aceh's provincial capital.
Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.
A giant 9.1-magnitude quake off the country on December 26, 2004, triggered a tsunami in the Indian Ocean that killed 230,000 people in 13 Indian Ocean countries, including Thailand, Sri Lanka and India.