Indonesian authorities say five people had heart attacks and died after two large earthquakes triggered back-to-back tsunami warnings.

No other deaths were reported from Wednesday's temblors, but several people were injured as they tried to flee to high ground.

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The main quake, an 8.6-magnitude quake, struck at 8.38pm (NZT) off the west coast of northern Sumatra, Indonesia, according to the US Geological Service (USGS). It had initially been graded as an 8.9.

The second, an 8.2, struck in the same area at 10.43pm (NZT). Seventeen aftershocks of magnitude 5.0 and above followed.

Aside from a few cracks in the walls of houses in hardest hit Aceh province and structural damage to one bridge, amazed residents said Thursday it almost felt like nothing had happened

Aceh was the area decimated by the 2004 Boxing Day disaster, in which a giant 9.1-magnitude quake triggered a tsunami in the Indian Ocean that killed 230,000 people, nearly three-quarters of them in the province.

However, some areas close to the epicentre were remote so it could take some time to find out if there was any damage.

NEW ZEALANDERS SAFE

All of the New Zealanders who were registered as being in Indonesia were safe and well, said the Ministry of Foreign Affairs (Mfat).

While Stuff had received a number of reports of people back home in New Zealand not being able to reach their loved ones, the ministry confirmed it had been in touch with all those it was aware were over there, and no one had needed any help.

"There have been no requests for assistance from any New Zealanders in Thailand or Indonesia following last night's earthquake and tsunami warning.

"The Ministry of Foreign Affairs and Trade has made contact with all 288 registered New Zealanders in Indonesia," an Mfat spokesman said.

The ministry was advising Kiwis in the quake zone areas to monitor local media and do what the authorities told them in the case of aftershocks.

The Mfat spokesman said he was not aware of any casualties and Indonesia had made no requests for international assistance, however they were monitoring the situation were in close contact with Indonesian authorities.

Fairfax employee Bevan Ngan has been in Panang, Malaysia for the past week, and said the atmosphere there was "one of confusion".

Talking from Batu Ferringhi beach in the North of Malaysia, he said there wasn't too much panic but tourists there were unable to find out much of what was going on.

"The beach is closed off, normally it's just full of people, and there is a car with a loudspeaker which is driving up and down saying things in Malaysian, but we're getting most of our updates off Google."

Ngan said he and his family were in their 6th floor hotel room when the first quake struck, and the shaking went on for about a minute.

"Half an hour later, hotel staff were still not aware whether there was a quake or not, and then confusion broke out when there were tsunami warnings."

He said no one had been moved to higher ground, but things had calmed now the warnings were cancelled.

EVACUATIONS

Indonesia's disaster management agency said power failed in Aceh and people gathered 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 warning system worked," Indonesian President Susilo Bambang Yudhoyono said.

Warning sirens also rang out across the Thai island of Phuket, a tourist hotspot that was one of the worst-hit areas in the 2004 tsunami.

"Guests from expensive hotels overlooking Phuket's beaches were evacuated to the hills behind and local people were driving away in cars and on motorcycles. Everyone seemed quite calm, the warning had been issued well in advance," freelance journalist Apichai Thonoy said.

Many people were frightened of further tremors.

"It's dark out here but I am scared to go home," said Mila, a 41-year-old woman taking refuge in the grand mosque in the town of Banda Aceh, the provincial capital.

"I just want to stay alert because I fear there will be more quakes coming. We are human, it is only natural that we have fear, but I really wish we will all be safe."

Waves of up to one metre high were seen near islands off Aceh, but Indonesia cancelled a warning of fresh tsunamis. It said the worst-hit area was the sparsely-populated island of Simeulue, off Aceh's southern coast.

The first major quake was 435 kilometres from Aceh's provincial capital, USGS said.

The tsunami watch that followed from the Pacific Tsunami Warning Center in Hawaii advised countries all along the rim of the Indian Ocean, from Australia and India to as far off as Africa, that a tsunami could be generated.

But just as the region was sighing relief, the 8.2-magnitude aftershock hit.

"We just issued another tsunami warning," Prih Harjadi, from Indonesia's geophysics agency, told TVOne in a live interview.

His countrymen were told to stay clear of western coasts.

Again, the threat quickly passed.

Experts said both quakes were geologically different than the one that spawned the 2004 tsunami, occurring horizontally, with the tectonic plates sliding against each other, creating more of a vibration in the water.

The other type of earthquake, a "mega thrust", like the one that hit off the coast of Japan last year, caused the seabed to heave and displaced water vertically, sending towering waves racing toward shores.

Roger Musson, seismologist at the British geological survey who has studied Sumatra's fault lines, said initially he'd been "fearing the worst."

"But as soon as I discovered what type of earthquake it was ... I felt a lot better."

The tremors were felt in neighbouring Malaysia, where high-rise buildings shook. Singapore, Thailand, Bangladesh and India also were rattled.

Jason Williams, a Kiwi working in the Malaysian city of Georgetown, Penang, felt the quake as a long rocking motion.
It went on for over a minute, he said.

"We thought 'gee this thing is not going to let up'."

Williams said he had felt more severe aftershocks in Christchurch, but the length of the earthquake was unusual.

Along with locals, he rushed for the exits of Penang International Airport, where he was working.

"We stood on the apron and watched the planes rocking back and forth."

There were no signs of damage in Georgetown, he said.

But it was the streets of Aceh where real chaos broke out.

Patients poured out of hospitals, some with drips still attached to their arms. In some places, electricity was briefly cut.

Hours after the quake, people were still standing outside their homes and offices, afraid to go back inside.

"I was in the shower on the fifth floor of my hotel," Timbang Pangaribuan told El Shinta radio from the city of Medan. "We all ran out. ... We're all standing outside now."

He said one guest was injured when he jumped from a window.

Thailand's National Disaster Warning Center issued an evacuation order in six provinces along the country's west coast, including the tourist destinations of Phuket, Krabi and Phang-Nga.

India's Tsunami Warning Center issued a warning for parts of the eastern Andaman and Nicobar islands. In Tamil Nadu in southern India, police cordoned off the beach and used loudspeakers to warn people to leave the area.

The quake was felt in Dhaka, Bangladesh, where many people in the city's commercial Motijheel district left their offices and homes in panic and ran into the streets. In Male, the capital of the Maldives, buildings were evacuated.

Indonesia straddles the "ring of fire", a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.

- AP, Reuters and STACEY KIRK/Stuff