- Tsunami warning issued after 8.7 magnitude quake strikes off Aceh coast
- Aftershock hits hours later as similarly powerful magnitude of 8.2
- Fears of repeat of 2004 disaster, which killed 31,000 people in province
- Panic in streets as buildings shake for four minutes and people flee to hills
- David Cameron, visiting 1,600 miles from Aceh, says UK is 'ready to help'
Thousands of people in Aceh - 270 miles from the epicentre of the first 8.7 magnitude quake - fled to the hills fearing a repeat of the deadly 2004 Boxing Day disaster which devastated the province.
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| Rush: Clogged streets in Banda Acey as people grab whatever vehicles they can find and head to the hills. There were fears that a tsunami could be as bad as the one on Boxing Day in 2004 |
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| Panic: People in Banda Aceh scramble to ecsape the city after a tsunami warning was issued |
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| Rush: Residents of Banda Aceh evacuate following a tsunami warning after two massive earthquakes |
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| Piled up: A father loads his wife and four children on to his moped and drives off |
In the main city of Banda Aceh, terrified residents screamed 'God is great!' as they jumped into cars and the backs of motorcycles, clogging streets as they fled to high ground.
Buildings shook for four minutes and there were reports of people jumping from windows in a desperate attempt to escape.
Then, four hours later, a massive aftershock - with a similarly huge magnitude of 8.2 - struck only 110 miles further out to sea, unleashing even more panic.
A tsunami alert was issued for other countries across the Indian Ocean today, including India, Sri Lanka, Australia, Burma, Thailand, the Maldives and other Indian Ocean islands, Malaysia, Pakistan, Somalia, Oman, Iran, Bangladesh, Kenya, South Africa and Singapore.
There are fears of a repeat of the 9.1-magnitude quake seven years go that triggered a tsunami that killed 230,000 people. Nearly three quarters lived in Aceh, which is on the Sumatra island.
The first quake, which was centred 20 miles beneath the ocean floor, was later thought unlikely to have triggered a fatal wave.
However, the aftershock, which was centred 10 miles beneath the ocean around 380 miles from the provincial capital, Banda Aceh, may yet unleash a tsunami.
Roger Musson, seismologist at the British geological survey who has studied Sumatra's fault lines, said the first tremor was a strike-slip quake, not a thrust quake, which causes the sea bed to flip up.
Mr Cameron is visiting the country’s capital, Jakarta, which is 1,600 miles south-east of the province and on a different island, Java. No tremors have been felt there and the city is unlikely to be hit.
He told President Susilo Bambang Yudhoyono: ‘Our thoughts should be with those who are affected.
‘Britain of course stands ready to help if help is required.
‘We will stand with you and your government and your people at this time of worry.
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| Fears: Lines in the ocean show how far tsunami waves could travel in just a few hours |
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| Strike point: The spot where the earthquake hit. The bubbles represent previous quakes |
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| Frail: An elderly woman is helped by her family as they attempt to flee the city |
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| Elsewhere: Tourists in Phuket, southern Thailand head to high ground amid fears a tsunami could strike there again |
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| Moving out: Sri Lankans leave their houses after the government announced a tsunami warning. It was later repealed along with most other countries in the Indian ocean |
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| Leaving: Office workers evacuate from buildings in Colombo, Sri Lanka after tremors were felt there |
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| Gone in a blur: Acehnese people escape in motorcycles and tuk tuks |
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| Concern: An Acehnese woman with a child tries to stop a car to go to higher ground after tremors are felt |
Mr Yudhoyono moved to calm nerves, saying that there appeared to be no serious casualties and local residents had been taken to safety.
He added that ‘as of this time there is no threat of tsunami’ - despite an international warning being issued.
He added: ‘The situation is under control so far. It is a very different situation from 2004 when Aceh was faced with a deadly tsunami.’
The tremor was felt in Singapore, Thailand, Bangladesh, Malaysia and India. A tsunami warning has been issued for cities all along the coast of Sumatra.
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| Visit: David Cameron talks to Indonesian President Susilo Bambang Yudhoyono after his arrival at the Presidential Palace in Jakarta today |
There were also reports of the water level dropping in the Thai resort of Phuket - a sign that the sea is drawing back in preparation to launch an enormous wave.
A tsunami watch means there is the potential for a tsunami, not that one is imminent. Since 2004 such warnings are issued after every earthquake in the Pacific.
Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.
Last year's devastating tsunami in Japan was triggered by an earthquake with a similar hypocentre depth 20 miles below the surface.
The earthquake that struck off the coast of Aceh today has chilling parallels with the disaster that obliterated the region on Boxing Day, 2004.
More than 230,000 thousand people were killed by the resulting tsunami that swept inland and washed away homes.
A further 250,000 people were injured by the catastrophe, which was the sixth deadliest ever recorded.
But so unpredictable is the whim of nature that today's earthquake - despite being almost as powerful - has not had the same fatal effects.
The tsunami that struck in the Indian Ocean in 2004, was caused by a 9.1 magnitude earthquake. Today's earthquake measures 8.7 on the Richter scale.
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| Flood waters in Thailand lap around the damage homes that were crushed by the tsunami that struck off Indonesia in 2004 |
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| Damage: Residents are swept away in the raging torrents that engulfed parts of Sri Lanka after the earthquake off the coast of Indonesia in 2004. The quake was more catastrophic than the recent one |
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| Power: A wall of water crashes over a street in Miyako City, Japan during last year's devastating tsunami there |
Although there is a slight difference in the strength, it would still be strong enough to cause significant damage.
The biggest tsunami ever recorded was triggered by an 8.3 magnitude earthquake in Lituya Bay, Alaska on July 9, 1958 - less powerful than the most recent quake in Indonesia.
And the Japan 2011 tsunami was brought on by 9.0 magnitude quake.
So why has today's quake not caused a killer tsunami?
The quake struck 20.5 miles below the sea bed whereas the one in 2004 was slightly closer to the sea at just 19 miles.
However, due to continual movement of the plates over the last eight years there position today will be different to when disaster struck before. The position of the plates when one of them breaks and causes seismic waves is a vital part of what causes the tsunami.
Although both earthquakes formed on the fault line where the India and Burma plates meet, today's earthquake did not cause disruption in the water.
Roger Musson, seismologist at the British geological survey who has studied the fault lines off Sumatra in northern Indonesia said that today's tremor was a strike-slip quake, not a thrust quake which causes the sea bed to flip up.
It is not impossible for strike-slip earthquakes to generate vertical uplift of water,
'When I first saw this was an 8.7 near Sumatra, I was fearing the worst,' Mr Musson said.
'But as soon as I discovered what type of earthquake it was, then I felt a lot better.'
Mega tsunamis strike when the plates create changes in the water pressure. The plates will have moved under each other differently today than how they did in 2004, creating less change in the water pressure.
Moreover, researchers studying the 2004 quake found small but significant jumps of between 5 and 10 millimeters at stations as far as 3000 kilometres from the epicentre. They also found the rupture was 1000km long and spread rapidly northwards from its origin.
This damage makes a significant difference because it is the 'snapping' motion of the earth's crust below the ocean that causes movement in the water above it. If the plates do not displace a large amount of water, the power of the underwater waves will be less.
In this case it appears that even though an underwater earthquake has been recorded, the disruption to the sea bed has not been strong enough to create momentum in the water.
The analyusts from Ecole Normale Supérieure (ENS) in Paris studied GPS data showing that the damage caused by the earthquake was at least 200km north of its epicentre.
The severity and location of the damage to seabed caused by today's earthquake is still unclear.
One aspect of tsunamis that is within man's control are the warnings that can be put in place. In 2004 there were no alerts telling people that a tidal wave was on its way.
Following the damage, The Indian Ocean Tsunami Warning System was established following a United Nations conference in Kobe, Japan, in January 2005.
Seventeen seismic stations were placed in the ocean to detect plate movements and tremors. These sensors then pass the information to two recording stations which sound sirens and even make automated phone calls and send text messages and emails to residents.
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