Tsunami: Anatomy of a disaster
A year after the massive tsunami of Dec 26, Helen Lambourne of BBC analyses why it happened
At 0059 GMT on 26 December 2004, a magnitude 9.3 earthquake ripped apart the seafloor off the coast of northwest Sumatra.
Over 100 years of accumulated stress was released in the second biggest earthquake in recorded history.
It unleashed a devastating tsunami that travelled thousands of kilometres across the Indian Ocean, taking the lives of more than 200,000 people in countries as far apart as Indonesia, the Maldives, Sri Lanka and Somalia.
A new BBC One programme, featuring the harrowing stories of survivors, gives a scientific account of the disaster.
The earthquake
Two hundred and forty kilometres (150 miles) off the coast of Sumatra, deep under the ocean floor, at the boundary between two of the world’s tectonic plates, lies a 1,200km (745 miles) trench called the Andaman-Sumatran subduction zone.
At about the same speed as your fingernails grow, the lower plate, carrying India, is being forced or subducted beneath the upper plate, carrying most of South-East Asia, dragging it down, causing huge stresses to build up.
These stresses were released on 26 December. Shaking from this giant mega-thrust earthquake woke people from sleep as far away as Thailand and the Maldives.
Unlike the more frequent strike-slip earthquakes of Kobe or Los Angeles, which last for a matter of seconds, subduction zone quakes last for several minutes.
The shaking during the Indonesian event went on for eight minutes.
Nobody knows how many died in the actual quake itself, but scientists have since visited the nearby island of Simueleu and found something astonishing.
The whole island has been tilted by the force of the earthquake, causing coral, submerged beneath the ocean for thousands of years, to be thrust out of the water on the east side; bays in the west have been drained.
“We were astonished to find ourselves walking through a pristine marine ecosystem, missing only its multitude of colours, its fish, and its water,” said Professor Kerry Sieh, from the California Institute of Technology, US.
Yet, when the shaking from the earthquake subsided, no-one had any idea that the tremors had set in motion something far more deadly - a tsunami.
The tsunami
Deep under the Indian Ocean, at the epicentre of the quake, the 20m (65ft) upward thrust of the seafloor set in motion a series of geological events that were to devastate the lives of millions.
Billions of tonnes of seawater, forced upward by the movement of the seabed now flowed away from the fault in a series of giant waves.
The only people in the world to have any idea what had happened were thousands of kilometres away on the island of Hawaii.
But, relying on seismic data alone, the scientists at the Pacific Tsunami Warning Center had no idea the earthquake had unleashed an ocean-wide tsunami.
It was a full 50 minutes after they first picked up the tremors that they issued a warning of a possible local tsunami.
Thirty minutes after the shaking had subsided, the first wave, travelling eastwards, crashed into Sumatra.
On the shores directly facing the epicentre, the waves reached heights of 20m (65ft), stripping vegetation from mountain sides 800m (0.5 mile) inland, capsizing freighters and throwing boats into the trees.
The city of Banda Aceh, just a few kilometres further round the coast was almost completely destroyed, killing tens of thousands of people in just 15 minutes.
What the elephants kenw
Leaving a devastated Sumatra behind, the series of waves continued across the Andaman Sea towards Thailand.
A herd of elephants in the mountains seemed to know it was coming. They began behaving strangely, stamping the ground and tugging at their chains, eventually breaking away to run to the hills.
Elephants have special bones in their feet that enable them to sense seismic vibrations long before we can. Animals taking to the hills was not the only sign that something was about to happen.
Due to the complex way in which the seafloor ruptured, some waves set off travelling with the crest first, others travelling trough first.
The trough, reaching the shores of Thailand, caused the sea to disappear off the beaches. It is one of the classic warning signs of an approaching tsunami.
Tragically, many tourists went down to the beach to look, some to rescue fish left flapping on the sand. A few minutes later, the first wave hit Thailand.
A thousand tonnes of water crashed down on each metre of beach. At Khao Lak, the wave reached 10m (30ft) and caused billions of pounds of damage. The human cost was far greater - nearly 5,000 confirmed dead and 3,000 still missing.
At the same time, the westbound series of waves were heading for Sri Lanka. In the deeper waters of the Indian Ocean, barely noticeable at just a 30cm (1ft) above the surface, they were travelling at some 800km/h (500 miles per hour).
Sri Lanka
The first wave hit Sri Lanka with no recede and no warning. The waves, up to six of them, weighing over 100 billion tonnes, rushed inland like a giant tide.
As they hit Sri Lanka’s southern tip, they began to change direction, an effect called refraction.
The part of a wave closest to the shore slowed down in the shallow water, leaving the outer part, travelling at faster speeds, to bend around the island. The southwest coast of Sri Lanka, the side that should have been safe, was suddenly in the waves’ direct line.
Cities such as Galle were destroyed; over 4,000 people died in this region alone.
The waves carried on further north to India, where they killed 10,000 people.
The Maldives
Next in the waves’ line, was one of the lowest lying countries on Earth - the Maldives.
Miraculously, although 80 people died here, this country escaped relatively unscathed.
It seems that due to their unique geography, being the tips of underwater volcanoes and without a continental shelf to push the wave height up, the tsunami just washed through.
Coral reefs are also thought to have protected the country, acting like a giant underwater colander, stripping the waves of energy.
As the waves left the Maldives, they passed through a narrow gap between the island chains, focusing their energy directly at Somalia, where 300 people lost their lives.
In Kenya, the waves, when they hit were small; their energy further removed by the land masses of the Seychelles and Diego Garcia.
They had also seen the news reports and evacuated the beaches; only one person died.
The last victim of a natural disaster that had claimed 300,000 with hundreds still unaccounted for.
Tsunami: mangroves ‘saved lives’
Healthy mangrove forests helped save lives in the Asia tsunami disaster, according to a new report, writes Mark Kinver
The World Conservation Union (IUCN) compared the death toll from two villages in Sri Lanka that were hit by the devastating giant waves.
Two people died in the settlement with dense mangrove and scrub forest, while up to 6,000 people died in the village without similar vegetation.
Many forests in the past were felled to build prawn farms and tourist resorts.
The IUCN said it showed that healthy ecosystems acted as natural barriers.
‘It saved a lot of lives as well as properties,’ said Vimukthi Weeratunga, the union’s biodiversity coordinator in Sri Lanka.
‘We have carried an out ecological assessment of the damage caused by the tsunami. In some areas the damage was very minimal, and mangrove vegetation had played a role.’
Research has shown mangroves are able to absorb between 70-90 per cent of the energy from a normal wave.
There is, however, no reliable data on how the trees mitigate the impact of a tsunami.
Many people living in coastal areas now want to see their communities benefit from the apparent protection offered by mangrove forests.
‘People tend to respect these natural barriers even more, especially after the tsunami,’ Mr Weeratunga said.
‘Now everyone is keen to plant a lot of mangroves in the coastal areas but unfortunately we cannot plant mangroves everywhere.’
Slow recovery
Coral reefs were also in the direct path of the tsunami. Fears for these ecosystems were allayed after initial surveys found that there had not been widespread, long-term damage.
But they did not escape unharmed. Debris and silt from the shore was washed out to sea and covered the reefs
Twelve months later, the IUCN has found that reefs which were in good shape before the waves struck are recovering much more quickly than degraded sites.
Lucy Emerton, head of the union’s ecosystems and livelihoods group in Asia, explained why.
‘Healthy coral reefs are much more robust in terms of recovering from either natural or man-made disasters,’ she told the BBC News website.
‘Physically, you see a beautiful coral reef that is coming back to life rather than one that is still smothered in debris.’
Many of the reefs in the Indian Ocean had been damaged from dynamite fishing, coral mining and bleaching.
The protection that healthy marine and coastal ecosystems provided during the disaster highlighted the need for effective environmental policies, Ms Emerton said
It was immediately obvious what an important role mangrove forests, wetlands and coral reefs played in mitigating the impact [of the tsunami],’ she argued.
‘It has led to a real step forward in looking at integrated coastal management systems.’
She said strong conservation laws already existed but there had been questions about how effective they had been enforced.
One example of a local government flexing its conservation muscles is the recent declaration to establish two new turtle sanctuaries in southern Sri Lanka.
Elsewhere, four international conservation groups, led by Wetlands International, have launched a project called Green Coast.
Working alongside local governments and construction companies, the project hopes to rehabilitate the habitat in areas affected by the disaster.
Fishing fears
Both mangrove forests and coral reefs found in coastal areas provide vital protection and breeding grounds for fish - a key source of income and nutrition for people in the region.
A report published by the Malaysian-based WorldFish Center has warned that misplaced investment by donors could do more harm than good in the long term.
When the giant waves swept inland in December 2004, between 80-90% of the fishing fleet was destroyed.
This prompted a massive effort by the international community to replace lost vessels and gear.
While welcoming the overwhelming response, the centre’s director general, Stephen Hall, said it was important for donors to coordinate their efforts with the governments.
‘There is a real danger that we are going to set these communities back on the downward spiral of unsustainable fish stocks,’ he said.
‘The Indonesian government estimates that 10,500 boats were lost [in Aceh]. Of those, around 2,500 were repaired. Recent estimates put the number of boats that have been built or in the process of being built at around 10,800.
‘So we now have 2,800 more boats than we started with.’
Dr Hall said this created the potential to place even greater pressure on already overexploited stocks.
This view is shared by the IUCN’s Vimukthi Weeratunga.
‘Immediately after the tsunami, a lot of the NGOs and other well wishers were distributing boats.
‘But we saw the distribution of too many boats and an increase in fishing.’
Latest figures from the UN Farming and Agriculture Organization (FAO) show fish catches in Aceh for 2005 are down 41% for marine fishing and 26% for brackish water aquaculture.
Balancing act
The United Nations Environment Programme (Unep) published a report in November highlighting the challenges of meeting the demands of the mammoth relief effort without exacerbating the damage to the environment.
It warned that haphazard groundwater extraction, unsanitary disposal of waste, chaotic rebuilding of homes and unsustainable timber harvesting could result in more environmental damage, leading to an increase in poverty and greater vulnerability to future disasters.
A number of agencies, including the FAO and Unep, are working closely with the governments of nations affected by the disaster.
Help is being provided in a number of areas, such as offering technical assistance to overstretched environment ministries and coordinating the mobilisation of funding.
As attention begins to focus on the future, Vimukthi Weeratunga warns that there are no quick fixes when it comes to repairing the environmental damage.
‘It will take five to seven years, at least, to get to the pre-tsunami stage - and that is a conservative estimate.’
— BBC
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