March 25, 2005
By Helen Lambourne
BBC

At two minutes to eight in the morning on Boxing Day 2004, a magnitude 9.3 earthquake ripped apart the seafloor off the coast of North-West 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 nearly 300,000 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, at the boundary between two of the world's tectonic plates, lies a deep trench called the Andaman-Sumatran subduction zone.

At about the same speed as your finger nails 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 Boxing Day of last year. Shaking from this giant mega-thrust earthquake woke people up in their beds as far away as Thailand and the Maldives.

Corals were lifted clear of the water, here at Simeuleu in Indonesia

Unlike the more frequent strike-slip earthquakes like the Kobe or Los Angeles quakes, which last for a matter of seconds, subduction zone quakes last for several minutes.

The shaking during the Indonesian event lasted for eight minutes.

Nobody knows how many died in the actual earthquake, 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 and bays in the west to be 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 into motion a series of geological events that were to devastate the lives of millions.

Billions of tonnes of seawater, propelled by the upward movement of the seabed now flowed away from the fault in a series of giant waves.

Everything can be traced to the collision of two tectonic plates(Image: Royal Navy UK)

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 had first picked up the quake 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, losing 200,000 people in just 15 minutes.


THE RECEDE

Leaving a devastated Sumatra behind, the series of waves continued across the Andaman Sea towards Thailand.

Due to the complex way in which the seafloor ruptured, some waves set off travelling with the crest first, others travelling trough first.

The damage across the Indian Ocean region was immense

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.

Heading for Sri Lanka, in the deeper waters of the Indian Ocean, the westbound waves, barely noticeable at just a 30cm (1ft) above the surface, were travelling at some 800km/h (500 miles per hour).


SRI LANKA

When the wave hit Sri Lanka there was no recede and no warning. The waves, up to six of them, weighing over 100 billion tonnes, rushed inland like a giant tide.

As the wave hit Sri Lanka's southern tip, it began to change direction, an effect called refraction.

The part of the wave closest to the shore slowed down in the shallow water, leaving the outer part of the wave, 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 wave's direct line.

Cities such as Galle were destroyed; over 4,000 people died in this region alone.

The wave carried on further North to India where it killed 10,000 people.


THE MALDIVES

Next in the wave's 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 wave of energy.

As the wave left the Maldives, it passed through a narrow gap between the island chains, focusing the wave directly at Somalia, where 300 people lost their lives.

In Kenya, the wave, when it hit was small; its 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: Anatomy of a Disaster tells the story of 26 December 2004, why the waves were so devastating and what scientists are doing to prevent it happening again. The programme is broadcast on BBC One on Sunday at 1835 GMT.

http://news.bbc.co.uk/2/hi/science/nature/4381395.stm