Almost a year since the tragic events in the Indian Ocean, we look back at the tragic events of 26 December 2004 – and ask: could it happen again?

by Stuart Carter

This is the story of a force of Nature so powerful that it killed over 300,000 people, most of them in Sumatra. The destruction it caused has been likened to a nuclear attack. What made it so powerful? And could it happen again?

It’s now almost a year since the tsunami struck and the world is still struggling to come to terms with the enormity of the disaster.

The story starts in the early morning of 26 December 2004. In Sumatra, fishermen are already on the water. In Sri Lanka, 1500 people begin their train journey from Colombo to Galle. In Thailand, beach-front restaurants are opening up. And in Hawaii, scientists at the Pacific Tsunami Warning Center are testing their systems. All these people are totally unaware that they are about to be caught up in one of the worst ever natural catastrophes.

Shortly after dawn and entirely without warning, a huge earthquake rips across the ocean floor south of Sumatra.

Traditional Japanese view of a tsunami

It happens at the junction of two of the great rafts of rock – tectonic plates – that make up the Earth’s crust. Driven by convecting heat from within the planet, these plates are always on the move. And where they collide, massive forces are unleashed.

Under the ocean off Sumatra, the Indo-Australian plate is grinding underneath the Eurasian plate, in a process called subduction. It’s been going on for 20 million years. On 26 December last year, something snapped.

Phil Cummins of Geoscience Australia has been studying earthquakes in the region for over 15 years. He fills in the details: “As the Indo-Australian plate is subducted beneath Indonesia, it pulls it down and deforms the upper plate. The process builds up strain energy. Eventually the stress on that contact exceeds the strength of the contact, and the upper plate snaps back into position”.

The earthquake’s epicentre is 155 miles to the southwest of Aceh province which lies in the northern part of Sumatra. The provincial capital, Banda Aceh, feels the full force of the earthquake; and buildings collapse. The town is wrecked 20 minutes before the tsunami arrives.

Minutes after the earthquake rips through the Earth’s crust, its waves are recorded by a seismometer over 8000 miles away in California. The seismogram shows that the earthquake continued for over four minutes. Later analysis will confirm that it measures 9.0 on the Richter scale, making it one of the most violent on record. It’s more powerful than all the world’s earthquakes over the previous five years put together.

Halfway across the world in Hawaii, barely a minute after the earthquake, computers at the Pacific Tsunami Warning Center spring into life as they pick up the seismic signals. Any earthquake over magnitude 6 prompts an automatic paging sequence to scientists on duty alerting them in an instant.

Dr Stuart Weinstein of the Pacific Tsunami Warning Center recalls “I noticed that the trace of the seismogram was very full. It appeared as just kind of one thick line going across the screen - and that told me that it was a substantial earthquake”.

The warning centre relies on data from a network of seismic stations to detect earthquakes, but their system of coastal tide gauges and deep-ocean pressure sensors triggered by tsunamis are located only in the Pacific. Thanks to this system, the centre has successfully warned Pacific coastlines against a series of killer waves for over 50 years – and saved countless lives.

But – in December 2004 - the Indian Ocean has no such system.

In the first few minutes after the earthquake off Sumatra, data from seismic stations has already alerted Weinstein and his colleague Dr Barry Hirschorn to its location and size – enough to cause them grave concern.

They send a bulletin to alert Pacific coastlines of the earthquake, but with no network of tsunami sensors in the Indian Ocean, Weinstein and Hirschorn have no way of knowing if a tsunami has been triggered there. Hirschorn later explains: “The seismic analysis can only take you so far. We also looked into what water level might be available, but because this was the wrong ocean the answer was none”.

As they watch events unfold, their frustration would turn to despair. When an earthquake occurs under the ocean, its energy can dissipate as shockwaves through the crust – or as tsunamis through the ocean. In this case, just seconds after the earthquake, the energy it released – the equivalent of 23,000 Hiroshima-sized atom bombs – was instantly transferred to the water column above the ocean floor. As the seafloor lifted, it displaced billions of tons of water above it.

Receding flood waters in Sri Lanka CREDIT: DigitalGlobe

In Sumatra, thousands begin their day, unaware of the approaching wave as it charges towards them at over 600 mph. In the open ocean tsunamis are virtually undetectable. Fishermen out here may not even notice a tsunami passing underneath their boat. But as it nears land, it rears up into a monster…

Simon Boxall of the Southampton Institute of Oceanography explains: “as the wave approaches the land the front of the wave slows down, to four hundred, three hundred, two hundred miles an hour. The back of the wave is still going at five hundred miles an hour, so the back of the wave catches up with the front, and then you get this big build-up of water.”

The amplified wave can have a curious side-effect. Before it hits land, it draws water from its leading edge, exposing the seabed for as much as a mile. It’s a phenomenon that lures thousands to their deaths as people rush down to pick up fish stranded on the exposed rocks.

Tsunamis rarely roll towards land like a classic surfer’s wave. Instead they surge forward like an onrushing flood. The leading edge builds into a wall of water. They’re so deadly because the volume of water behind them is so great.

Professor Bill McGuire of University College, London, is an expert in the behaviour of these lethal waves. He explains: “tsunamis have wavelengths - the distance from the crest of one wave to the next wave - of hundreds of kilometres. It will keep coming in maybe for five minutes or more and it has enormous mass behind it.”

Just 15 minutes after the earthquake, northern Sumatra is the first to experience the full impact of the tsunami. Virtually no video footage is taken of the wave that hits Sumatra. It is simply so big that anyone filming it would have been killed.

The devastation in the provincial capital, Banda Aceh, is unprecedented. Even those used to disaster zones have never witnessed anything like it. And residents living well inland are also dwarfed by the wave – tragically, there’s nowhere to run.

Banda Aceh before (left) and after (right) the tsunami… CREDIT: DigitalGlobe

Three-quarters of all the tsunami’s victims die in Sumatra. But the great wave is far from finished. Just 15 minutes later, the Andaman and Nicobar islands are hit. Over 7000 die or go missing. Forty-five minutes after the earthquake, scientists at the Pacific Tsunami Warning Center are still unaware of the tsunami. But they are desperately trying to analyse the earthquake data.

Another 15 minutes passes, and the tsunami strikes Thailand. One hour has passed since the earthquake. Cities are waking up and beaches are beginning to stir into life. People everywhere have remained oblivious to the thousands of deaths in Sumatra – and to the approaching tsunami, until the sea rushes inland.

Fifteen minutes after the tsunami first inundates the Thai coast, a deadlier second wave strikes. Thousands are caught by surprise. According to Bill McGuire: “in any particular earthquake you’ll get a different tsunami train. In other words, there isn’t just one wave there are usually several waves, and it doesn’t follow any particular rule. Sometimes the first wave is the biggest one, sometimes there are maybe two, three or even more relatively small waves and then you get a huge one in the middle and then some smaller ones. But normally you do get more than one wave, they are of different heights and they arrive over different periods of time. It can be tens of minutes, even hours, between the first wave and the last wave.”

It is 9.00am in Sri Lanka, two hours since the birth of the waves. British holiday maker Shenth Ravindra is crammed into a train with 1500 others. He is unaware that he is about to be caught in the worst rail disaster in history.

The track follows a coastal route, from Colombo to Galle – never far from the ocean. Ravindra later recalls: “I heard the water and then all of a sudden it came and started hitting the train, to such an extent that the carriage got detached from the other carriages behind us and in front of us. The water started to spill in, and I felt it come all the way up to my chin. I thought ‘I’ve got to get out of here’. I was able to climb through this doorway and make my way onto the corner of the roof and the wall. And I was helping put these children onto the train.”

No help is forthcoming to the occupants of the train, nor to thousands of others along the coast of Sri Lanka and India. And the survivors of the first wave are completely unaware that a second, much bigger, tsunami is minutes from impact.

According to Ravindra: “When I saw the second wave coming, all I could see was a wall of water that took up about 80 to 85 % of the horizon – the sky was blocked. There were a lot of children surrounding me and they were clinging onto me and hugging me and I was losing my balance a little bit on the train. But luckily it just pushed the carriage quite neatly along to the point where it wedged against a house.”

Ravindra jumps from the train onto the roof of the house. He is one of only a handful who would survive. As the train disappears beneath the water, almost everyone on board is drowned – almost 1500 people.

Throughout India and Sri Lanka over 45,000 lives are lost. It is still only three-and-a-half hours since the earthquake. At the Pacific Tsunami Warning Center, the horror of the disaster is becoming apparent as an internet news report confirms their worst fears. The earthquake they’ve been monitoring has generated a massive tsunami.

Weinstein realises that they have to warn Africa. “We then created a tsunami travel time map for the Indian basin. It told us where the wave was presently, and gave us an idea of how much time we had in order to warn people. And then immediately we started to try to contact nations that were ahead of the wave.”

Just under four hours after the earthquake – and the Maldives are next on the tsunami’s path of destruction. With the wave charging across the ocean at 600 mph, it seems like the Pacific Tsunami Warning Center is fighting a losing battle. But Weinstein’s travel time map does prove effective. Seven hours after the earthquake struck, the east coast of Africa – on the far side of the Indian Ocean – is next in line for the terror of the tsunami. Using data from the travel time calculations, the scientists at the Pacific Tsunami Warning Center are at last able to predict its path and raise the alarm. In Kenya, this advance warning leads to the evacuation of the shoreline – and there’s only one fatality.

Over the next 24 hours, disturbances are detected throughout all the world’s oceans. But the tsunami’s energy is dissipated as it progresses beyond the Indian Ocean.

Its wake, however, will resonate for years to come.

Richard Gross of NASA’s Jet Propulsion Laboratory used seismic data to make some startling calculations on the results of the record earthquake: “it has caused the mass of the plates to be out of balance, which has in turn caused the Earth to wobble by about two-and-a-half centimetres. This is just like the tyre on your automobile. If the tyre is not balanced it will not spin smoothly.”

Not only did the earthquake push the planet off balance, it also shortened the length of the day by about 3 millionths of second!

For most people, the tsunami was totally unexpected, even though they are all too common in southeast Asia. Last December there was no warning system in the Indian Ocean. Now, sensors for a tsunami network are being installed around Indonesia.

Despite the promise of this early warning system, there are still many thousands of people living near the sea in Banda Aceh. They remain exposed to danger, there’s no way to provide an alert quickly enough if another earthquake occurs so nearby. The only possible way they can be protected is to either move much further inland or to build a large protection sea wall as they do in Japan.

Protection against tsunamis however remains a matter of cost and politics, even though these destructive waves are a global hazard. Tsunamis pose a direct threat to 80 percent of the world’s population. And it’s not a case of ‘if’ but ‘when’.

December 2004 was a stark reminder of the precariousness of our co-existence with Nature. In reality, with only 15 minutes to warn and evacuate northern Sumatra, even the most sophisticated warning system may not have been enough to save many of the people who died there.

But it would have saved many of the thousands who died in eleven other countries…

More information:

- maps, animations and more from America’s National Oceanic and Atmospheric Administration

- satellite images of the devastation