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.
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 llooked 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.