Down To Earth revists the causes of the massive earthquake that triggered tsunami 12 years ago and the progress scientists have made towards understanding and predicting its trigger
In the aftermath of Tsunami
On December 26, 2004 the third-largest earthquake (9.5-magnitude) ever recorded jolted India, Indonesia, Sri Lanka and 11 other countries. It triggered a series of devastating tsunamis along the coasts of most countries bordering the Indian Ocean, killing 230,000–280,000 people.
Authorities were taken by surprise as the rapidly rising tide devoured everything that came its way, leaving them incapable to prevent human and economic loss. What has been our progress in predictive science and disaster management more than a decade after the tsunami?
India has been trying to advance its tsunami prediction mechanism and early warning system. Most recently, a network of 35 motion accelerometers and Global Navigation Satellite System (GNSS) receivers were set up on the Andaman and Nicobar Islands. According to media reports, the network will help assess the rupture, area and direction of an earthquake, allowing quick estimation of tsunami and its range.
Twelve years after the disaster, bodies of victims still remain unidentified. Around 400 victims of the disaster are yet to be identified in Indonesia.
As the world pays homage to those who lost their lives, Down To Earth puts together a series of observations made in the aftermath of the Tsunami.
On the trail of tsunami
For over five centuries the earth had struggled to maintain its composure, bowing here and bending there, yet somehow withstanding the mounting pressure. Far beneath the surface, the Indo-Australian tectonic plate was subducting under the Sunda plate, but near the surface these plates had locked together, accumulating historic levels of tension along a huge stretch of the Sunda megathrust. Then, in a decisive moment on December 26, 2004, the earth shrugged off its strain. In a matter of seconds, 1,600 kilometres of ocean floor broke free and lurched upward, like a compressed spring suddenly released. The displacement was later calculated to be as high as six metres. It was this incredible force of displacement that triggered the tsunami of 2004, killing 23,000 people and causing incalculable loss of livelihood.
The tsunami could not have been prevented, but it could have been anticipated, and the loss might have been greatly reduced. Despite the uncontrollable chaos we associate with natural disasters of this scale, there are rules and patterns governing even the greatest earthquakes and tsunamis. Over the past decade, scientists studying the Sunda megathrust have made great strides in understanding some of these patterns (see ‘Many faults of Sunda megathrust’).
Supercycle of earthquakes
The Sunda megathrust, which arcs all the way from Myanmar to Australia hugging the western coast of Sumatra, is replete with seismic activities throughout much of its 5,200 kilometres length. Scientists have long known that the strain accumulated in megathrust faults such as the Sunda are responsible for earthquakes of moment magnitude 9.0 (a measure of total energy released by an earthquake) or higher.
But very few earthquakes are that powerful—only five have occurred since 1900; the 2004 earthquake was by far the deadliest one.
In 2005 when another earthquake hit the Aceh region of the Sunda megathrust that had ruptured the year before, it was thought to have released remainder of the accumulated tectonic strain in that region. Attention then turned to the portion of the Sunda megathrust just south of the equator, now thought to have the greatest accumulated strain and high earthquake potential. This danger zone lay between the Mentawai islands and the city of Padang in Sumatra.
To predict the threat of earthquakes and tsunamis in the Mentawai region, scientists led by Kerry Sieh, director of the Earth Observatory of Singapore, started measuring “uplifted coral micro-atolls” off the Mentawai islands. These micro-atolls are colonies of corals that protrude from the water due to the upliftment of the ocean floor during a major earthquake. The micro-atolls off the Mentawai islands provided an excellent measure of the timing and strength of recent earthquakes, as well as ones that occurred hundreds of years ago (see ‘Coral clock’).
Sieh and his team used this coral data to reconstruct the region’s seismic history. And in the process, they noticed an interesting pattern. At the time, it was already well established that powerful earthquakes return in a cyclic pattern. Says Sieh’s colleague, Belle Philibosian, “Tectonic stress on a fault builds up over a long period of time—usually hundreds or thousands of years—before being released in an earthquake, after which stress builds up again and the cycle repeats.” But with the temporal and spatial precision offered by the corals, Sieh and his colleagues noticed an additional feature: during each cycle, a cluster of earthquakes occurred together, much like a series of hiccups, over the course of a few decades. Sieh termed the phenomenon a “supercycle”. “The reason I coined that term is that we had been talking for a century or so about earthquake cycles—about strain accumulation and release. And here we had strain accumulation and partial, partial, partial release,” Sieh told Down To Earth.
Since the last cluster of large earthquakes in the Mentawai region had occurred in 1797 and 1833, the coral data indicated that a new set of earthquakes was overdue. “Just after we made the discovery—the paper had gone to the press, but not yet been printed—the first of the most recent cycle began in September 2007,” Sieh recalls. The earthquake that struck the Mentawai region was of 8.5 magnitude. Sieh’s work on supercycles which had anticipated the earthquake was published soon after in Science in 2008. Now, there is widespread consensus that the 2007 earthquake marks the beginning of the latest cluster, and that there is at least one more great earthquake to come. Philibosian explains that “in previous supercycles, the ruptures have not been more than 40 years apart, so it is very likely that at least one more major earthquake will occur by 2047.”
The prediction is not based on historical precedent alone. In a 2014 paper in Journal of Geophysical Research, Philibosian and her group confirm that the majority of the Mentawai segment of the Sunda megathrust continues to bear significant accumulated stress which could cause a massive rupture at any moment. What matters is the strain that has built up since the last rupture, she says.
Several other portions of Sunda megathrust also have significant earthquake potential. Of particular risk to India is its northern extension near Myanmar.
Impacts on India
The last giant earthquake near Myanmar, known as the Arakan, occurred in 1762. As per a 2013 paper by Wang Yu of California Institute of Technology, published in Journal of Geophysical Research, the supercycle period of the northern region is estimated to be 500-700 years. However, Wang Yu cautions, “It is very likely that we would have some earthquakes smaller than the magnitude of the 1762 earthquake strike this section in between these giant events.”
Explaining the potential for tsunamis, he says, “An earthquake of magnitude greater than 7.0 results from a megathrust rupture at a shallow depth and is very likely to generate a tsunami wave in the Bay of Bengal. It does not necessarily mean that the wave would be big, because the water depth in the Bay of Bengal is quite different from the water depth near the Andamans.”
There is also evidence of a continued risk of earthquakes in the Acehnese portion of Sunda megathrust that ruptured in 2004, according to a study by Aron Meltzner of Caltech, which was published in Journal of Geophysical Research in 2010. The last great earthquake cluster in that portion probably occurred between 1390 and 1450.
Meltzner and his colleagues have estimated the amount of strain stored in the fault since 1450, and it amounts to double the estimated amount released in 2004. Besides, Meltzner explains, “In the earthquake sequence between 1390 and 1450, the total amount of strain released was much more than in 2004. If such a large amount of strain was released over a matter of decades at some point in the past, it can happen again.”
Link discovery to mitigation
The improved understanding of earthquake patterns in the past decade has no doubt enhanced scientists’ ability to monitor and predict earthquakes and tsunamis. “Virtually all we know of offshore Sumatra (the Sunda megathrust) has been learned in the past 10 years,” says Sieh. The scientific breakthroughs over the last decade have heavily relied on the GPS data combined with measurements of coral micro-atolls. The Sumatran GPS Array has been greatly expanded, from just six GPS stations during the 2004 earthquake to 49 stations now, many of which lie directly above the locked sections of the Sunda megathrust. Countries have also put tsunami warning systems in place. The Indian Ocean Tsunami Warning System, which became active in 2006, may have reduced the damage of the 2012 Indian Ocean earthquakes.
Despite these improvements, the fact remains that scientific discoveries alone cannot reduce loss or damage until public awareness, emergency response preparedness and infrastructural resilience occupy a prominent place in the government agenda. Consider Padang in Sumatra. The city, home to almost a million people, is likely to be near the epicentre of the upcoming earthquake in the Mentawai region, and would face the brunt of the resulting tsunami. “Some non-profits have been working on public awareness and evacuation exercises. But our study shows that only 10 per cent of the 500,000 exposed would be able to get out of the flooding zone by the time the tsunami hits the coastline,” says Sieh. This is because preparation efforts are much less than the effort required, he adds. “Padang is a big town and there is too much to do. Those in power do not want to think about an event that may only occur in 25 years. It’s somewhat like our response to climate change—we make our living, burn our coal, and if it happens it happens.”
In an article written prior to 2007 earthquake, Sieh stated that, “Maintenance of this status quo throughout South and Southeast Asia will prove tragic and expensive, for without strong chains linking scientific discovery to mitigation, other events as profoundly disturbing to human well-being as the 2004 tsunami will strike elsewhere in the coming century.” His words remain relevant today.
|Many faults of Sundamegathrust
Where to expect giant earthquakes and tsunamis
A. Mentawai region has a supercycle of 200 years, meaning cluster of earthquakes return every 200 years. Previously, it had occurred in 1350-1380, 1600s, 1797-1833. The earthquake in 2007 marked the beginning of the latest cluster. Stress has accumulated here since 1833. At least one major earthquake will occur by 2047. It may affect India but not as hard as the 2004 tsunami
B. Aceh/Andaman region is spread over 1,600 km and has a supercycle of 500 years. Last major earthquake of 2004 caused a massive tsunami. It was followed by another next year. More earthquakes may occur in near future but they are unlikely to trigger tsunami
C. Northern extension has a supercycle of 500-700 years. Since the earthquake near Myanmar was in 1762, a cluster of earthquakes may hit the region in 2202-2402. They may trigger tsunamis in the Bay of Bengal. But smaller earthquakes could still occur
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Scientists move closer to understanding triggers for tsunamis
It’s been a decade since one of the largest ever recorded earthquakes struck off the coat of Indonesia, triggering a massive tsunami which devastated coastal regions around the Indian Ocean and killed over 230,000 people.
While the world observes the passing of 10 years by holding memorial services for those who lost their lives, scientists have been working to improve their understanding of tsunamis and earthquakes.
The tsunami of 2004 was caused by a rupture in the 1,600 km stretch of the Sunda megathrust fault between Aceh (Indonesia) and Andaman islands. It was an event waiting to happen, say scientists. For more than five centuries, the Indo-Australian tectonic plate was subducting under the Sunda plate, but near the surface these plates had locked together, accumulating extremely high levels of tension along a huge stretch of the Sunda megathrust. The earth struggled to maintain its mounting pressure.
Then, in a decisive moment on December 26, 2004, the earth shrugged off its strain. In a matter of seconds, 1,600 km of ocean floor broke free and lurched upward, like a compressed spring released suddenly. The displacement of the sea floor was later calculated to be as high as six metre. It was the incredible force of this displacement that triggered the great tsunami of 2004.
The tsunami could not have been prevented, but it could have been anticipated, and the loss might have been greatly reduced. Despite the uncontrollable chaos we associate with natural disasters of this scale, there are patterns and rules governing even the greatest earthquakes and tsunamis.
Over the past decade, much scientific research has taken place along the Sunda megathrust, including the segment that ruptured in 2004.
The term megathrust refers to an extremely large and active fault between convergent tectonic plates. These geological structures are responsible for all earthquakes of moment magnitude 9.0 or higher. Very few earthquakes are that powerful—a grand total of five have occurred since 1900, and the 2004 earthquake was, by far, the deadliest of these.
The 1,600 km stretch that ruptured during the 2004 earthquake was only one section of the 5,200 kilometer Sunda megathrust, which arcs all the way from Burma to Australia, hugging the western coast of Sumatra.
The 2004 earthquake was followed by a nearby earthquake in 2005, which was thought to have released the remainder of the accumulated tectonic strain in that region (recent evidence suggests the region may still have earthquake potential). After the 2004/2005 earthquakes, attention turned to the portion of the Sunda megathrust just south of the equator, now thought to have the greatest accumulated strain, and high earthquake potential. This danger zone is between the Mentawai islands and the city of Padang, Sumatra.
A team of scientists led by Kerry Sieh, Director of the Earth Observatory of Singapore, had been measuring "uplifted coral micro-atolls" off of the Mentawai islands. These are colonies of coral that protrude from the water due to the literal uplift of the ocean floor that occurs during a major earthquake. This region is south of the Aceh region which was raised in 2004. They have come to resemble small islands, or more precisely atolls, and they provide an excellent measure of the timing and strength of recent earthquakes, as well as earthquakes long since passed. Sieh and his team used this coral data to reconstruct a vastly improved seismic history of the region. In the process they noticed a very interesting pattern to the past earthquake occurrences.
At the time it was already well established that the great earthquakes of the past have arrived in cycles: as Sieh's colleague Belle Philibosian explained, "tectonic stress on a fault builds up over a long period of time—usually hundreds or thousands of years—before being released in an earthquake, after which stress builds up again and the cycle repeats."
But with the additional temporal and spatial precision afforded by the coral micro-atoll data, Sieh and his colleagues noticed an additional feature: in each period of the cycle, a cluster of earthquakes occurred together, like series of hiccups, over the course of a few decades. In particular, every 200-230 years an earthquake-cluster had occurred in the Mentawai region going back at least three cycles. They termed this phenomenon a "supercycle". While speaking to Down To Earth, Sieh elaborated, "The reason I coined that term is that we'd been talking for a century or so about earthquake cycles—about strain accumulation and release. And here we had strain accumulation and partial, partial, partial release."
Next big one
Since the last cluster of large earthquakes in the Mentawai region (in this case there were two) had occured in 1797 and 1833, the records indicated that a new set of earthquakes was past due. Sieh recalled: "Just after we made this discovery— the paper had gone to the press, but not yet been printed—the first of the most recent cycle began in September 2007." The earthquake that struck the Mentawai region had a moment magnitude of 8.5. Sieh's work on supercycles which had anticipated the earthquake was published soon after in Science in 2008.
There is widespread consensus that the 2007 earthquake marks the beginning of the latest cluster. There is widespread consensus that there is at least one more great earthquake to come in the Mentawai region.
Sieh has worked with multiple NGOs on public education and preparedness for the upcoming earthquake and tsunami, which are likely to have devastating consequences for the city of Padang, Sumatra, home to almost a million people. Despite these efforts, Sieh says, "A recent study we did showed that only 10 per cent of the 500,000 exposed would be able to get out of the flooding zone by the time tsunami hits the coastline." Sieh adds, "The effort to prepare is much less than the effort required. Padang is too big a town, there's too much to do, and those in power don't want to think about an event that may only occur in 25 years. It's not unlike our response to climate change I suppose--we make our living, burn our coal, and if it happens it happens.”
Vijay Ravikumar is a mathematician and educator with an interest in ecology and environmental justice
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