Understanding triggers for tsunamis
It’s been a decade since one of the largest ever recorded earthquakes struck off the coast 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 metres. 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 occurred 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, the main contributor to this piece, is a mathematician and educator with an interest in ecology and environmental justice. Views expressed are personal.)