The elephant and the tectonic plate
Evidence suggests the Sumatra region may be hit by a big quake within the next 20-30 years
Kerry Sieh Kusnowidjaja Megawati & Pan Tso-Chien, Straits Times 9 Mar 08;
WHY does Sumatra keep sending earthquakes our way? And what might be coming next?
These are questions that many Singaporeans are asking again these days, as the shaking that began in 2000 resumed two weeks ago.
The world's biggest earthquakes occur in places where giant tectonic plates beneath the oceans are diving down under neighbouring land masses.
Singapore is fortunate to lie several hundred kilometres from the nearest such 'subduction' zone, in the middle of a tectonic plate that includes all of its South-east Asian neighbours.
Still, the western edge of that plate, on the other side of Sumatra, generates such big earthquakes that gentle swaying is commonly felt in Singapore.
All of the earthquakes felt here in the past seven years happened because the upper surface of the downward moving oceanic plate, called the Sunda megathrust, is sticky.
As the oceanic plate descends northward at about 60mm a year beneath Sumatra, it clings to Sumatra's underside along the megathrust until it reaches depths of 50km or so, where it finally becomes hot and gooey enough to slip steadily into the deep earth.
The great earthquakes of 2004, 2005 and 2007 occurred because very large patches of the megathrust suddenly became unstuck.
Imagine a rope tied to an elephant on one end and held by a team of men - some large, some small, some strong, some weak - on the other.
The elephant is the dense oceanic plate sinking inexorably into the deep earth. The men are individual patches of the megathrust. The more the elephant pulls on its end of the rope, the more each man has to resist at the other end. Eventually, the weakest man lets go (the first earthquake, in 2000).
He is relieved, but his teammates now have to carry his burden, to keep the elephant from winning the tug-of-war. Sometime later, another man lets go (the 2002 earthquake). He feels relieved, but those remaining have to resist the elephant with even more force. Finally, half of the team gives up in rapid succession (the giant Aceh earthquake of 2004). And then a neighbouring contingent yields (in 2005).
Now the elephant's tug is being resisted by far fewer men.
As of a year ago, only one major part of the megathrust between Myanmar and Java remains unbroken - that 600km-long section nearest to Singapore.
Global Positioning System (GPS) instruments in western Sumatra show that the megathrust of this 'Mentawai' section continues to resist failure.
Ominously, the corals of the Mentawai island reefs show that this section breaks in sequences of great earthquakes about every two centuries and that the last such sequence occurred about two centuries ago, between 1797 and 1833.
And so it appears that even the strong men still holding the rope are, in fact, nearing the end of their rope.
For several years now, we have been saying that the elephant will win, and probably sometime within the next two or three decades.
The large earthquakes of last September were, in fact, our first indication that the Mentawai section has finally begun to fail.
On Feb 20 this year, a tiny patch between the great 2004 and 2005 ruptures failed in a magnitude 7.4 quake. Curiously, that rupture may have been on the same patch that caused the 2002 foreshock.
The more recent small quakes between Feb 22 and March 3 indicate that the Mentawai section continues to lose its grip.
The parts that remain unbroken are large enough to generate a magnitude 9 earthquake, if they all go at once.
One might hope that the remaining men will let go one by one (a flurry of smaller earthquakes, like the previous week's), so that the elephant doesn't win in one dramatic flash.
But we consider this piecemeal scenario quite unlikely since it would entail thousands of such earthquakes. The historical and geological records suggest that within the next couple of decades one or two additional great earthquakes will end the contest.
The fallout
THE implications of this are serious for coastal communities of western Sumatra. If the great earthquake were to happen today, strong seismic shaking would likely damage many buildings and other infrastructure.
Moreover, scientific estimates of tsunami inundations at Padang and Bengkulu, the two largest cities along the Sumatra coast, illustrate the urgent need for strong emergency planning and management. Fortunately, some efforts towards these ends are being made by local government and NGOs, including new projects by Singaporean researchers.
However, it is too early to tell whether these efforts will lessen losses to levels below those that occurred in Aceh in 2004.
What about the effect of this coming great earthquake on Singapore? It is inevitable that more high-rise buildings will be built in Singapore as its economy grows. High-rise buildings respond to earthquakes, especially shaking from distant earthquakes. The low-frequency seismic waves can travel longer distances than high-frequency waves, just as the bass frequencies of a musical instrument are heard farther than the treble. The low frequency of the earthquake shaking experienced in Singapore from Sumatran earthquakes can be amplified at sites with soft soils.
The amplified low frequency shaking may be close to the natural frequencies of high-rise buildings in the city, creating a socalled resonance effect.
High-rise buildings sway back and forth during the earthquake. It is therefore more likely for people living or working on upper floors of high-rise buildings to sense these earthquakes than those living in low-rise houses.
However, this greater sensitivity to Sumatran earthquakes does not mean that high-rise buildings are more likely to be damaged.
The potential of a structure to be damaged is not related to the amplitude of shaking at the top of the building, but rather to the so-called inter-storey drift - the amount of deformation between one storey and the next.
While the inter-storey drift generated by distant earthquakes may be within the limits of a well-designed building, the response of buildings to ground shaking depends on the details of structural configurations, and further research work will be required to better quantify dynamic building responses.
It would therefore be prudent to consider the dynamic effects of distant earthquakes in the design of future buildings.
For scientists, engineers, emergency responders and urban planners, the ongoing series of Sumatran earthquakes and tsunamis is a rare and fascinating opportunity to observe how earth's tectonic plates work and to improve the resilience of our engineered structures and our communities to these dynamic natural events.
We in Singapore are very wellpositioned to learn from the Sumatran activity. As the tectonic motions continue, we and our Indonesian colleagues will also continue to monitor and interpret it with our GPS and seismic networks.
Professor Kerry Sieh (sieh@gps.caltech.edu), from the Tectonics Observatory, California Institute of Technology, is Visiting Professor, Nanyang Technological University. Assistant Professor Kusnowidjaja Megawati (kusno@ntu.edu.sg) teaches at the School of Civil & Environmental Engineering, NTU. Professor Pan Tso-Chien (cpan@ntu.edu.sg) is the Dean of the College of Engineering, NTU. They have ongoing research programmes on the earthquake geology of Sumatra and on seismic hazard and risk for Singapore and the region.
