Grace Chua Straits Times 22 Apr 12;
When a massive, 8.6-magnitude earthquake struck off Sumatra on April 11, people there headed for the hills.
They feared a repeat of 2004's horrors, when a 9.1-magnitude quake off Aceh caused a tsunami that left more than 230,000 people dead in countries across the Indian Ocean, from Indonesia to Somalia.
For several hours, the region braced itself for killer waves. But there were none.
So what made this month's quake and its 8.2-magnitude aftershock so different from what happened in 2004?
Dr Paul Tapponnier, one of the world's foremost seismologists, is a pioneer in the field of tectonics, unravelling the puzzle of how the forces acting on the Earth's plates cause them to move, slide and collide.
Now with the Earth Observatory of Singapore (EOS), he explained that earthquakes can occur at three main types of faults: When two pieces of the Earth's crust split from each other, slide horizontally away from each other, or collide.
Typically, these faults are found at the boundaries between tectonic plates, which are large pieces of the Earth's crust. But lesser faults can also develop within plates, at weak spots that form when the plate grows and cools.
Most of the large (magnitude 7-plus) quakes off Sumatra are along the Sunda megathrust, the boundary formed by the Indo-Australian plate thrusting into and under the Eurasian plate.
But, Dr Tapponnier explained, the story is more complex. The Indo-Australian plate is actually trying to shear apart into Indian and Australian plates, producing stresses and weak spots.
And within the Indo-Australian plate, there are many smaller faults running north to south.
Scientists believe the April 11 undersea quakes were the result of these small faults interacting with the stresses, as well as the megathrust. The quakes occurred within the Indo-Australian plate, when two strips of the Earth's crust slid horizontally past each other, a type of sidelong motion called a strike-slip fault.
Because they did not push large amounts of sea water upwards, they did not cause much of a tsunami. Indeed, news reports indicate that the April 11 wave was generally less than a metre high in Aceh.
In contrast, the Dec 26, 2004 earthquake caused such a powerful tsunami and massive damage in all directions from its epicentre off Sumatra because it was what is known as a 'megathrust' earthquake.
That occurred when the Indian and Australian tectonic plates suddenly dived under the Eurasian plate with such great force that it moved the sea floor and sea water up or down by many metres.
That generated a series of large waves - a tsunami - capable of travelling right across the Indian Ocean to Africa.
Although there was no tsunami on April 11, coastal communities did the right thing by evacuating to higher ground.
'It takes time for scientists to determine the tsunami potential of a large earthquake, and warnings do not reach all areas in time,' said Dr Tapponnier, 65.
The largest strike-slip earthquake ever recorded?
The 8.6-magnitude earthquake on April 11 might have been the largest strike-slip earthquake ever recorded, said Dr Tapponnier, who joined EOS as head of its Tectonics Group in 2009.
The $287 million centre was set up in 2008, four years after the 2004 disaster, to study the triple threat of earthquakes, volcanoes and climate change.
Now, he and his colleagues are working to reverse-engineer the April 11 quake, much like crime scene investigators reconstructing a murder from the surrounding evidence.
They are using data from a network of 50 Global Positioning System stations in Sumatra and its outer islands.
The data from the land-based stations shows how some points in northern Sumatra were shunted north-east as much as 35cm on April 11, while other points in central Sumatra were shifted west.
The scientists are trying to model how deep the fault went, how far the strips of crust slid along each other, and the amount of force involved.
Earthquakes come in cycles, and there are more to come
Historic records show that earthquakes seem to come in cycles. Indeed, Sumatra saw megathrust earthquakes in 2004, 2005 and 2007 and those were larger than any other recorded off the island by seismographs.
But scientists at EOS and the Indonesian Institute of Sciences' geotechnology division, in Bandung, have been looking even further back in time.
For clues to understand the history of earthquakes in the area, they have been scrutinising coral growth patterns on the chain of outer islands off western Sumatra.
As the land mass rises or sinks in successive earthquakes, the sea level changes too. This affects the growth of corals, as exposed portions of coral die off.
In Aceh, they found that large earthquakes happened in 1394 and 1450. In the intervening centuries, the strain between the moving tectonic plates built up till the 2004 earthquake released much, but not all, of that strain.
Mirroring the two-stage release of strain in 1394 and 1450, there might be a second large earthquake and tsunami in the coming decade, say scientists.
And they are watching a 400km section of the megathrust along the Mentawai Islands and the city of Padang in West Sumatra.
That is where a series of large earthquakes has occurred about every two centuries. The scientists say a new series began there in 2007 and is likely to conclude with a quake as strong as magnitude 8.8 and a large tsunami.
But Dr Tapponnier said: 'Science cannot predict the day, month or year of an earthquake, but can show us which areas face high earthquake and tsunami potential.'
The immediate concern: individual earthquakes can trigger more
Besides these historical cycles of earthquakes, individual earthquakes occur and change the stress on neighbouring faults.
In a span of just three years, the large Sumatran megathrust earthquakes of 2004, 2005, and 2007 triggered one another in short order.
Taken together, scientists say, they have effectively pulled on the Indo-Australian plate, perhaps triggering the April 11 earthquakes.
And the April 11 quakes themselves may have changed the stresses on the megathrust in ways that scientists are now trying to anticipate, said EOS researcher Emma Hill.
She said: 'We can't say that it's suddenly going to cause an earthquake tomorrow - it can happen tomorrow or in 20 years - but it's good to know if we should be more worried than we were before the April 11 quakes.'
Dr Tapponnier, who has studied earthquakes for about four decades now and has observed the destruction they can cause, pointed out that greater awareness of quakes and tsunamis can save lives.
'Always move to higher ground. The water does not always recede before a tsunami, so waiting to watch the ocean is not a safe thing to do after a large earthquake,' he said.
Earthquakes on land worry him more than those at sea, because it is falling buildings that tend to kill people.
He pointed out, somewhat ominously, that there is a fault in Myanmar due for a rupture, and another in western China, under a new high-speed rail line.
More quake studies can help.
'Society can take many steps to reduce vulnerability to these hazards, from the individual and community to national and international levels,' he said.
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