SINGAPORE (Reuters) - The powerful undersea earthquake off the Indonesian island of Sumatra this week was a once in 2,000 years event, and although it resulted in only a few deaths, it increases the risks of a killer quake in the region, a leading seismologist said.
Wednesday's 8.6 magnitude quake and a powerful aftershock were "strike-slip" quakes and the largest of that type recorded, Kerry Sieh, director of the Earth Observatory of Singapore, told Reuters.
"It's a really an exceptionally large and rare event," he said.
"Besides it being the biggest strike-slip earthquake ever recorded, the aftershock is the second biggest as far as we can tell," said Sieh, who has studied the seismically active, and deadly, fault zones around Sumatra for years.
Strike-slip quakes involve the horizontal movement of colliding earth plates, and are typically less powerful than those where there is vertical movement. They are also less likely to trigger big tsunamis, or tidal waves.
A magnitude 9.1 quake in roughly the same region on Boxing Day in 2004 decimated Aceh province on Sumatra and killed over 230,000 people in 13 countries around the Indian Ocean.
Sumatra, the westernmost island in the sprawling Indonesian archipelago, has a history of powerful quakes and tsunamis, most triggered by an offshore zone along its entire length, where the Indian-Australian tectonic plate is forced under the Eurasian plate.
This creates a deep ocean trench as one plate slides under the other at a rate of several centimeters a year. In this zone, called the Sunda megathrust, stress builds up when the subducting Indian-Australian plate bends the Eurasian plate like a spring board as it moves down into the Earth's crust.
Eventually enough stress builds up that the edge of Eurasian plate suddenly jolts upward, triggering an earthquake. The sudden uplift of the seafloor and huge pulse of seawater triggers a tsunami.
Over the centuries, repeated magnitude 8 and 9 quakes have struck along portions of the megathrust zone off the coast of Sumatra, flattening towns and killing thousands of people.
Wednesday's event was different, Sieh said, because it occurred further west from the megathrust zone in a fault that runs north-south. This strike-slip fault involved a sudden horizontal movement of the Indian and Australian plates along hundreds of kilometers, preliminary data suggest.
Sieh said the Indian plate and Australian plate are moving relative to each other horizontally at about 1 cm a year.
"If all of that ... is taken up on this one fault and if you make some crude calculations about how much slip occurred during this earthquake, say 20 meters. It means that this earthquake shouldn't happen more than once every 2,000 years."
Wednesday's quake caused few casualties and triggered very small waves, despite its magnitude. But the sting in the tale is that it likely to have increased stress on the plate boundaries near Aceh, increasing the risks of another major earthquake in the same area as the 2004 disaster.
In addition, research by Sieh and colleagues published in 2010 showed that the 2004 Aceh quake only relieved about half the stress that has built up over the centuries along a 400 km portion of the megathrust faultline.
That makes another major quake in the area a matter of time.
Adding to concerns, further south along another 700 km portion of the megathrust fault under the Mentawai islands, Sieh and colleagues in a separate 2008 research said so much stress was building up on this section that one or more major quakes were likely within years.
The Mentawai islands, a popular surfing destination, are a chain of about 70 islands off the western coast of Sumatra. They face the city of Padang on Sumatra, home to about one million people and likely to be in the path of any tsunami that is triggered.
"I am very confident that we are very likely to have within the next few decades to have this great Mentawai earthquake that will have a magnitude at least as big as yesterday's," said Sieh.
And when it does, history shows there will be more than one quake within a few years.
He said a magnitude 8.4 quake in 2007 that struck this part of the megathrust relieved only a small portion of the pent-up pressure. The last time it ruptured was a magnitude 9 quake in 1833 and an 8.4 quake in 1797.
"We've had so many big earthquakes around in Sumatra in the past few years that it seems like an awful lot of the faults around there seem ready to go."
Editing by Sanjeev Miglani and Raju Gopalakrishnan