World

Long-term tsunami risk rises after Indonesia quake

Last week's powerful earthquake off western Indonesia increased pressure on the source of the devastating 2004 tsunami that killed 230,000 people in and around the Indian Ocean, seismologists say.

'The spring was pushed a little bit tighter,' seismologist says

A woman grabs hold of two children as people flee to higher ground in Banda Aceh, Indonesia, following last week's quake. (Chaideer Mahyuddin/AFP/Getty)

Last week's powerful earthquake off western Indonesia increased pressure on the source of the devastating 2004 tsunami that killed 230,000 people in and around the Indian Ocean, seismologists say.

As a result, forces have been ratcheted up on a fault that could unleash another monster wave sometime in the next few decades.

"The spring was pushed a little bit tighter," said Kerry Sieh, director of the Earth Observatory of Singapore.

The timing of another so-called megathrust earthquake, if it's on the way, "could have been advanced by a few years," he said.

Last week's 8.6-magnitude earthquake off the west coast of Indonesia's Sumatra island also showed that tsunami-ravaged Aceh province, close to the epicentre, remains unprepared for the next big one. Though the quake caused little damage, the country's disaster-management chief acknowledged that evacuation efforts were "a big mess."

Indonesia, located on the Ring of Fire, an arc of volcanoes and fault lines encircling the Pacific Basin, has unleashed some of the deadliest seismic events of the past century. But the 9.1-magnitude quake that struck in 2004, triggering a 30-metre-high tsunami and killing 230,000 people, caught scientists off guard because its fault, west of Sumatra, had long been quiet.

Since then scientists have conducted a flurry of research. By observing past patterns of quakes, which tend to be cyclical, they better understand what might happen next, though it's impossible to make predictions with any certainty.

Tectonic pressures still high

The two last behemoth quakes occurred around 1393 and 1450, and Sieh said the 2004 earthquake may be just the first part of a similar couplet.

Stresses loading up on the fault for centuries were relieved only about halfway eight years ago, Sieh said. And last week's tremor effectively squeezed the overlapping tectonic plates that form the fault.

"The next megathrust rupture could be in 50 years or in five," he said. "It's impossible to know."

He said a separate section of the fault, hundreds of kilometres to the south, also could snap within the next 30 years, sending a tsunami slamming into Padang, a low-lying Sumatran city of one million residents.

Last week's quake was a "strike slip" quake, which means it thrust from side to side, not vertically, and therefore did not generate a large tsunami.

Danny Hilman Natawidjaja, a geologist with the Indonesia's Institute of Science, agreed that last week's quake piled a small amount of new stress onto the megathrust, and that "both Aceh and Padang need to be prepared."

"Authorities need to take a good look at what didn't work well last week and find a way to fix it," he said. "Hopefully, this is an opportunity to learn."