Another quake can't be ruled out: scientist

PARIS, Wednesday (AFP, Reuters) A prominent seismologist said he could not rule out the risk of a third big quake off the Indonesian island of Sumatra, where two massive temblors have occurred in just three months.

"The probability of a third quake in the coming months and years, cannot be excluded," said Mustapha Meghraoui, in charge of active tectonics at the Institute for Planetary Physics in Strasbourg, eastern France.

"The theory is that this particular region has seismic cycles of between 150 years and 200 years. The December 26 event caused extreme disruption, and one possibility is of a cascade of quakes."

Monday's 8.7-magnitude quake - one of the biggest in a century - came just over three months after a 9.0 event further to the north which unleashed the tsunami that scoured the coastline of the northern Indian Ocean, killing more than 273,000 people.

The two events occurred in so-called subduction zones where plates of Earth's crust overlap, bumping and grinding.

The December 26 event occurred at a stress point where the Indian plate slips under a tongue called the Burma microplate.

That quake unleashed a huge amount of energy to a Sunda Trench, the undersea fault that runs to the west of Sumatra, where there were big quakes in 1833 and again in 1862.

"It's like two metal springs which are adjoined," Meghraoui said. "If you tense one spring and then release it, some of the energy is transmitted to the neighbouring spring."

In this region, the Indian Ocean is sliding beneath Indonesia at the rate of seven centimetres (2.8 inches) a year, he said.

But this is not a smooth movement. Tension builds up as the plates jam, and when the tension is suddenly and violently released, the result is an earthquake.

What significantly ratcheted up the tension, explained Meghraoui, was the energy imparted on December 26.

"Cascade earthquakes" - a series of earthquakes that decline in magnitude until the tension is eased - are a known phenomenon in seismology.

In the Nankai Trough southeast of Japan, five of the seven large earthquakes of the past 1,500 years unleashed earthquakes in the fault's next section within the following five years.

In Turkey in 1999, a 7.4 earthquake in Izmit, southeast of Istanbul, was caused by the stresses of previous temblors on the Anatolian fault. In turn, this placed stress on the adjoining section of the same fault, unleashing a 7.1 quake at Duzce three months later.

Although tsunami alerts were issued after Monday's event, no big wave occurred - or more exactly, nothing as big as the wall of water up to 10 metres (32.5 feet) high that caused so much devastation on December 26.

The reason, said University of Ulster seismology professor John McCloskey, was Monday's quake was around 12-15 times smaller in magnitude than the December 26 behemoth.

"That's crucial, because the bigger the energy released, the greater the chance that the seabed will move," said McCloskey.

And it occurred relatively far below the surface - at a depth of bout 30 kilometers (18 miles), according to the US Geological Survey (USGS).

In addition, it is unclear whether the seabed was thrust up vertically, which is one of the ideal conditions for making a tsunami.

In a study published on March 17 in the British science weekly Nature, McCloskey's team had warned of the potential for a very large, imminent quake of up to 8.5 magnitude, either on the Sumatran fault, which slices across the Indonesian island, or in the Sunda Trench as a result of the December 26 disaster.

Meanwhile experts said the powerful quake that struck off Indonesia probably did not generate a killer tsunami because it was deep and in a fortuitous location.

But they said the uncertainty over whether there would be a tsunami after the 8.7 magnitude quake under the Indian Ocean floor showed just how little is known about earthquakes and their effects.

"That is why a tsunami early warning system is still badly, badly needed for this area," said Jian Lin, a marine geophysicist at the Woods Hole Oceanographic Institution in Massachusetts.

But there is a big difference between a magnitude 8.7 and a magnitude 9 quake on the logarithmic scale used by geologists, said Lin.

"This earthquake was significantly smaller, even though it was a great earthquake," he said in a telephone interview.