3rd quake possible

2005-03-29 21:31

Paris - A prominent seismologist said on Tuesday 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, who is 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 more than 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 happened in so-called subduction zones where plates of Earth's crust overlap, bumping and grinding.

Burma microplate

The December 26 event happened 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 that 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 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 ratched 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.

Quake was smaller

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.

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 10m high that caused so much devastation on December 26.

The reason, said University of Ulster seismology professor John McCloskey, was Monday's quake was about 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.

- AFP