By Arijit Ghosh and Jean Chua

Sept. 18 (Bloomberg) — The world’s largest earthquake in more than two years that struck six days ago off Indonesia’s Sumatra island increased the risk of another major temblor occurring in the region, seismologists said.

The 8.4 magnitude quake on Sept. 12 was in a 4,500 kilometer (2,800 mile) so-called subduction zone, an area where one tectonic plate is forced under another. It occurred when the Indian plate slid under the Australian plate, a process known as a rupture. The area started rupturing in 2004 after a 9.1 magnitude quake hit Aceh in northern Sumatra.

Last week’s temblor ruptured a 300-kilometer area. Another 200-kilometer stretch north of Bengkulu and extending to Nias Island that was struck by an 8.6 magnitude quake in 2005 is now facing increasing stress, said Suleyman Nalbant, a lecturer at the University of Ulster in the U.K.

“Now that this has happened, there is a significantly higher chance that the rest will blow too,” Kerry Sieh, a geologist and seismologist with the California Institute of Technology, said from Singapore. “Our biggest concern now is that there will be another one. How big and when, whether it’s in the next 10 minutes, 10 hours, 10 days, nobody can say.”

The Sept. 12 earthquake northwest of Bengkulu in southern Sumatra was the world’s biggest since the temblor at Nias on March 28, 2005.

The last big quake that occurred near Bengkulu was 174 years ago and the last large temblor to hit south of Nias occurred in 1797, according to Nalbant.

New Shift

Historically, plates that have ruptured shift again after about 150 to 200 years, said Paramesh Banerjee, a scientist with the Indian government-funded Wadia Institute of Himalayan Geology. Banerjee has written papers with University of California scientists on quakes in the Sumatra region.

“Once the rupturing is complete, we can safely expect there will not be a very big quake in the area” for some time, said Banerjee. “Ultimately, the entire subduction zone has to be ruptured, that is inevitable.”

The subduction zone stretches from the Andaman Islands in the Indian Ocean to the Banda Sea bordering Papua.

The quake off the coast of Aceh in northern Sumatra in December 2004 caused a tsunami that devastated coastal communities in countries across the Indian Ocean, killing more than 220,000 people. Last week’s quake at a depth of 15 kilometers killed 23 people and damaged more than 45,000 houses.

“If it were a giant, then last week’s quake was just the tail of it. The head may be yet to come,” said Danny Hilmi, a geo-technology researcher at the Indonesian Institute of Sciences. “From the geological data, we should be worried the next one would be in the Siberut and Pagai islands.”

Fault Lines

Indonesia is the world’s largest archipelago and is located on the so-called Pacific “Ring of Fire,” an arc of volcanoes and geologic fault lines surrounding the Pacific Basin. The archipelago lies in a zone where four tectonic plates meet. These plates constantly shift, sometimes causing earthquakes that can produce tsunamis.

The 2004 temblor caused the Indian tectonic plate to subduct, increasing stress on the Sumatra fault line near the province of Aceh, John McCloskey from the University of Ulster said in a study released three months after that disaster.

The Sept. 12 quake spurred volcanic activity near the epicenter, Indonesia’s Volcanology and Geological Hazard Mitigation Agency said.

“There were an increased number of tremors in three volcanoes in Sumatra, which indicate the movement of magma,” said Saut Simatupang, head of the agency.

Mount Talang, the closest to the epicenter, recorded 30 to 40 times the usual number of volcanic shocks a day after the quake, he said.

To contact the reporter on this story: Jean Chua in Singapore at jchua4@bloomberg.net ; Arijit Ghosh in Jakarta at aghosh@bloomberg.net .

Source: Bloomberg Site, 18 September 2007

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