A third killer earthquake may be lurking beneath the Indian Ocean, seismologists say. Last Monday’s quake was caused by an increased geological stress set up by the giant earthquake in December, and they fear the process will repeat itself.
Phil Cummins, a seismologist with Geoscience Australia, said: “There is a chance that the next segment further to the south-east could rupture sooner than we expected. But we can’t predict the time.
“Rather than 100 years it might be 20 or 50 years.” It could strike in the next three months, he added.
The fault where both seismic slips occurred is like a zip gradually coming undone, geologists say. When a quake relieves stress along one stretch, it piles dangerous levels of stress on the adjoining zones, which eventually produce another rupture.
Cummins said a third earthquake was expected because the latest tremor could have raised the geological stress further.
Monday’s quake occurred 171km south-east of the December 26 event, on the Sunda fault, which runs across the Indian Ocean, pushing its sea floor under the Indonesian archipelago.
Scientists are now working to understand why Monday’s quake did not start a destructive tsunami. The Australian Cocos Islands in the Indian Ocean recorded a small wave, but there have been no other reports.
The latest earthquake — the seventh in as many days — happened at a depth of about 30km, about the same as the December 26 event.
It was much smaller, with a magnitude of 8,7, but powerful enough, experts said, to shift the seabed upwards and cause a tsunami.
Dave Tappin of the British Geological Survey said: “My guess is that although it was a giant earthquake, the vertical movement was minimal and the slip was mainly horizontal.”
The southern section of the fault, where it occurred, has had several big earthquakes in the past 300 years. December’s was the first major seismic activity on the northern section for much longer. “The southern segment ruptured numerous times in the 19th and 18th centuries,” Tappin said. “I suspect the stresses that had built up and allowed the vertical displacement in the north hadn’t done so in the south.”
n On Wednesday the United Nations estimated that 500 people had died on the Indonesian islands in Monday’s quake, but local politicians said the toll was at least twice that figure. Many more are believed to be trapped under the rubble.
“There are still areas we haven’t got to, so [the death toll] will probably rise,” said a UN spokesperson.
On the island of Nias survivors used hacksaws and hammers to dig under crumpled buildings in search of their missing relatives.
The UN and other agencies said they planned to divert relief supplies from Sumatra, where they had been stockpiled to help victims of the Boxing Day tsunami. But they immediately hit problems, finding roads from the airport on the island badly damaged.
Another big problem was finding fresh water and food for the survivors. Paul van Tongeren of Oxfam said: “The water situation [in Gunungsitoli, the main town on Nias] is grave: 20 000 people have no access to clean water.” — Â