Sumatra Poised for Another Tsunami, Study Says

John Roach
National Geographic News
June 8, 2005
The earthquake- and tsunami-battered region of Sumatra, Indonesia, is at
risk for more temblors and killer waves, seismologists cautioned today in a new study.

Study co-author John McCloskey, a seismologist at the University of Ulster in Northern Ireland, said the finding adds urgency to the push for greater earthquake and tsunami preparedness in the Indian Ocean region.

"We should assume it will happen in the near future and do as much as quickly as we can," he said. "For governments to take their eye off the ball of preparedness would be irresponsible."

The seismologists calculated the stress changes induced by the March 28 earthquake—the second giant quake within three months along the Sunda trench west of Sumatra. Their findings are reported in tomorrow's issue of the journal Nature.

The calculations suggest the region of the Sunda trench that is beneath the Mentawai islands is now at the greatest risk of rupture. This section is south of the epicenters of the December 26, 2004, and March 28 earthquakes that rattled the region.

The magnitude 9.1 December earthquake triggered the tsunami that killed an estimated 300,000 people in Asia.

According to McCloskey and colleagues, stress changes induced by the December earthquake caused a magnitude 8.7 earthquake in March, killing about 2,000 people on the island of Nias, west of Sumatra.

"The chance of the earthquakes not having been related is vanishingly small. There's no doubt one was caused by the other," McCloskey said.

Greatest Threat

According to the latest calculations by McCloskey and colleagues, the stress change at Nias induced by the December 26 earthquake was tiny: between 0.07 and 0.17 bars. One bar of stress is equivalent to atmospheric pressure at sea level—about 14.7 pounds per square inch (6.7 kilograms per every 6.5 square centimeters).

"There are many examples of a very small stress triggering a large earthquake. This was one of them," McCloskey said.

This concerns the seismologists. Their calculations show that the stress change beneath the Batu and Mentawai islands, caused by the March quake, is similar to the earlier change in Nias that triggered the March event.

The Batu section of the Sunda trench, south of the Mentawai islands, last ruptured in 1935 and has slowly slipped ever since. As a result, McCloskey said, the total stresses there are probably too low to cause a giant rupture.

Of greater concern to McCloskey and colleagues is the section of the trench south of Siberut, which is at the northern end of the Mentawai islands.

This section last ruptured in 1797, which means it has more than 200 years of accumulated stress waiting to be released. The seismic history of this section indicates that major quakes strike there about every 230 years.

According to the seismologists' calculations, the stress buildup south of Siberut is sufficient to produce an earthquake of magnitude 8.5, with a high potential for a tsunami, McCloskey said.

Banda Aceh

In addition, the new calculations show that the March earthquake expanded Sumatra fault' stressed section by about 125 miles (200 kilometers).

The fault runs the length of Sumatra. Previous calculations from the stress induced by the December earthquake suggested the region immediately south of the hard-hit city of Banda Aceh was at great risk for further temblors.

"That [threat] hasn't gone away," McCloskey said. "There's no sense [that the March event] has relieved any stress there. Rather, it has increased the area of stress along the Sumatra fault."

Roger Bilham, an earthquake hazard specialist at the University of Colorado at Boulder, said McCloskey and his colleagues are doing the right thing by making their findings public. But he cautioned that the findings are not a prediction of when the next earthquake will occur.

"There's no question the strain changes have increased in the region on the faults they mention," Bilham said. But, he added, seismologists lack sufficient knowledge of how the Earth moves beneath its crust to predict when the next earthquake will occur.

The Mentawai section of the Sunda trench might rupture next week, or it might rupture in 50 years. "Basically, we'd be fools to ignore this" stress buildup, Bilham said.

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