18 March 2005

New Asian quake threat warning

The earthquake that triggered the December 26 tsunami has increased stress on nearby faults, making another major South Asian quake more likely, scientists reported today.

The magnitude 9 earthquake was centered off the west coast of Sumatra, an Indonesian island. The quake shifted nearly 97,000 square miles (250,000 square kilometers) of terrain along the Sunda trench subduction zone, where the Indonesian and Australian tectonic plates dive beneath the Burma tectonic plate.

In places, the earthquake caused the ocean floor to shift as much as 65 feet (20 meters). This displacement triggered the tsunami in the Indian Ocean that, according to the United Nations, killed nearly 300,000 people in South Asia and East Africa.

A team of seismologists led by John McCloskey at the University of Ulster in Northern Ireland, say more shaking could be in store.

McCloskey and colleagues found that the so-called Sumatra-Andaman earthquake increased the stress on adjacent sections of the Sunda trench and the nearby Sumatra fault, which runs the length of Sumatra.

The team cannot predict when another earthquake will occur. But they say the increased stress, combined with historical evidence, raises the likelihood of another big quake in the region. History, they say, shows that one earthquake tends to follow another in the same region.

"People believe lightning never strikes twice in the same place," McCloskey said. "Earthquakes do. Earthquakes cluster in space and time. When you get an earthquake, you are more likely to get another, and our calculations show the stress interaction [in South Asia] is very high."

McCloskey and colleagues Suleyman Nalbant and Sandy Steacy published their calculations in tomorrow's issue of the science journal Nature.

The greatest risk, the scientists say, is for a magnitude 7.5 earthquake on the Sumatra fault near the already devastated town of Banda Aceh. The town is northeast of the epicenter of the December 26 earthquake.

Stress Interaction
McCloskey and colleagues used a map of the slip and displacement from the Sumatra-Andaman earthquake to calculate the stress buildup on the Sunda trench and neighboring Sumatra fault.

The map was created by Chen Ji, a seismologist at the California Institute of Technology in Pasadena. According to Ji's work, portions of the ocean floor along the Sunda trench moved as much as 65 feet (20 meters) in an area stretching from the epicenter north for about 310 miles (500 kilometers).

Subsequent calculations by McCloskey and colleagues show this displacement and slip resulted in a large buildup of stress in a 31-mile (50-kilometer) stretch of the Sunda trench just south of the rupture zone. They saw an even stronger loading of stress along a 186-mile (300-kilometer) section of the Sumatra fault near Banda Aceh.

"We are very confident in our calculations, and it is clear that there is a relationship between these increased stresses and the increased risk of further activity on either fault," McCloskey said. "However, how this relationship works out in detail is not known."

For example, he added, there is not a one-to-one relationship between earthquake occurrence and stress increases. More research is needed to understand why.

Kerry Sieh is a geologist at the California Institute of Technology who has studied the Sumatra tectonic system for the past ten years. He said the findings of McCloskey and colleagues will come as little surprise but are nevertheless a nice quantitative analysis of the stress buildup in the region.

"Whether or not that means an immediate threat of another big earthquake to the south is a much more complex question to answer," he said. "When a fault is being loaded up by a nearby event, a number of factors go into making the evaluation of whether it will rupture soon."

Pending Earthquake?
Ross Stein is a seismologist with the U.S. Geological Survey in Menlo Park, California. He said the stress calculations by McCloskey and colleagues are reasonable but cautioned that they are not a calculation of the earthquake risk.

The last time a major earthquake struck the southern section of the Sunda trench was in 1866. "In other words, it has stored up quite a bit of stress on its own. Add to it the sudden hit from the earthquake and it would surprise no one if it would rupture again," he said.

According to Sieh, the last large and devastating earthquake anywhere along the Sunda trench was in 1907. Though seismologists are uncertain as to the exact 1907 epicenter, it appears to be the section immediately south of the 2004 rupture.

Sieh added that he believes the section of the trench at most risk for failure lies between about 186 and 620 miles (300 to 1,000 kilometers) south of the 2004 epicenter. That section fails about every 200 years. It last ruptured in a couplet of giant earthquakes in 1797 and 1833.

"A repeat of that episode would produce devastation along the West Sumatran coast similar to that along the coast of Aceh last year," he said.


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