Another 2004-Force Tsunami Likely Within 30 Years, Study Shows

By Ryan Flinn and Simeon Bennett

 

Dec. 12 (Bloomberg) -- Another tsunami-triggering earthquake off Indonesia is “extremely likely” in the next 30 years and may claim a higher toll than the 2004 surge that killed more than 220,000 people, according to a study in the journal Science.

 

Researchers studying changes over the past 700 years in coral reefs near Indonesia’s Mentawai islands found evidence the region is due for a quake as big as magnitude 8.8, and that a smaller temblor last year is probably a harbinger of bigger ones.

 

A large shock would set off a tsunami that would strike cities and towns in western Sumatra, scientists led by Kerry Sieh of the California Institute of Technology in Pasadena said in the study, to be published today.

 

“Somewhere in the next 30 years it’s extremely likely,” Sieh told reporters in Singapore on Dec. 9. “Whether it’s one big rupture or several, we can’t say. It will certainly result in a tsunami.”

 

Indonesia is the world’s largest archipelago and forms part of the “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. Those plates constantly shift, sometimes causing earthquakes that can produce tsunamis.

 

The 2004 tsunami that swept across the Indian Ocean, devastating coastal communities in Indonesia, Sri Lanka, India, Thailand and other countries, was caused by a 9.1 temblor in a fault seismologists call the Sunda megathrust. There have been hundreds of earthquakes in Indonesia since then, including a 5.9- magnitude quake reported yesterday in the Kepulauan Mentawai region by the U.S. Geological Survey.

 

Sunda Megathrust

 

Earthquakes push up the sea floor, lowering the water level. That’s recorded in coral growth rings, which Sieh and colleagues studied in a 700-kilometer-long (435-mile) section of the Sunda megathrust, south of the 2004 epicenter, off the coast of the Mentawai islands. They discovered four “supercycles” of temblors dating to about 1350.

 

Each supercycle comprised the build-up of tectonic strain over time that was released by two or more large earthquakes. Because a strain-relieving quake hasn’t rocked the so-called Mentawai patch since 1833, an 8.4-magnitude earthquake that struck in September 2007 is probably the first of a new series, Sieh said. The next may be as big as 8.8-magnitude.

 

Because the length of time between each event in past series varied from a few decades to more than a century, predicting precisely when the next will occur is impossible, he said. Still, the quake will trigger a tsunami that may be as high as 5 meters (16 feet) at Padang, a city of about 750,000 people in western Sumatra, Sieh said. The 2004 tsunami was between 5 and 12 meters high when it struck Banda Aceh in northern Sumatra.

 

Higher Death Toll

 

“Losses of life and property could equal or exceed those in Aceh province in 2004,” Sieh and colleagues wrote in the study.

 

About 30 percent of the people in Padang have been trained in how to respond to an earthquake and tsunami and when to evacuate, said Patra Rina Dewi, executive director of Kogami, a non-government organization working to prepare for the next tsunami. Last year’s quake was a learning experience, she said.

 

“Our people in Sumatra, especially in Padang city, know how to respond now, because on 12 September we got that kind of earthquake, and people just evacuated immediately,” she said from Padang in a telephone interview yesterday.

 

To contact the reporters on this story: Ryan Flinn in San Francisco at rflinn@bloomberg.net; Simeon Bennett in Singapore at sbennett9@bloomberg.net