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The December earthquake off the coast of Indonesia, and the tsunami it created, wreaked massive destruction and loss of life. It also increased stress on nearby faults in the area. A report published today in the journal Nature indicates that the region may now be primed for another big quake, one that could spawn a second tsunami. The news follows on the heels of another reminder of the area's geologic restlessness: early Wednesday a 5.4 earthquake 100 kilometers west of the city of Banda Aceh was recorded. (Early media coverage reported no casualties.)
John McCloskey and his colleagues at the University of Ulster analyzed data from the 2004 Sumatra-Andaman earthquake and found two zones of increased seismological stress in the surrounding area. The first region lies in the Sunda trench, which is a 50-kilometer-long section off the northern tip of Sumatra, and the second is on the Sumatra fault, which runs under the length of the island and ends near Banda Aceh. "Our results show a stress increase of up to five bars in the Sunda trench next to the rupture zone and a strong positive loading of nine bars for 300 kilometers of the Sumatra fault," McCloskey remarks. As a point of comparison, it is believed that the recent Izmit earthquake in Turkey, which measured a magnitude 7.4, was triggered by stress increases of about two bars over an area of 50 kilometers.
The authors report that their results suggest an earthquake of magnitude 7-7.5 on the Sumatra fault "would seem to represent the greatest immediate threat." From preliminary data collected on Wednesday's quake, it remains unclear whether the faults analyzed in this study were involved or whether it occurred on a fault that is not as well categorized. "Every earthquake that occurs [in the region] is going to relieve some of the stress," notes John Bellini, a geophysicist with the United States Geological Survey. But because of the logarithmic nature of the Richter scale, he notes, "you'd need 1,000 earthquakes of magnitude five to release as much as one magnitude seven [quake]." McCloskey and his colleagues note that their findings underscore the need for a warning system: "It is vital that disaster fatigue does not delay the implementation of the Circum Indian Ocean Tsunami Warning System."
