WASHINGTON, Dec 3 (Reuters) - An area off Indonesia's Sumatra island that has been the source of disastrous earthquakes still carries a lot of pent-up pressure that could result in a strong quake, researchers reported on Wednesday. Measurements of the quake zone around the Mentawai islands off Sumatra's west coast show it has not moved as much as in the past, suggesting the potential for a large "megathrust" remains, the geologists report in the journal Nature. Indonesia is part of a belt of intense seismic activity known as the "Pacific Ring of Fire" and was the site of a huge earthquake in December 2004. It triggered a devastating tsunami in the Indian Ocean that killed more than 230,000 people in the region. There have been many smaller quakes in the area and another large one of 8.4 magnitude, followed by a 7.9 event, which killed more than 20 people in 2007. Kerry Sieh, Jean-Philippe Avouac and colleagues at the California Institute of Technology in Pasadena have been watching the area, using satellites to measure ground movements and also seeking geological evidence of what has happened there in the past. They are concerned because even more monstrous quakes occurred in 1833. "Two salient questions are why the 2007 sequence did not duplicate the 1833 event and why it released only about 25 percent of the deficit of moment that had accumulated since 1833," they wrote. "We use Global Positioning System (GPS) measurements, field measurements of uplift, synthetic aperture radar (SAR) interferometry (InSAR) and seismological records to estimate the source parameters of the two large earthquakes of 12 September 2007," they added. They found the 2007 quakes released only some of the pent-up movement and pressure that is occurring as one tectonic plate rubs against another there. "The potential for a large megathrust event in the Mentawai area thus remains large," they concluded. Tectonic plates are large masses of solid rock that float on the Earth's crust. The Sunda megathrust is a tectonic plate boundary near the coast of Sumatra, where a patchwork of creeping and locked areas of crust rub up against one another. Because they are rough, slips and slides occur in bursts that shake the surface. The researchers say their measurements suggest another big slip could occur. (Reporting by Maggie Fox, editing by Will Dunham)
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