Earthquakes occur at 'subduction zones,' such as the one west of Indonesia, when one tectonic plate is forced under another--or subducts. Instead of sliding across one another smoothly, the plates stick, and energy builds up until they finally slip or 'rupture', releasing that stored energy as an earthquake. These earthquakes can generate tsunamis when the seafloor moves up or down rapidly. Sean Gulick, a geophysicist from the University of Texas at Austin joined an international research team to try to figure out why there were two quakes in 2004 and 2005, and what made them so different. The scientists used seismic instruments to study layers of sediment beneath the seafloor with sound waves. The researchers found that the fault surface where the two tectonic plates meet, called a décollement, has different properties in the two earthquake rupture regions
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