Researchers found differences between the rocks in the regions where the 2004 and 2005 Sumatran earthquakes occurred. In the southern part of the region where the 2004 earthquake occurred, the earthquake rupture began closer to shore and did not reach as far seaward. The 2005 earthquake ruptured farther seaward beneath a thick wedge of compacted sedimentary rocks. These differences help explain why the 2004 earthquake and tsunami were more severe than the events in 2005.
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Image Courtesy of Nicolle Rager Fuller, National Science Foundation
Composition of Rock Sediment Affected the Strength of Earthquakes in Sumatra
Earthquakes occur regularly in Sumatra, Indonesia because it is located near the boundary of two of Earth's tectonic plates. These earthquakes occur at 'subduction zones,' which are areas where one tectonic place is forced under another, and there is a subduction zone west of Indonesia.
Instead of sliding across one another smoothly, the plates stick, and energy builds up until they finally slip or 'rupture', releasing energy as an earthquake. Earthquakes can generate tsunamis when the seafloor moves up or down rapidly. Some earthquakes create large tsunamis and others do not.
On December 26, 2004, an earthquake in the Indian Ocean created an enormous and destructive tsunami known as the "Boxing Day Tsunami." Three months later, another strong earthquake occurred to the south, but this earthquake triggered a much smaller tsunami.
"Many people wondered why the 2004 quake was so large," said Sean Gulick, a geophysicist from the University of Texas at Austin. "Perhaps a more interesting question is: why wasn't it larger? Why did the rupture occur as two events instead of one large one?"
Gulick joined an international research team to try to figure out why there were two earthquakes, and what made them so different. The scientists used instruments to study layers of sediment beneath the seafloor. They found that the fault surface where the two tectonic plates meet has different properties in these two different earthquakes.
Scientists believe the difference in the composition of the rocks of the two earthquakes resulted in the fault slipping over a much wider part of the margin and farther seaward in the 2004 event. They think that because more earth moved, more of the seafloor moved and more water was forced to move, which created a larger tsunami.
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