for National Geographic News
The force of the magnitude 9 earthquake that struck northern Sumatra on December 26, 2004, may have caught much of the world by surprise. But scientists say the region has a violent geologic past and is ripe for more cataclysmic earthquakes and volcanic eruptions in the millennia to come.
The Indonesian island sits in an area of the Indian Ocean where several large chunks of Earth's crust, known as tectonic plates, collide. Tectonic plates can slip past, beneath, and over the top of each other. In the Sumatra region, the Indian and Australian plates are slowly creeping alongside andin a process called subductiondiving beneath, the Burma plate, part of the larger Eurasian plate.
The plates meet at the Sunda trench, a subduction zone that runs 3,400 miles (5,500 kilometers) from Myanmar (Burma) south past Sumatra and Java and east toward Australia. The trench runs roughly parallel to the west coast of Sumatra about 125 miles (200 kilometers) offshore.
In addition, the Sumatra fault spans the entire length of the island. Akin to California's San Andreas Fault, the Sumatra fault is a strike-slip fault, where two sections of rock pass horizontally by each other in opposite directions, like the pieces of slide rule.
Earthquakes occur along both the Sunda trench and the Sumatra fault.
"So Sumatra gets at least a double [earthquake] whammy, and then there's the volcanoes associated with all subduction zones," said Rob McCaffrey, a geophysicist at the Rensselaer Polytechnic Institute in Troy, New York. McCaffrey has studied the Sumatra tectonic system since the 1980s.
Indeed, McCaffrey and other scientists say Sumatra sits in the midst of one of the world's most geologically active regions. Earthquakes of greater than magnitude 8 struck Sumatra in 1797, 1833, and 1861. Quakes of more than magnitude 7 rocked nearby islands in 1881, 1935, 2000, and 2002.
"But I don't think anyone was expecting this segment of the subduction zone to produce a [magnitude] 9 [earthquake]," McCaffrey said.
The region of contact between plates in a subduction zone is called an earthquake fault, or a megathrust. The Sunda trench is considered a megathrust fault.
At the Sunda trench near Sumatra, the Indian and Australian tectonic plates creep north-northeast at 2.4 inches (61 millimeters) each year as they slip past and beneath the Burma plate. But the process is not smooth, explains James Dewey of the U.S. Geological Survey's National Earthquake Information Center in Golden, Colorado.
The plates push against each other, building up pressure "until the stress reaches a critical level, and that's when it starts rupturing," Dewey said. The ruptures, which are felt as earthquakes, do not occur along the entire length of the subduction zone, but rather in patches.
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