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Quake in December Set Stage for More Upheaval

By Rob Stein
Washington Post Staff Writer
Tuesday, March 29, 2005; Page A10

Yesterday's massive earthquake occurred in a region that is prone to temblors because large segments of Earth's crust are colliding there, creating enormous pressures that are released periodically in cataclysmic jolts, geologists said yesterday.

Scientists had been expecting that another large quake might strike soon in the region because the massive undersea upheaval that triggered December's tsunami generated even more pressure on the region's already volatile geology, experts said.

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"What happened today was not a surprise. A number of scientists have been talking about an increased likelihood of more earthquakes in this area because of the rupture that happened in December," said Lori Dengler, a geologist at Humboldt State University in Arcata, Calif. "And it may not stop here."

Earthquakes occur along the boundaries between sections of Earth's surface known as plates. These plates are constantly moving, slowly but inexorably pushing against one another.

"They are squeezing together over geologic time at about the rate your fingernails grow," said Alan L. Kafka, a geophysicist at Boston College. "The movement causes tremendous amounts of force to build up and up and up."

Eventually, rock in one of the plates gives way under the pressure, causing a section to snap and the plates to suddenly lurch.

"It's on these plate boundaries that we have the world's largest earthquakes," said Bruce Presgrave, a geophysicist at the U.S. Geological Survey's National Earthquake Information Center. "The rocks are stressed and stressed and stressed until finally they can't sustain it anymore and they snap, shifting to a new position. That's what we feel as earthquakes."

The area where yesterday's quake occurred is particularly troublesome because of the speed at which plates there are converging, and their relative positions. Unlike the San Andreas fault in California, where two plates are moving past each other horizontally, the region west of the island of Sumatra is a "subduction zone," where plates are sliding over and under one another.

"These are the places where we have most of the world's earthquakes, and lots and lots of volcanoes," Dengler said.

In December, a section of one plate about 700 miles long suddenly plunged about 30 feet beneath another, causing a magnitude 9.0 earthquake that created the devastating tsunami. That event probably increased pressure on the next section of the plate boundary just to the south, causing yesterday's similar sudden thrust of one plate beneath the other, this time apparently involving a smaller section of perhaps 200 to 300 miles.

"We think it's very likely that the extra stresses put on by the quake happening to the northwest is very likely to have triggered this earthquake. It would have happened sometime, but the timing may have been moved up by the quake to the north," Presgrave said.

There is now an increased possibility that yet another massive quake could occur soon, farther along the same plate boundary, experts said.

"Think of a crack in your windshield that propagates over time," said Kate Hutton, a seismologist at the California Institute of Technology. "Once there's a break, the two ends are the most highly stressed and it eventually keeps growing."

The timing of the next event is unpredictable because it depends on a host of factors, including whether pressure had been released along the boundary by previous quakes. But more quakes are inevitable.

"It's just a question of when, and how big," Dengler said.

Scientists were unsure yesterday why this quake apparently did not cause a tsunami, but there could be a number of explanations.

If the temblor caused the seafloor only to vibrate, instead of suddenly shifting upward, that would have prevented a tsunami.

"The question is whether it actually shifted the seafloor itself. If the quake doesn't actually move the seafloor, and just shakes it, then you're not moving the water," Presgrave said.

Another possibility is that the force of whatever water movement occurred was dissipated by nearby islands or just moved out to sea. But scientists are studying the event for clues that might help them predict future tsunamis.

"This is a really important question," Dengler said.

Work is underway to create a tsunami warning system for the Indian Ocean similar to the one in place for the Pacific.

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