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INDONESIA: Sumatra Earthquake No Surprise, Expert Says |
| January 7, 2005 |
 | According to seismic experts, the severity of the Sumatran earthquake was influenced by its location along an uninterrupted fault line and its proximity to the Earth's surface. Click image to enlarge.
Map courtesy NASA |
The difference between a major earthquake and a minor tremor often depends on its location. And the location of the recent earthquake that triggered a deadly tsunami in the Indian Ocean came as no surprise to geologists, according to Anne Meltzer, a world-renowned seismologist at Lehigh University.
"Earthquakes like this one happen only once every 50 to 100 years and they happen in very specific locations," said Meltzer, who has supervised two major international seismology research projects in the Himalayas.
The earth's surface is covered by seven to nine major tectonic plates and a number of minor plates, or platelets. The plates – huge slabs of semi-solid rock beneath continents and oceans – range from a few hundred to thousands of kilometers wide and 15-200 km thick.
Earthquakes, volcanos and landslides occur when tectonic plates collide, drift apart or slide past each other. Subduction zones, which are caused when tectonic plates collide, produce the most powerful earthquakes, said Meltzer, because they have long continuous fault lines.
The earthquake that struck the Indonesian island of Sumatra on Dec. 26 occurred along a subduction zone where the Indian tectonic plate is subducted, or pulled beneath, the Burma tectonic platelet. The quake caused a rupture stretching 1,000 km along the fault line separating the Indian and Burma plates.
The effect of the Sumatra earthquake was exacerbated by the fact that the rupture occurred only 10 km below the earth's surface, said Meltzer. Had the earthquake occurred at a depth of 100-300 km, its energy would have been absorbed by the Earth’s crust.
Scientists cannot predict when an earthquake will strike, but they have developed technology to warn people to flee ahead of the tsunamis that often follow an event like the Sumatra earthquake. Buoys, tide gauges and satellite images of sea surfaces are used in early warning systems that notify people of pending danger. Such a system is not in place in the Indian Ocean, but an early warning system was installed in countries on the Pacific Rim, which is ringed with subduction zones, after an earthquake in Alaska in 1946 sent a tsunami crashing into Hawaii.
"An early warning system cannot prevent a tsunami," said Meltzer, "but it certainly would have reduced the loss of life from the recent tsunami."
 | The maps above show estimates of how much the Earth moved as a result of the quake. A black star marks the earthquake epicenter.
On the left is vertical displacement, or uplift. Positive values (red, yellow, and green) show areas that rose during the earthquake, and negative values (blue) show regions that dropped. The maximum rise, along the subduction zone where the Burma Plate is moving over the India Plate, was over 5 meters.
On the right is horizontal motion. The greatest motion on the sea floor was 11 meters. The coast of Sumatra moved as much as 3 meters, and the north end of Simeulue Island moved 2 meters.
Map and Information courtesy NASA |
Source: Lehigh University
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