Subduction zone, shallow depth make lethal mix in earthquake that triggered Asian tsunami

01/05/05

The position of the earth's tectonic plates is key, says Lehigh University seismologist

The location of the recent earthquake that triggered a deadly tsunami in the Indian Ocean came as no surprise to geologists, says 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," says Meltzer, who has supervised two major international seismology research projects in the Himalayas.

The earthquake that struck the Indonesian island of Sumatra on Dec. 26 occurred along a "subduction zone" where the Indian tectonic plate is being subducted, or pulled beneath, the Burma tectonic platelet, says Meltzer.

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 vary in size from a few hundred to thousands of kilometers across and in thickness from 15km to 200km.

Earthquakes, volcanos and landslides result 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 Sumatra earthquake caused a rupture stretching 1,000 km along the fault line separating the Indian and Burma plates.

By contrast, the San Andreas Slip Strike Fault, which extends from the California-Mexico border to Northern California, is broken into segments, which diminish the magnitude of its earthquakes. The San Francisco Earthquake of 1906 ruptured the northernmost 430 kilometers of the San Andreas fault line and has been estimated at just over 8.0 on the Richter Magnitude Scale.

Because the Richter scale is logarithmic, an earthquake measuring 9.0 has a magnitude 10 times greater than an earthquake measuring 8.0.

The effect of the Sumatra earthquake was exacerbated by the fact that the fault rupture occurred only 10 km below the earth's surface, says Meltzer. Had the earthquake occurred at a depth of 100-300 km, its energy would have been attenuated.

The Sumatra earthquake registered 9.0 on the Richter scale, making it the fourth largest in the world since 1900 and the most powerful since the 1964 Alaska earthquake. The tsunami unleased by the earthquake has killed more than 150,000 people in Asia and Africa.

A global network of seismometers monitors earthquake activity, says Meltzer, and can determine within minutes of an event the location and magnitude of the earthquake and the likelihood that it will trigger a tsunami.

Scientists cannot predict when an earthquake will strike, says Meltzer, but they have developed technology to warn people to flee ahead of the tsunamis that often 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.

The Alaska earthquake of 1964 unleashed a tsunami that reached as far as California. The tsunami triggered by the Sumatra earthquake was recorded in New Zealand and as far away as the west coasts of South and North America.

"An early warning system cannot prevent a tsunami," says Meltzer, "but it certainly would have reduced the loss of life from the recent tsunami."

Meltzer has studied earthquakes and deformation of the earth in the Himalayas, the western U.S., the Caribbean and South America. Recently, she completed a three-year term as chair of the executive committee of IRIS (Incorporated Research Institutions for Seismology), a federally funded consortium of 100 institutions that have research programs in seismology.

Meltzer and Peter Zeitler, Lehigh professor of earth and environmental sciences, led a five-year, international study of the vicinity of Nanga Parbat, highest peak in the western Himalayas, and are leading a second five-year international study of Namche Barwa, the highest peak in the eastern Himalayas.

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