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Powerful quake shakes South Pacific.

Powerful Quake Shakes South Pacific

A scant six hours after a severe earthquake shook the seafloor southwest of New Zealand last week, seismologists had sketched a portrait of the fault fracture that generated this quake, the Earth's largest in at least 12 years.

"This is sort of the beginning of a new era in seismology," says Harvard University's Adam M. Dziewonski. "We have a nearly real-time capability to assess not just quake location -- which has been done for a long time -- but also the mechanism and the type of forces."

The May 23 earthquake, measured at 8.2 to 8.3 on the Richter scale of magnitude, struck along the Macquarie ridge about 500 miles from New Zealand. "It was an extremely large earthquake. But it was just out in the middle of nowhere and there appears to be no damage," says Bruce W. Presgrave from the U.S. Geological Survey's National Earthquake Information Center in Golden, Colo.

Seismologists define the geologic structures that produce quakes by analyzing seismic waves traveling through the Earth. Among other things, the waves indicate whether horizontal or vertical motion caused the quake and characterize the orientation of this motion. To perform these calculations, seismologists usually must wait several months for seismic data from stations around the world to reach large information networks.

Dziewonski and his colleagues cut this time to hours by retrieving digital data from two seismic stations on either side of North America via computer modems -- his group's first opportunity to test the technique on such a large quake. Researchers at the University of Paris and at the California Institute of Technology in Pasadena use the same process.

Based on limited data, these estimates serve only as first approximations but can provide important information. "An earthquake like this occurring anywhere near a populated region could cause tremendous damage, and you really want to know, very soon afterward, the tectonic significance of the earthquake in order to assess the likelihood of large aftershocks," Dziewonski says.

The fault that spawned last week's tremor sits along the Macquarie ridge complex, a line of underwater ridges and troughs that form the boundary between the Pacific plate and a southern tongue of the Australian plate.

Richard G. Gordon, a geophysicist at Northwestern University in Evanston, Ill., says the Macquarie region is tectonically complex. The Indian plate is moving northeast relative to the Pacific plate, and if the plate boundary pointed exactly parallel to that motion, the plates would slide past each other. However, models suggests the boundary points about 20[deg.] counterclockwise from the direction of motion, so the plates are crunching together as they slide, says Gordon.

Earthquakes in the area support this idea because some release horizontal stress by sliding and others release compressional stress by forcing one crustal block to override the other. According to the Harvard researchers, last week's tremor involved both types of motion.

Because the earthquake relieved a substantial portion of the stress in that area, it will help scientists resolve some questions about the region's tectonics. Some studies suggest that the southern extension of the Australian plate may not attach firmly to the main plate because the extension appears to be migrating westward relative to the main plate. Gordon says the plate may deform to allow this motion, or the southern piece may be an independent microplate. The recent quake, he says, "will help tell us whether the Australian plate is behaving rigidly or not."
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Author:Monastersky, R.
Publication:Science News
Date:Jun 3, 1989
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