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Great quake followed slow precursor.

Seismologists studying records of a great earthquake that struck south of New Zealand in 1989 have detected evidence that the tremor started off extremely slowly, taking many minutes to build before releasing its main burst of energy in the largest earthquake of the last 14 years. While the discovery does not immediately offer a method for predicting seismic disasters, it bolsters the hope that scientists may someday be able to discern when the Earth is preparing to generate a major quake.

Pierre F. Ihmle, Paolo Harabaglia, and Thomas H. Jordan of the Massachusetts Institute of Technology found the evidence of a precursor while studying the magnitude 8.2 jolt that occurred May 23, 1989, on a submerged fault along the Macquarie Ridge. Although large, the remote tremor caused little damage.

Analyzing records of the seismic waves detected around the world after the quake, Ihmle and his colleagues found unusual features in the lowest-frequency vibrations. To explain the anomaly, they suggest that such waves began emanating from the undersea fault more than six minutes before the actual earthquake started. They call this early activity a "slow earthquake" and calculate that it released 20 to 25 percent of the energy liberated by the entire shock. On its own, the slow precursor would have had the same strength as the magnitude 7.6 temblor that struck east of Los Angeles last June.

The researchers suggest that the early low-frequency waves may have come from a part of the fault deep beneath the ocean floor, where high temperatures make rock more ductile. Rock on either side of the fault could have started to slip in the deep zone, moving slowly until the rupture spread into the cool upper crust, where it sparked a fast tear that unleashed most of the quake's energy in a 20-second burst, says Ihmle.

Researchers have detected slow precursors of other large oceanic earthquakes. But this is the first known example that started on a strike-slip fault, where land on each side of the fault slides horizontally -- the same type of motion seen along California's San Andreas fault. The other examples were seen in subduction zones, in which one plate slides beneath another. Scientists have not yet found slow precursors to any San Andreas quake or to jolts on any other continent. But the new discovery offers clues about how large earthquakes start, and it should spur research on precursors, says Ihmle.
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Title Annotation:1989 New Zealand earthquake
Author:Monastersky, Richard
Publication:Science News
Date:Jun 5, 1993
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