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FURTHER EVIDENCE OF DIFFERENT RUPTURE PROCESS FOR DEEP EARTHQUAKES

Note: This release is embargoed until 6:00 p.m. EDT Thursday, August 4, 1994. The study will appear in the August 5 issue of the journal Science.

SANTA CRUZ, CA--For the third time in three years, researchers at the University of California, Santa Cruz, have unearthed evidence that the world's deepest quakes rupture the planet in a markedly different way than their shallow and more damaging cousins.

For their latest study, seismologists Heidi Houston and John Vidale examined how deep earthquakes release their energy over time. They found that on average, the deepest events--up to 430 miles beneath the earth's surface--get started quickly, build rapidly to a crescendo, then end just as quickly, creating a symmetric pattern of energy release. In contrast, quakes closer to the surface last longer and do not end nearly as abruptly, on average. Instead, their energy output tails off more gradually and in a more varied way.

The difference, the researchers say, stems from a mysterious change in how earthquake faults rifle through deep rock. Higher temperatures and pressures far below the surface may create a more uniform environment that a quake can rupture in one smooth pulse. Alternatively, the faulting process for deep quakes may be radically different, triggered by changes in the structures of minerals rather than brittle failure along a plane of rock.

"There appears to be something fundamentally different happening on the fault plane at depth," says Houston, a seismologist at UCSC's Institute of Tectonics. "There's a difference in the degree of complexity, but we don't know whether it's in the bulk material or in the faulting process itself." Even with that uncertainty, she says, geophysicists studying this hot topic now have another constraint for their models of the physical mechanisms by which deep quakes occur.

Houston and Vidale, who is affiliated with both UCSC and the U.S. Geological Survey in Menlo Park, published their work in the August 5 issue of the journal Science.

The paper complements two previous UCSC studies on the seismic characteristics of deep earthquakes. In a 1991 report in Nature, Houston and UCSC mineral physicist Quentin Williams described evidence that deep quakes, once they start, reach their peak motions almost twice as quickly on average as shallow ones. Last year in Nature, Vidale and Houston showed that the same deep quakes typically last only about half as long as ones closer to the surface.

The new study yielded a more qualitative characteristic: the "envelope" of an earthquake's energy release, a curve that traces out how much energy the quake produces at each point in time from beginning to end. However, because the precise ending time of an earthquake is a challenge to discern, deriving those shapes was a massive chore.

The researchers gathered recordings of 169 large earthquakes at least 60 miles deep that struck around the Pacific Rim between 1980 and 1992. With seismic traces available from hundreds of seismometers in California and Washington, the amount of data was formidable: Vidale estimates that during a two-month span, he looked at 50,000 seismic traces--rejecting the noisiest ones--and picked earthquake start times for 20,000 of them. Technical reasons made 47 of the earthquakes unusable, leaving 122 events in the final study.

Then, Houston and Vidale relied upon a statistical phenomenon to nail down the end of each quake. By adding together many seismic traces in a computer and aligning them to the same start time, they reduced the echoes, noise, and other local effects that normally obscure the point at which a quake actually stops. That reduction goes as the square root of the number of traces; stacking together 100 traces for the same earthquake, for instance, cuts the noise by a factor of 10. The result: a clean "average" seismogram for each event.

Next, the researchers computed the envelope of each average seismogram. Envelopes for simple events looked like bell-shaped curves, with most of the energy release in the middle. For complex events, many envelopes had peaks and valleys, showing how the earthquakes waxed and waned. Finally, to explore differences in the envelopes of earthquakes at various depths, Houston and Vidale stretched all of the envelopes to the same length and averaged groups of them together.

For the deepest events (325-435 miles), the average envelope was quite symmetric, ending almost as abruptly as it began. Between 220 and 325 miles deep, endings became more gradual. Envelopes for the shallowest events studied (60-220 miles, but still regarded by seismologists as "intermediate" in depth) were the least symmetric of all.

"These intermediate-depth earthquakes end more gradually than deep ones," says Houston. "That's new information. For the deep events, it's interesting to see how simple they look on average. But we're looking at averages of an average, and any one earthquake can deviate significantly from that pattern."

The trend toward asymmetric energy release might continue with even shallower earthquakes, says Vidale, if the planet's outermost 60 miles grows less and less uniform. The researchers hope to conduct such a study, but seismic records from quakes close to the surface are much noisier and harder to analyze than those from deep events, Vidale notes.

This new approach could shed light on shallow and damaging ruptures, says Houston: "If shallow quakes are asymmetric, we would infer that heterogeneity is important to the rupture. This has implications for understanding the physics of ruptures and possibly for earthquake prediction."

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