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12/01/92

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DISPUTE REVOLVES AROUND POSSIBLE NEW MOJAVE FAULT . . . the simplest of several hypotheses is always the best in accounting for unexplained facts. William of Occam (c. 1285-c. 1349)

STANFORD -- In science, broad new ideas usually touch off heated controversies, especially if they sound simple and cross the boundaries between scientific fields. In a classic example, geologists are pinned against geophysicists over Amos Nur's recent hypothesis that a major new fault line is building in the Mojave Desert.

Some critics find the concept of a new fault "crazy" or "bizarre," but Nur, Stanford professor of geophysics, is confident that his model will be accepted.

Armed with more data - including seismic measurements of small earthquakes that are lined up along the postulated new line, and the finding of newly fractured rock in that area - Nur and his colleagues Hagai Ron, Greg Beroza and graduate student Laura Alfonsi will present their hypothesis to the 1992 Annual Meeting of the American Geophysical Union to be held Dec. 7-11 in San Francisco.

In 1989, Nur and colleagues proposed that a major fault was developing in the central Mojave region east of Los Angeles, running in a northerly direction. He based his model on the observation that older faults in the area apparently had rotated away from the direction of tectonic motion and are now oriented too unfavorably to absorb stress built up as the Pacific and North Atlantic plates grind past one another.

He expected future earthquakes along the new fault, and three years later, two strong quakes struck Southern California. Both the Joshua Tree (which measured 6.1 on the Richter scale) and the Landers (7.4) tremors of 1992 fell neatly in line with the proposed fault.

"That's extremely unusual," Nur said. "It's one of the few times when some really complicated observations, that otherwise can't be understood, fall into place so simply."

According to Nur, the Mojave case is intriguing because the orientation of the slip directions of the recent quakes also lines up.

"This is very hard to explain by coincidence," he said.

The Landers shock was special because its rupture line makes a sharp turn; earthquakes usually travel along a straight course and stop propagating at bends. During the Landers event however, one part of the rupture was along an old fault running northwest-southeast and the other part raced more northward along the suggested new fault line. According to Nur, no conventional model can explain this kink, but his idea does.

"What happens if an old fault rotates and gets stuck?" he said. "A new one develops, and during the transition period, both faults can equally slide. We anticipated exactly this mixed behavior."

Not all earth scientists, particularly geologists who map the area, find this argument compelling.

University of Southern California professor of geology Greg Davis, a Stanford graduate, concedes that the northward-running part of the Landers quake puzzles geologists. However, it alone doesn't make a valid case for a new fault, he said, because most of the Landers rupture occurred on preexisting faults.

Davis also questions the basic premise on which Nur's hypothesis is built, namely, that older faults in the central Mojave region have rotated enough to become inactive. Geological evidence rather shows that faults in the Landers area have rotated only slightly and are still "quite young and well," he said.

Davis agrees that plate motion is transferred into the Mojave Desert but he believes that this process has already been going on for a long time. The postulated fault may be very old but could have gone unmapped because it runs through a valley where faults, covered with gravel, are hard to find. He said that Nur didn't fully appreciate how long it takes for faults to build.

"Sometimes we don't know whether a fault is present until we have an earthquake," Davis said, "then we investigate it and may find that it had moved multiple times before. If the recurrence intervals of ruptures are long, maybe as long as tens of thousands of years, geologists may not see the fault despite its long history."

Nur counters that if the proposed fault was indeed old but quiet, then other faults cutting across it almost perpendicularly would have offset it into segments because these other faults have slipped several kilometers over the last 10 million years. Geological evidence, such as fractured rocks, would accordingly be found in segments instead of the coherent line indicated by recent geological data, Nur said. The researchers are beginning to look into this in more detail.

According to Nur, a part of the problem stems from a lack of sufficient communication between geologists and geophysicists, who use different approaches to study the earth and often don't read each other's journals.

For example, while geophysicist Nur contends that some geologists do not appreciate the mechanical approach of his model, geologist Davis said that Nur ignored previous geologic evidence contradicting this model.

Nur said the simplicity of his model upsets some of his colleagues. It follows William of Occam's razor, essentia non sunt multiplicanda praeter necessitatem - i.e., that the simplest idea is generally the best one.

"Some geologists who have worked in the Mojave for a long time," Nur said, "think: This is so simple; if it is there, I should have seen it. Since I haven't seen it, it can't be true."

These geologists, however, say that the model is based on incorrect simplifications. They feel more comfortable with Aldous Huxley, who said about Occam's principle:

". . . entities sometimes ought to be multiplied beyond the point of the simplest possible explanation. For the world is doubtless far odder and more complex than we ordinarily think."

That's how science works.

-jns/nur-

This story was written by Gabrielle Strobel, a science writing intern at the Stanford News Service.

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