Friday, Jan. 21, 2000 | 10:18 a.m.
Stanford University geologists see the San Andreas fault and its earthquake threat as weakening, while the Mojave Desert is becoming much more active.
There's evidence that the Mojave Desert is laced with strong, new faults, capable of producing earth-shattering quakes for a distance of 200 miles southwest of Las Vegas.
Stanford scientists, who were publicizing their findings this week, reported that the faults ruptured in patterns that at first puzzled researchers, who expected quakes along known faults.
But evidence of quakes at the surface may not tell the whole story. The Mojave study could unearth a new way for scientists to learn how new faults form.
Some of the ruptures broke the Earth's surface, such as the Hector Mine quake on Oct. 1, the latest big mover and shaker, geophysicst Gregory Beroza said.
Others did not.
"What caught our eye about the Mojave earthquakes was that they set off several quakes, not on the same fault," Beroza said.
Over the past 67 years seven moderate-to-large quakes have erupted along a 62-mile line through the Mojave.
The 1992 Landers temblor was the largest in the series at 7.4 magnitude.
A 5.6 magnitude temblor at Little Skull Mountain June 29, 1992, the day after the Landers shaker, was noted by the scientific team, but they concentrated on the largest earthquakes of 6.0 magnitude or more.
The Little Skull Mountain quake erupted on a fault next to Yucca Mountain, the proposed high-level nuclear waste repository 90 miles northwest of Las Vegas, damaging a U.S. Department of Energy building at Yucca.
It may take 10,000 years or more to prove the scientists' theory, and they did not consider the impacts of larger, newer quakes shaking Yucca Mountain.
Quake potential is part of the studies by the DOE and the Nuclear Regulatory Commission, which must license Yucca if it passes scientific muster.
The theory proposed by Stanford's geophysicists Beroza, Hagai Ron and Amos Nur begins with visualizing Earth's surface as a net. Tug on a corner and all the points in the net move.
At a meeting of the American Geophysical Union in December, the scientists reported their findings.
Quakes occur when faults slip. In a single earthquake more than one fault may rupture, but not always along a straight line.
The Hector Mine quake near Twentynine Palms, Calif., ruptured within the U.S. Marine Corps Air Ground Combat Center, a sparsely populated area.
But at least six major faults cross California's Interstate 40 and new faults cut across old ones.
"It has been a misconception that faults in the Mojave could not produce such large earthquakes," said Ron, visiting professor of geophysics from Geophysical Institute of Israel and Hebrew University of Jerusalem.
To visualize the movement of a land mass, imagine removing the bookends from a row of books. The books will topple, just as faulted blocks of Earth's crust can rotate and deform. Like the books sliding to one side, crust blocks can stop slipping and lock up.
Old faults in the Mojave may be 5 million to 6 million years old. The new faults may be only 10,000 years old. While new faults need greater force to slip than old ones, both faults can slip when activity shifts from old to new faults.
Could a new fault stop old faults dead in their tracks?
"Ask us in 100,000 years," Beroza said.
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