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06/30/92

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Stanford team predicted new fault was building in the desert

STANFORD -- A team of Stanford geophysicists three years ago predicted a new fault system in the desert area east of Los Angeles and "anticipated" that it could be the site of a powerful earthquake.

Two destructive earthquakes roared through that area Sunday morning, June 28, killing a child and injuring hundreds of other people.

Now, said Amos Nur, professor of geophysics, scientists must greatly expand their model of the activity in the area and confront the notion that a new fault is forming that may compete with the well-known and deadly San Andreas.

The paper was published in the American Geophysical Union series of monographs in 1989 and generally ignored. In it, Nur and his colleagues suggested that the well-documented and geologically well-developed strike slip faults in the Mojave desert area have become "so unfavorably oriented relative to the directions of prevailing principal tectonic stresses that a new fault system must develop to accommodate future faulting."

They based their analysis on paleomagnetic and geological evidence suggesting that the old faults have rotated over the last few million years to their present orientation, at angles to the tectonic push in the area. This locks them in place, building pressure.

Further, Greg Beroza - then of the Massachusetts Institute of Technology, now at Stanford - found that some of the motion between the Pacific and North American tectonic plates, which gave form to most of the geology of California, had to be accommodated east of the San Andreas.

Nur and his colleagues found that two previous quakes in the area, one near Galway Lake in 1975 and another in Homestead Valley four years later, happened on faults that were either unknown or barely known. The two moderate quakes were on a single north-south line, hinting that a north-south fault existed. The previously identified faults in that region ran northwest-southeast.

Geophysicists began taking the paper seriously in April when another quake, hitting 6.8 on the Richter scale, struck near Joshua Tree, right on the same north-south line, Nur said.

The quake prompted the Stanford scientists to go back to earlier quakes in the century, where they found one near Manix (6.2 on the Richter scale) in 1947, also on the same line.

Nur and Beroza were completing the first draft of a new paper on the new fault when the two quakes hit the area Sunday morning. The first of the quakes, which measured 7.4, was exactly on the same line, running from Manix through Galway Lake, Homestead Valley and Joshua Tree.

(The second quake Sunday morning was further to the west, and Nur said it is not clear how it is connected to the first temblor.)

"This may suggest that not only are the old and local faults in the Mojave region being replaced by newer, better oriented ones, but that the newly emerging fault is competing with the San Andreas itself," Nur said.

"The San Andreas has a large kink in its shape in the Mojave- Transverse ranges of California, so that it is obliquely oriented to the slip of the North American and Pacific plates. And it can accommodate slip in this orientation only because it is exceedingly weak.

"The new Mojave fault, in contrast, is favorably oriented, but because it is new it may also require much higher shear stress to make it slip. It is possible that it is just becoming equally easy for the earth to form this new fault, rather than to cause slip on the oblique kink on the San Andreas."

Nur said it would be useful to continue the research into other past quakes in the area and into Nevada to draw the new fault line further.

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