From: clarinews@clarinet.com (LIDIA WASOWICZ, UPI Science Writer)
Newsgroups: clari.tw.science,clari.news.disaster,clari.local.sfbay,clari.local.california,clari.news.top
Subject: Quake study warns time running out for southern California
Keywords: research, science, earthquakes, trouble
X-Supersedes: <ca-quakeU3J73pp@clarinet.com>
Date: Thu, 7 Jan 93 15:08:03 PST
ACategory: usa
Slugword: quake
Priority: major
Format: regular
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Note: (adv 6 pm est)

	SAN FRANCISCO (UPI) -- The next major earthquake could jolt 10 million
southern California residents much sooner than has been anticipated,
researchers projected Friday.
	A four-year study found evidence that large temblors -- measuring
between 7.2 and 7.8 on the Richter scale -- rocked the notorious San
Andreas fault at Wrightwood, 40 miles northeast of Los Angeles, in 1857,
1812, 1690, 1610 and 1450.
	``Evidence for five large earthquakes during the past five centuries
along the San Andreas fault zone...northeast of Los Angeles...indicates
that the average recurrence interval...are significantly smaller than
previously thought,'' the scientists wrote in the journal Science.
	Since this segment last trembled in a big way in 1812 and absorbed
additional stress from the June 1992 blaze of shakers that climaxed in a
7.5 main shock in the Landers area, about 100 miles northeast of Los
Angeles, ``it appears to be at just the right point for the next large
earthquake,'' said David Schwartz, a geologist for the U.S. Geological
Survey in Menlo Park, and one of the reseachers who conducted the study.
	``By excavating trenches up to 18 feet deep across the fault to look
for the signature of past earthquakes, we have been able to date the
last five big ones,'' Schwartz said in an interview.
	``Our data show the average repeat time of 100 years for large
earthquakes on this San Bernardino section of the San Andreas fault --
versus the 135-year interval now commonly used as a guide in
predictions.''
	The fault section that bore the last big shaker 180 years ago ``seems
to produce 4.5 meters of movement when it slips and moves,'' said
Schwartz. ``Since 1812 that segment has accumulated 4.5 meters of slip,
so it is essentially at the point where we think it fails.'' A meter is
equivalent to just over 1 yard.
	This interpretation contradicts a report presented last month by
University of California, Santa Cruz, scientists at the fall meeting of
the American Geophysical Union in San Francisco. That study used a
computer model of the San Andreas fault to suggest earthquakes occur far
less regularly than currently assumed.
	In support of their conclusions, the Science authors wrote: ``The
recurrence times of large earthquakes are most accurately determined
from historical records and geologic site investigations of
paleoearthquakes.
	``These new observations emphasize the need to re-evaluate
probabilistic estimates of earthquake occurrence along the southern San
Andreas fault,'' they said.
	In the next six months, the researchers plan to utilize their data to
re-examine earthquake hazards for southern California.
	``Some of these probabilities will be revised upward, but exactly by
how much will need to await analysis,'' Schwartz said.
	The study was conducted by Schwartz and Thomas Fumal, of the USGS,
and University of Oregon geologists Silvio Pezzopane and Ray Weldon.

****