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The following item just came in via a news tracking service I subscribe to.
Interesting reading for all who follow earthquake policymaking in California,
and the impact that recurrence intervals have on such decisions.

Frank Lew, SE
Orinda, CA
================================================

Tuesday March 17 10:43 AM EST 

Earthquake rules change
UPI Science News 
PARKFIELD, Calif., March 17 (UPI) _ The long ``overdue'' shaker in the
earthquake capital of the world is not much more than a myth. 

Not only that, earthquakes follow a far less structured schedule than they're
given credit for and do not necessarily increase in likelihood with the
passage of time. 

Those unorthodox conclusions are drawn by a University of California, Los
Angeles, seismologist from data gathered by the myriad earthquake- monitoring
instruments set up in Parkfield, Calif., a decade ago. This tiny town's claim
to fame is its location on the notorious San Andreas fault and what has been
thought of as a pretty regular earth-shaking pattern. 

In fact, asserts David Jackson, professor of earth and space sciences at UCLA,
the treasure-trove of data coming out of Parkfield strongly indicates the
long-held belief that magnitude 6 shakers jolt the area just about every 22
years is not much more than a myth. 

Says Jackson, also sciencie director of the Southern California Earthquake
Center: ``Earthquakes are not like comets that can be reliably predicted to
return. We have been warned for more than 10 years that an earthquake is
coming to Parkfield any time now. I'm not holding my breath. I would not be
surprised if Parkfield goes another 20 or 30 years _ or even longer _ without
another magnitude 6 earthquake.'' 

Jackson's view is bound to shake up the scientific community, which has a near
consensus that the probability of another quake measuring 6 on the Richter
scale in Parkfield is as high as 10 percent each year. Jackson puts the odds
at less than 1 percent. 

The view favored by the vast majority is based on data _ sketchy though some
of it may be _ that indicate Parkville has been jolted by magnitude 6 shakers
at pretty regular intervals over the past 140 years. Since the town last
rattled and rolled to that tune in 1966, it is commonly thought the time is
more than ripe for an encore. 

Jackson, however, points out the earthquake data prior to 1922 are sadly
lacking. In his scientific opinion, ``I think in Parkfield, people have found
an apparent pattern in random data. It's easy to imagine patterns in
retrospect.'' 

To him the ``pattern'' is nothing more than coincidence. 

He also questions the assumption that earthquake probability increases with
time. 

As he puts it, ``Scientists have not been able to establish that, and while
some earthquakes are said to be 'overdue,' many others occur where they are
'way underdue.''' 

Jackson disputes the notion that Parkfield, and the entire state of California
for that matter, face an ``earthquake deficit'' that must be made up in the
coming decades with more large shakers. 

The earth may decide to do just one big bang-up job with a magnitude 8 jolt in
place of more frequently scheduled magnitude 6 or 7 temblors. In Jackson's
view, it may be centuries before the really big one hits. 

The assumed Parkfield predicament of a magnitude 6 shaker every 22 years is
based on two assumptions _ both of them unproven, according to Jackson. One is
that a large fault such as the San Andreas can be divided into independent
segments, each with its own magnitude and interval between earthquakes. The
second is that those earthquakes will follow a relatively regular routine. 

Says Jackson: ``These ideas seem reasonable, but none has been validated by
any objective test. They just don't seem to work.'' 

He points out the next quake was already overdue in 1989 when the Parkfield
monitoring system was being instigated. 

Now, Jackson says, ``it's so far overdue that instead of saying it is
imminent, we should reflect on whether the whole idea of the Parkfield
earthquake is valid. There will certainly be earthquakes on the San Andreas
and some future earthquake will visit Parkfield, but it won't be a copy of the
last one. Furthermore, I don't believe Parkfield is the most likely place in
California to have an earthquake in the next decade or the next year.'' 

While Jackson advocates earthquake preparedness, he points out that more
Californians are killed in car accidents each year than all Americans who have
ever lost their lives to an earthquake. 

Jackson made his presentation at the annual meeting of the Seismological
Society of America in Boulder, Colo. 

(Written by UPI Science Writer Lidia Wasowicz in San Francisco) _- 

Copyright 1998 by United Press International. 

All rights reserved.