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- To: seaoc(--nospam--at)seaoc.org
- Subject: Shift in the seismic paradigm?
- From: FLew98 <FLew98(--nospam--at)aol.com>
- Date: Tue, 17 Mar 1998 14:00:18 EST
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.
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