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Hayward Fault quake called less likely

Keay Davidson
  Aug. 18, 2000

Scientists' paper revises downward chances of temblor on northernsegment

The northern Hayward Fault is less likely to experience a major quake in the next three decades than previously thought, scientists said, reaffirming a similar conclusion issued in late 1999.

Between now and 2030, there's only a 16 percent chance - less than one in six - of another major quake on the northern segment of the Hayward Fault in the East Bay, they say in an article published in Friday's issue of Science


By major quake, they mean one that's at least as bad as the 1994 Northridge quake in Southern California. That temblor killed 57 people and caused $20 billion in damage.

That upholds a similar estimate in late 1999 by the U.S. Geological Survey and other Bay Area geoscientists. Back then, they issued a so-called "probabilities" report estimating the likelihood of a 6.7-magnitude quake in the Bay Area by the year 2030. That report estimated a 16 percent chance of such a quake on the northern segment of the Hayward Fault, which runs from Berkeley to San Pablo Bay.

A decade earlier, the prospects looked somewhat worse for the northern Hayward Fault. Back then, a 28 percent estimate was issued in a 1990 USGS report.

The Science article doesn't report fundamental new information, cautioned seismologist Bill Ellsworth of the USGS office in Menlo Park. The article mainly reports the same research data used in the 1999 forecast, albeit in more refined fashion following "peer review" by outside experts, he says.

"It's bringing into print some of the information that was utilized by the 'Working Group '99,' " Ellsworth said in a phone interview.

In any case, lay people shouldn't take too literally the 12-point difference between the 1990 and 1999 estimates. That's because the two forecasts, issued almost a decade apart, were based on significantly different data sets and assumptions.

For example, the 1990 report estimated the probability of another quake like Loma Prieta, which occurred in 1989 and measured about 7.0 magnitude. By contrast, the 1999 report used as its model a weaker quake, the 6.7-magnitude Northridge shake of 1994.

Even so, "both of those (the 6.7 and 7.0 quakes) are very damaging earthquakes," said Bob Nadeau, one of the co-authors of the Science article. The lead author was UC-Berkeley geophysicist Roland Burgmann. CQ

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