Odds say big quake inevitable

0 Comments | Oakland Tribune, Apr 16, 2008 | by Betsy Mason

California has virtually no chance of escaping a major earthquake during the next three decades, according to scientists.

A new statewide forecast released Monday puts the probability at 99.7 percent for a quake of at least magnitude 6.7, the size of the 1994 Northridge quake, which killed 72, sometime before 2037.

"It's near certainty that we're going to have a potentially damaging earthquake somewhere in the region in the next 30 years," said geophysicist Ned Field of the U.S. Geological

Survey.

This is the first time a forecast has been put together for the entire state using the same methods so that risks for different parts of the state can be compared.

The study shows that Southern California's risk is slightly higher for a magnitude 6.7 or greater at 97 percent, compared with Northern California's 93 percent.

For even larger quakes of magnitude 7.5 or greater, the odds are tipped more toward Southern California at 37 percent versus 15 percent for Northern California.

The work is the result of a joint effort by the U.S. Geological Survey, the California Geological Survey and the Southern California Earthquake Center.

"In development of the building code, this new seismic hazard estimate will be used to ensure that the buildings and infrastructure are designed to consider the appropriate level of ground motion," said engineering geologist Chris Wills of the California Geological Survey.

The new forecast will also be used as a basis for seismic hazard maps that CGS produces to designate areas at risk of landslides and liquefaction in an earthquake. Any new development in these areas must get a licensed geologist to evaluate the hazard and mitigate the risk in the construction.

"Two faults that we are particularly concerned about with respect to high probabilities are the southern San Andreas and the Hayward- Rogers Creek," Field said. "Both have elevated probabilities relative to their long-term averages."

The northern San Andreas fault relieved a lot of stress on Bay Area faults during the 1906 earthquake. The stress has