IS L.A. ON SHAKY GROUND? FIVE LESSER-KNOWN FAULTS MIGHT BE UNDERESTIMATED.

Byline: Charles F. Bostwick Staff Writer

Using computer models compiled by the Southern California Earthquake Center, a pair of University of Massachusetts researchers warned on Tuesday that the seismic hazard along five lesser-known faults in the Los Angeles Basin might be underestimated.

Professor Michele Cooke and graduate student W. Ashley Griffith compared measurements of movement in the Southern California landmass to other researchers' theories about what the earth's movement does to seismic faults under Los Angeles.

``We know the San Andreas produces earthquakes. We need to see what's going on in the Los Angeles Basin, so we can see what the hazards are,'' Cooke said in a telephone interview from Amherst, Mass.

``What we have here isn't a hidden San Andreas or even a hidden Sierra Madre (Fault). But we have some some aspects we need to look at more closely because they could surprise us.''

The San Gabriel, Hollywood, Raymond, Peralta Hills and Chino faults - all apparently big enough to produce earthquakes of 5.0 to 6.0 magnitude - could be moving at up to 1 millimeter a year, according to the computer model that best fits the geologic measurements, Cooke said.

Other researchers have determined how quickly some of those faults are slipping horizontally, but not how quickly they are slipping vertically. The probability of an earthquake is generally proportional to the slip rate.

``There are a whole lot of faults for which we don't have all the measurements,'' Cooke said.

By comparison, the Northridge Fault that produced the deadly 1994 earthquake moves 1 to 2 millimeters a year, and the 800-mile-long San Andreas Fault about 30 to 40 millimeters a year.

The study, published in the August issue of the Bulletin of the Seismological Society of America, was a further step in seismologists' efforts to understand Southern California's peculiarly complicated three- dimensional underground jigsaw puzzle.

Beneath Los Angeles, a web of faults is moving in different directions and different rates, squeezed by the pressure of the Pacific tectonic plate pushing against the North American plate at the San Andreas Fault.

``It adds to our knowledge of the earthquake hazards in the Los Angeles area,'' Tom Jordan, director of the Southern California Earthquake Center, said of Cooke's and Griffith's work, which his center supported.

But U.S. Geological Survey seismologist Susan Hough said the researchers' conclusions about the five faults should not cause Angelenos any extra alarm, even if they live nearby.

The 6.7-magnitude Northridge Earthquake, which caused heavy damage as far away as Santa Monica, illustrated that damage doesn't occur only near a fault, Hough said. In addition, researchers have had a good idea for a decade or so of how much energy is being stored up as the earth's movement squeezes Southern California.

``That's telling you about how many earthquakes you are going to get on average, and that's not going to change,'' Hough said.

Of the researchers' overall conclusions, she said, ``This is a nice study because it's putting the faults together and being systematic.''

The study looked into a continuing debate among seismologists about what the earth's movement does to Los Angeles and its faults - whether the pressure is primarily making Earth's crust thicker, raising mountains and hills such as the San Gabriels and the Santa Monicas, or whether the crust is squirting out horizontally, like a watermelon seed squeezed between thumb and finger.

None of the existing theories was a perfect fit for the measurements that researchers have acquired about fault movements, the study found. The best fit was a model that describes the basin as compressing from north to south, with only a very small degree, if any, of east-west extension.

That means the crust is thickening, raising hills and mountains, increasing the chance of earthquakes that are taking up the compression.

``Now that we know more about the tectonic setting of the basin, we can proceed with investigating the finer details of the model,'' Cooke said.

Charles F. Bostwick, [phone]

chuck.bostwick(at)dailynews.com

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SOUTHERN CALIFORNIA'S EARTHQUAKE FAULTS

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