Chance of Major Quake Within 30 Years Put at 86%

Scientists taking a more thorough look at the earthquake risks posed by the San Andreas and other large faults in Southern California sharply revised their earlier predictions and said Friday that there is an 86% chance of a very large earthquake in the region during the next 30 years.

A broad crescent from Santa Barbara through San Bernardino--where millions of people live--also faces an unusually high probability of ground shaking strong enough to jar china off shelves and begin to cause some structural damage, the scientists said.

The forecast of increased chances for an earthquake of magnitude 7 or stronger before 2024 is based on researchers’ new analysis of what they call the “horribly complex web of faults” underlying Southern California. There have been at least three earthquakes of that size in California since 1989.

The scientists’ work, made public a week after other researchers reported that the Los Angeles area faces a quake risk higher than previously believed, is the first comprehensive evaluation of Southern California’s largest fault systems since 1988. At that time, the likelihood of a large quake on the San Andreas and other regional faults was thought to be about 60%, about the same as the risk the San Francisco Bay Area faces from a powerful temblor.

“We are going out on a limb,” said UCLA earthquake expert David D. Jackson, who led the team of 14 researchers from the U.S. Geological Survey, USC, Caltech, Stanford, UC San Diego and UC Santa Cruz. “But we are convinced the logic holds water.”

The study, commissioned in the aftermath of the magnitude 7.3 Landers earthquake of 1992, was presented Friday at a conference held in Universal City by the state Office of Emergency Services and the Federal Emergency Management Agency to consider the lessons of last year’s 6.7 Northridge earthquake.

With more data, better scientific tools and the recent experience of several large earthquakes in the region, the scientists refined and broadened their hazard assessments for an area reaching from the Monterey County community of Parkfield in the north to El Centro in the south.

In assessing the potential effects of earthquakes, the geologists, seismologists and geophysicists determined that areas centered on Parkfield, Palmdale, Ventura and San Bernardino face the highest probability--at least 60%--of experiencing strong ground shaking Quake Losses

* Times on Demand has prepared a compilation from the Money Talk column of the most-asked questions and answers on how taxpayers should treat earthquake casualty losses, insurance payments and FEMA grants on their 1994 return. To order, call 808-8463, press *8630 and select option 1. Order Item No. 2823. $4, plus 50 cents delivery. Also, a complete guide to earthquake preparedness by the state Office of Emergency Services is available on the TimesLink on-line service. Sign on and “jump” to keyword “earthquakes.”

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during the next 30 years. The level of shaking was estimated at 20% of the force of gravity, far less than was recorded at the center of the Northridge earthquake.

Along the mountain ranges that run east-west between Santa Barbara and San Bernardino, the probability of strong ground shaking is about 50%, the scientists said. The probability is about 30% at Los Angeles City Hall. There is no place in Southern California entirely immune from some risk of the effects of an strong earthquake, they noted.

As rescue teams still searched for survivors of the Kobe earthquake in Japan earlier this week--with the news dominated by images of the terrible damage that can be caused when the earth trembles--scientists and state officials on Friday tried to downplay the need for any alarm over the state’s newest earthquake forecast.

“We need to be careful of overstating the risk, overstating our certainty, of overstating the cataclysmic damage that might result,” said Richard Andrews, executive director of the state Office of Emergency Services.

“From my perspective, I don’t think we know what is in store for us in terms of specific scenarios and specific impacts,” he said. “There is a tendency to believe that every new scientific forecast is more ominous and threatening than before.”

California state geologist James Davis, however, emphasized that “one of the overarching implications of the study is the vulnerability of the area--the possibility of a very large earthquake in the fraction of a human lifetime. There is no way we can overemphasize that.”

The study is the most recent effort to determine just how Southern California juggles the tremendous energy created by the jostling between immense pieces of earth’s fragmented crust.

The titanic force--enough to compress the entire Los Angeles area by half an inch every year--is generated by the collision of the Pacific Plate and the North American Plate, which press on the region like fingers squeezing a watermelon seed. When the pressure outweighs the friction and rocks suddenly slip, the ground can jerk unexpectedly.

It is how mountains are born and natural disasters triggered.

Scientist base their estimate of the region’s earthquake risk on satellite measurements and calculations that show there is more seismic stress built up in the rocks under Southern California than the known earthquakes could have vented in recent centuries.

Two years in preparation, the study for the first time incorporates local geology, quake records and satellite tracking of the tectonic energy building up in the region to gauge the hazards for all the region’s big faults. The scientists based their analysis on a detailed study of 65 distinct earthquake zones in the region.

They reduced by half their estimates of the quake probabilities for some faults that have not ruptured recently, including the Coachella Valley segment of the San Andreas Fault and the Anza segment of the San Jacinto Fault, while slightly increasing the predicted risk on other segments.

“What we find is that we have a deficit of earthquakes in all Southern California, not just in the Los Angeles Basin,” said Thomas L. Henyey of the Southern California Earthquake Center.

“It doesn’t have to be made up next year or in the next 10 years or even in the next 100 years, but it has to be made up sometime,” he said.

The researchers’ conclusions will eventually be incorporated into revisions of zoning regulations, building codes and seismic hazard maps.

State planners said the better understanding of seismic risk could lead to some changes in codes governing structures that were most vulnerable to the Northridge quake. Those include parking garages, structures with open first stories and steel-frame buildings.

Quake Losses

Times on Demand has prepared a compilation from the Money Talk column of the most-asked questions and answers on how taxpayers should treat earthquake casualty losses, insurance payments and FEMA grants on their 1994 return. To order, call 808-8463, press *8630 and select option 1. Order Item No. 2823. $4, plus 50 cents delivery. Also, a complete guide to earthquake preparedness by the state Office of Emergency Services is available on the TimesLink on-line service. Sign on and “jump” to keyword “earthquakes.”

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Shakey Ground

If you live virtually anywhere but the remote reaches of the Mojave Desert, you have at least a one in four chance in the next 30 years of experiencing earthquake movement strong enough to knock plates off shelves, or possibly require some precautions. Residents in some areas--notably San Bernardino, Palmdale or parts of Ventura County--have a much higher probability of feeling strong shaking.

SOURCE: Working Group on the Probabilities of Future Large Earthquakes in Southern California

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What Are the Chances?

Earthquake experts have released new forecasts of the probability of a quake on three major Southern California faults in the next 30 years. This chart shows the largest possible quake each segment could generate, the chance that the fault could generate a quake during the next 30 years and the prediction’s margin of error. The original probabilities calculated in 1988 are also listed.

Fault/Segment Max Prob Error Prob Mag 1994 Margin 1988 San Andreas-Carrizo 7.51 18 9 10 San Andreas-Mojave 7.56 26 11 30 San Andreas-San Bern. Mtns. 7.28 28 13 20 San Andreas-Coachella Valley 7.48 22 12 40 San Jacinto-San Bern. Valley 6.87 37 17 20 San Jacinto-San Jacinto Valley 6.96 43 18 10 San Jacinto-Anza 7.36 17 12 30 San Jacinto-Coyote Creek 6.94 18 13 NA San Jacinto-Borrego Mountain 6.77 6 8 10 San Jacinto-Superstition Mtns. 6.65 9 6 NA San Jacinto-Superstition Hills 6.63 2 6 NA Whittier-Whittier 6.91 5 3 NA Whittier-Glen Ivy 6.87 12 15 NA Whittier-Temecula 6.96 16 10 NA Whittier-Julian 7.26 5 5 NA Whittier-Coyote Mountain 6.91 1 4 NA

SOURCE: Working Group on Probabilities of Future Large Earthquakes in Southern California