BLASTS TO HELP GAUGE VALLEY'S QUAKE RISK.

Byline: Jesse Hiestand Daily News Staff Writer

Over several nights this October, scientists plan to set off dozens of explosions under the San Fernando Valley in their search for hidden earthquake faults.

The detonations also will rock the earth along a 60-mile line that stretches from Malibu to Reseda, the epicenter of the Northridge Earthquake, and out to the Mojave Desert.

Scientists said the blasts will not knock loose an earthquake or damage homes and property. Rather, they will help map the basin beneath the Valley, develop better building codes and provide clues about what to expect when an earthquake does eventually hit.

``If we can see how deep the basin is, we can say that homes and buildings in the area should be given more consideration in terms of their seismic retrofitting,'' said Mark Benthien, outreach specialist at the earthquake center, based at the University of Southern California.

``That translates directly into saving lives and reducing the economic loss,'' he added.

Scientists from the Southern California Earthquake Center and the U.S. Geological Survey said the explosives will range from as little as 20 pounds in the city to 2,000 pounds in uninhabited desert areas.

``What we're doing is very similar to an ultrasound, where sound waves are passed through the abdomen to create an image of a baby,'' Benthien said.

By mapping the shape and thickness of the sedimentary basin under the Valley, the Los Angeles Region Seismic Experiment will help show where quakes can hit and help better gauge the resulting shaking.

``The ultimate goal is to affect the building codes,'' said Robert Clayton, a California Institute of Technology geophysicist and one of the chief scientists on the project.

Finding faults

The blasts will be done to occur at least 60 feet below ground in fields, undeveloped properties and possibly some parks or schools.

The resulting force will be less than a 1.0-magnitude temblor and feel like a passing garbage truck, Benthien said.

Scientists are negotiating with government agencies and business and property owners to get permission to do the experiment on their land, said Gary Fuis, a USGS geophysicist.

Since scientists do not expect the explosions to do damage, they have no plans to specially warn residents who live near the blast sites.

Ideally, there would be one explosion every kilometer, or 0.62 miles, organizers said, but that cannot be done in the Valley, owing to the need to find open spaces far enough away from homes to avoid a hazard or nuisance.

Benthien and others are negotiating with landowners like Douglas Gross, a Los Angeles resident who own 520 acres just north of Lake Hughes. Gross agreed to let the scientists set off a charge on his property, as the nearest house is more than a half-mile away.

``We live in earthquake territory, and the more information we have about that protects all of us,'' Gross said.

The explosions will take place in eight-inch boring holes that will be dug this summer. The charges, a slurry of nitrate-based explosives, will be poured in and covered with dirt to the surface so the force stays contained below ground.

Twenty or so of the charges will be set off each night between about 1 a.m. and 5 a.m., meaning the 100 charges will be set off over four to six nights.

``If you were to set off one of these charges midday, you wouldn't see the vibrations because they would be hidden by all the other vibrations caused by traffic, construction, even jets landing around Southern California,'' Benthien said.

According to scientists, the experiment will help tally the faults in the Valley, where they are or if they extend from the underlying bowl-shaped basement rock to the sediment that has gathered atop it over millions of years.

Mapping the Valley

A fault previously unknown to scientists caused the 6.7-magnitude Northridge Earthquake on Jan. 17, 1994, which killed 57 people and left $20 billion in damage.

This experiment will also be used to map the shape of the basin under the Valley, which is analogous to a bowl of jelly in which a quake's seismic waves get trapped and bounce around.

Both the shape and depth of the basin affect how the surface moves, with deeper basins being prone to greater shaking.

Preliminary studies show the basin is just over a half-mile deep under Van Nuys and gradually gets deeper to the north until there is a sharp drop-off, perhaps 5 miles deep, north of Sylmar, Fuis said.

``The San Fernando Valley is very complicated in its structure, and it's going to take some (time) to analyze it,'' Fuis said.

It may take up to five years for scientists to completely analyze this data.

The experiment will also map the depth of the Ventura basin and look areas under Malibu, the Mojave Desert and a cross-section of the San Andreas fault.

A similar series of blasts were used in 1994 to map the basins under both the Los Angeles Basin and San Gabriel Valley.

By setting off detonations in a line from Seal Beach to Barstow, scientists discovered that the San Gabriel Valley basin was 3 miles deep, a mile deeper than they had suspected. That is why they believe the 1992 Landers quake north of Palm Springs had such an effect on that area.

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Map

MAP: (Color) San Fernando Valley

Boreholes are approximately every six-tenths of a mile along the line

Traci Wooden/Daily News