Seismic Mapping May Jostle Californians

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SOUTHERN Californians may feel some bumps in the night next month when scientists set off underground charges to map the region's web of underground faults.

None of the explosions, from Seal Beach to the Mojave Desert, should be stronger than what you might feel at a construction site, says Gary Fuis, a United States Geological Survey (USGS) geophysicist and one of the project's principal investigators.

``One of the first things people ask us is, `Are you going to trigger earthquakes?' The answer is no. These blasts are similar or smaller in size than those used in construction, quarry blasts, [and] mine blasts,'' Mr. Fuis says.

The project, conducted by the USGS and the Southern California Earthquake Council, will produce a seismic image of the Los Angeles region down to the depths of damaging earthquakes - six to 20 miles below ground.

``We're trying to see what the subsurface is that produces quakes in the L.A. basin and work out the earthquake-producing machinery,'' Fuis says.

The final cross-sectional maps should be completed within three years.

To generate the images, scientists will set off ammonium nitrate charges as deep as 140 feet. They'll record sound waves that are emitted and study the signals' paths and travel times.

They hope that such information can help predict strong earthquakes.