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In an effort to ferret out unmapped faults like the one responsible for the deadly 1971 Sylmar, Calif., earthquake, scientists plan to set off a series of underground explosions between the Mojave Desert and Seal Beach.

The explosives, to be detonated during two nights in October, also will help government and university scientists discover which areas of the Los Angeles Basin would shake the hardest during an earthquake, possibly leading to special building codes for certain sections of Southern California."Although we'll be placing some of these charges near faults, there is no need for alarm - it won't trigger an earthquake," said Gary Fuis, a researcher with the U.S. Geological Survey, which is heading the project.

"Earthquakes start about three to five miles below the surface. We'll be well away from the region of earth where Mother Nature generates earthquakes," he said.

The blasts will be produced by 250 to 4,000 pounds of ammonium nitrate-based explosives buried 50 to 180 feet deep. They will generate vibrations equal to the level of a 2.5-magnitude earthquake but will not be felt by most people, officials said.

"On any given day in California, we have about 10 earthquakes that measure about 2 or 2.5 magnitude," Fuis said.

The $600,000 project, which has not met any organized opposition, is the first of its kind in Southern California, although the Geological Survey has conducted similar tests in recent years in the San Francisco Bay Area and northern Alaska.

"Actually, the oil and gas industry has been doing this for decades to find oil deposits," said Tom Heaton, a Geological Survey seismologist in Pasadena, Calif. "Also, the blasts they do in mining operations are much bigger than what we will do. They measure those in tons of explosives."

In the project, the explosives will be detonated every two minutes, giving off vibrations or "P waves" that will be recorded by about 600 seismographs, most of which will be placed along the imaginary line between Harper Lake (about 20 miles northwest of Barstow, Calif.) and Seal Beach (south of Los Angeles), said Fuis.

Primary, or "P" waves, change speed as they pass through different types of rocks, which will give scientists a better idea of the Earth's composition, officials said.