The explosions are usually placed in spots perpendicular to fault lines to give the best readings and to allow differences in the rock to be easily discerned, said Jim Mori of the U.S. Geological Survey in Pasadena.
The northeast route followed by the blasts will roughly parallel the San Gabriel River, with up to 10 explosions set off in the valley, Mori said.
Using ammonium nitrate to create gentle shock waves, scientists will measure wave travel speeds to reveal the underlying geologic structure--the geological equivalent of a human X-ray. The speed at which the waves bounce off underlying rock and travel back to the surface allows scientists to determine what sort of rock and soil lies underneath. Mathematically, Mori said, the information gives a detailed picture of the fault location and the geologic structure.
Located at Caltech for the past three years, a computerized system stores Southern California quake facts culled from 220 sites. Small sensors the size of a soup can, with accompanying electronic equipment, are piled in garbage can-size barrels. These barrels are scattered 10 to 15 miles apart to allow scientists to carpet the area and get overall readings for the entire region.
A handful of sites are located in the flats of the San Gabriel Valley; another 20 are in the San Gabriel Mountains, where the bedrock gives better readings, Mori said.
"We try to hide them," he said, adding that solar panels sometimes used to power the equipment are very often hit by bullets from target shooters or are pulled off by thieves.
Sensitive seismographs, capable of recording zero-magnitude quakes--that is, the very smallest quakes--are set up to collect data. After major shocks measuring magnitude 5 or better, additional portable seismographs are hustled out to even more sites.
The system, called a "jukebox" by some scientists, enables researchers to retrieve data via computer and avoid traveling to Pasadena to spend weeks reading through computer tapes.
Jones and her husband, Egill Hauksson of Caltech's seismology lab, are network experts. Together or with other scientists, the couple have written on most major quakes in the San Gabriel Valley.
Their data has helped reveal the geometric structure of faults in the San Gabriel Valley, the direction of slippage in earthquakes, the geometric and geologic fit of faults and the determination of fault systems. Such information can yield the discovery of potentially hazardous faults or future earthquakes.
Despite all this research activity, much is unknown.
"We don't know what makes them start or stop," Jones said. "Or why they happen today instead of tomorrow. We don't know what is the trigger."