ost of the time the two plates block each other's movement and tremendous pressure builds. Frequently, one plate will move more than the other and the plates will slide past each other, causing the ground to shift--producing an earthquake.
A similar process goes on along smaller cracks in the crust outside the main fault line. While the earth slides horizontally along the San Andreas, many of the other fissures, including the one under Northridge, the epicenter of the last Californian quake and located within the San Fernando Valley, are called thrust faults because they cause the ground to move vertically. Given enough time, they help form mountains and valleys.
Surprisingly, these smaller faults were overlooked until 1983, when a intense tremor hit the small central California town of Coalinga. The fault turned out to be a deeply buried fault (four to 10 miles down) that had been undiscovered. Its only sign on the surface had been a fold, or buckling, in the earth's crust. These folds, formed by an imperceptibly gradual lifting of the ground, were thought to be harmless (Nash 46). However, when the seismic record of fold belts around the world were examined, it was found that folds grow through repeated earthquakes.
Other researchers have begun examining maps drawn up by petroleum geologists. Thrust faults, and the folds they form, are excellent traps for gas and oil, and many such subterranean spots have been found in the Los Angeles region. Since 1987, when the Whittier Narrows earthquake occurred, about six quakes of significant size have occurred along thrust faults beneath greater Los Angeles. All this activity may be a symptom that overall tectonic pressure in the region is increasing. Although temblors on secondary faults relieve stress locall
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