Along California's highly active San Andreas fault, experience has shown that smaller earthquakes can increase the probability of subsequent larger ones in the hours that follow, with the probability returning to normal after about three days. But in the Northwest, scientists haven't gathered enough information on fault activity to make such predictions.
The 1994 Northridge earthquake touched off a surge of interest in emergency preparedness. But today, given human nature, little remains but good intentions.
Discovery of a Hidden Fault in March of 1999 means that L.A. is at an even greater risk of major earthquakes than earlier thought. The fault has been named the Puente Hills Thrust. From downtown Los Angeles, the dangerous subterranean flaw appears to stretch 25 miles east and southeast to the Coyote Hills of suburban orange County, in three segments ranging in depth from 3 to 10 miles. The fault is being blamed for the Whittier narrows earthquake of 1987, which had a magnitude of 6.0, caused 8 deaths and $358 million in property damage. Judging from the estimated yearly slippage of rocks along the fault, any one of its three segments could produce an earthquake of at least 6.7 every 250-1000 years. If all three segments were to break at the same time, the resulting quake could reach the enormous intensity of 7.0 . An eathquake of that magnitude would release 30 times the energy of a 6.0 quake. This fault is probably one of the major players beneath the Los Angeles metropolitan region.
