For decades, Californians have lived with the knowledge that the San Andreas Fault could produce a magnitude eight earthquake. However, California's most recent earthquake, in early 1994, measuring 6.6 on the moment-magnitude scale--a measure of earthquake energy that has largely replaced the Richter scale--did not come from the San Andreas Fault (Nash 45). It was caused by a smaller fault in the San Fernando Valley that lay hidden deep underground, some nine miles down. While this was not considered the Big One (Nash 45) it still caused widespread damage: collapsed buildings and freeway bridges, fires, deaths and injuries, utility outages, and damage to property.
Soon after the first tremor, seismologists began trying to map out the newly revealed fault to determine how it is connected to other fissures in the region. To complete this task, the locations of hundreds of aftershocks will have to be tracked, which is a lengthy and tedious process. At first it was thought that the quake might have resulted from a previously unmapped extension of the Oak Ridge Fault, which angles past the city of Ventura and into the Pacific Ocean. However, as researchers explored the San Fernando Valley--the area hit hardest by the quake--other theories emerged, including the possibility that the fault was not connected to any known system. Individually, the faults that are not connected to the San Andreas are smaller and give rise to earthquakes that are less frequent and less severe, but collectively they represent a large hazard because they are so numerous (Nash 45).
California is an earthquake zone because it lies on the boundary, marked by the San Andreas Fault, between two huge sections of the earth's crust, known as plates. The Pacific plate--to which Los Angeles is attached--is slowly pushing its way north and west, past the North American plate to the east, which is moving in the opposite direction. M...