The next significant earthquake along California's San Andreas fault is most likely to occur in 1988 and to center near Parkfield, which is roughly halfway between Los Angeles and San Francisco.
This is the latest forecast of one of the boldest earthquake prediction experiments yet attempted. It emerged from an effort by the Interior Department's Geological Survey to monitor subterranean activity along the fault.
The fault is the joint where two large plates of Earth's crust meet edge to edge. The rock mass on the west side is moving slowly northward, rubbing against the rock on the east side. The west side moves at a rate of about 1 1/2 inches a year over much of the fault line except for a 15-mile stretch near Parkfield, where it appears to be stuck, or locked.
As geologists understand the situation, each year of northward creep adds pressure on the locked segment until, when the strain becomes great enough, the resistance is overcome and the west side suddenly lurches northward several feet, catching up with the moving part of the rock and then locking again.
The Parkfield cycle of lurch and lock, geologists have found, has occurred about every 22 years since records were first made in 1857 and all have been of about the same intensity, measuring around magnitude 6. This qualifies as a moderate earthquake, causing some damage. The last Parkfield earthquake was in 1966, so geologists W.H. Bakun and A.G. Lindh figure the next would be most likely to occur in 1988.
Large arrays of tremor-detecting and other devices are in place around Parkfield, recording all the hundreds of minor vibrations that accompany normal fault creep. The goal is to find patterns of events to indicate that strain along the locked segment is approaching the breaking point.