The average interval between magnitude 6 earthquakes at Parkfield, California, is 22 years. The town lies along the San Andreas Fault, where a sequence of six roughly magnitude 6 earthquakes occurred between 1857 and 1966.
Those earthquakes were separated by intervals ranging from 12 to 32 years, producing an unusually regular pattern for earthquakes. That apparent regularity led scientists to study Parkfield as a natural laboratory for earthquake prediction.
In 1985, the United States Geological Survey predicted that another comparable earthquake would occur by 1993. It eventually struck on September 28, 2004, with a magnitude of about 6.0—11 years later than predicted. The event supplied valuable data, but it also showed why earthquake timing remains difficult to forecast.
The 22-year figure describes a historical average, not a dependable schedule. Earthquakes do not follow a metronome, and the 2004 event’s delay is a useful reminder that statistical patterns cannot guarantee an exact date.