On September 28, 2004 a magnitude 6.0 earthquake struck Central California near the town of Parkfield. The quake caused no injuries and minimal property damage, but was of great interest to American geologists. In 1984 the United States Geological Survey predicted that a Magnitude 6 earthquake would occur on the San Andreas fault near Parkfield within five years of 1988. The prediction was based on a sequence of 6 similar earthquakes that occured every 22 years (on average) from 1857 to 1966. Although the 2004 Parkfield earthquake occured over a decade later than predicted, its magnitude and behavior fulfilled the prediction. In anticipation of this earthquake, geologists placed a large and varied suite of instruments along the Parkfield segment of the San Andreas Fault.
This image shows the topography of Central California surrounding the earthquake epicenter. Color represents elevation and shading (illuminated from the upper left) indicates slope. The San Andreas Fault appears as the trough that extends from the upper-left hand to lower-right hand corner. To the northwest of the epicenter, the San Andreas continually slips, causing frequent minor tremblors. To the southeast of the epicenter the fault has been motionless since a magnitude 7.9 earthquake in 1857. Another major earthquake is expected when the San Andreas finally breaks loose.