Answer
The SAFOD borehole has been drilled through the San Andreas Fault near the town of Parkfield, California. The 800-mile long San Andreas Fault zone is where the North American Plate and the Pacific Plate slide past each other. Along most of the San Andreas Fault, the two plats are locked together, creating large earthquakes when building stress causes the plates to lurch past each other. However, the plates in the region near Parkfield (and SAFOD) creep slowly past one another, generating many small earthquakes.