In April 1985, the U.S. Geological Survey issued a prediction that a magnitude 6 (M6) earthquake would occur before 1993 near Parkfield, Calif. (Figure 1). That September, the USGS and the California Division of Mines and Geology (CDMG) began developing the Parkfield Earthquake Prediction Experiment, which has become the most densely and comprehensively instrumented earthquake source region in the world. The experiment has two goals: a detailed understanding of the geologic processes that precede the anticipated earthquake and a public warning, if possible, shortly before the earthquake occurs. Additionally, since there is a high likelihood of a moderate-sized earthquake within a small area, several engineering experiments have been installed to test the performance of man-made structures.