The specific location of PFO was chosen because it was near active faults and because the the instruments required a large flat area not covered by alluvium. The flat area (Pinyon Flat) gave the name to the observatory. The surface material is decomposed granite, grading with depth into unweathered Mesozoic granodiorite. Work at PFO began in 1970, at which time the land was owned by the U.S. Forest Service; it was purchased by the University in 1980 with generous support from Cecil and Ida Green. Operation of the observatory is currently supported by the U.S. Geological Survey, the Southern California Earthquake Center (SCEC), and the Scripps Institution of Oceanography (SIO).