 In June 2004, the USGS in partnership with the National Science Foundation began drilling a deep hole to house instruments to monitor the fault at depth.This action was a part of the San Andreas Fault Observatory at Depth (SAFOD) program.Because of the regularity of large events (>5.5) at the Parkfield location (events in 1857, 1881, 1901, 1922, 1934, and 1966), and the fact that the waveforms from many of these events were almost identical, it was believed that the same segment of fault ruptured each time.