When, in 1966, an earthquake of magnitude 6 occurred on the San Andreas Fault near the town of Parkfield, in central California, it caused excitement among seismologists for two reasons. First, a number of new seismic stations had just been installed near the epicenter and had recorded the quake; and second, many residents and seismologists could recall similar earthquakes which had happened there in 1922 and 1934, and had been recorded at regional seismograph stations. With the data from the new stations, seismologists were able to determine that the 1966 event had started to the NW of the town of Parkfield and had ruptured the fault to the SE. Bakun and McEvilly (1979, 1984) compared seismograms from the 1922, 1934 and 1966 Parkfield earthquakes. They found similarities in waveforms of the 1922 and 1934 events as recorded by the Bosch-Omori seismograph at the seismograph station at the University of California, Berkeley (BRK), 265 km NW of the epicenter. For the 1934 and 1966 events, they compared and found similarities in records from Wood-Anderson seismographs at Mount Hamilton (MHC), 185 km NW of the event, and at Santa Barbara (SBC), 185 km to its SE. In addition, they looked at recordings of the surface waves for all three events from the Galitzin seismograph in the Netherlands at DBN, almost 10,000 km away. This was one of the few stations world-wide which had good recordings from the same instrument for all three earthquakes. Thus, Bakun and McEvilly (1979, 1984) suggested that these events could be members of a sequence of characteristic earthquakes. In 1985, based in part on the waveform similarities for these events, Bakun and Lindh (1985) proposed that another, similar earthquake might again happen in the same place, perhaps around 1988. As part of a prediction experiment (Bakun and Lindh, 1985), many instruments were installed around the town of Parkfield to record such a quake and to determine whether any precursors might be observed.
Berkeley Seismological Laboratory
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