Public Affairs Office
Pat Jorgenson (650) 329-4000
For Release: April 5, 1985
A letter summarizing the results of the scientific review of the Parkfield forecast was sent to Mr. William Medugovich, Director the California Office of Emergency Services, by Dr. Dallas Peck, Director of the U.S. Geological Survey.
Parkfield has been the site of a USGS earthquake prediction experiment that is using sophisticated distance measuring devices and other monitoring equipment in an attempt to determine and monitor signals that might presage an earthquake.
The research that led to today's statement has been carried out by William H. Bakun and Allan G. Lindh of the U.S. Geologcial Survey and Thomas v. McEvilly of the University of California. Their conclusions are based on analyses of reports of earthquakes in the Parkfield area in 1857, 1881, and 1901 and seismograph records of events near Parkfield in 1922, 1934 and 1966. The average interval between these events is 22 years and statistical analyses indicate a high probability (over 90 percent) of another earthquake in the region within in the 1985-1993 interval. The seismograph records of the last three Parkfield earthquakes are very similar, leading to the hypothesis of a characteristic earthquake in the Parkfield region of about magnitude 6 on the Richter Scale.
Parkfield is along the San Andreas fault in a sparsely populated region about 170 miles south of San Francisco and 180 miles north of Los Angeles. An earthquake of magnitude 6 is of moderate size, at the threshold of being able to cause modest damage to some structures that have not been designed for earthquake resistance.
The last characteristic Parkfield earthquake occurred on June 28, 1966, registered a magnitude slightly less than 6, and caused only minor damage to wood-frame houses in the region.
The results of the Parkfield studies by Bakun, Lindh, and McEvilly have recently been reviewed and endorsed by the National Earthquake Prediction Evaluation Council and the California Earthquake Prediction Evaluation Council. These bodies advise federal and state officials respectively on the validity of statements and studies regarding the occurrence of future earthquakes. The national council concluded that the findings at Parkfield constitute a long-term prediction, a term adopted by both councils to describe a statement on the occurrence of an earthquake at a specific place and within a time interval of a few years to a few decades.
In their evaluation of research, the two prediction review panels said that the potential exists for the next earthquake in the Parkfield region to be larger than the 1966 shock, and for the fault rupture to extend southeast into the adjacent 25-miles segment of the San Andreas fault. Both panels agreed, however the evidence for this larger earthquake was speculative and required additional data and review.
Under a program of earthquake prediction research, the US Geological Survey maintains an array of sensitive geophysical monitoring instruments in the Parkfield region in an attempt to predict the occurrence of the expected earthquake more precisely. The California Division of Mines and Geology also maintains a large number of instruments to measure the effects of the earthquake.
The California Office of Emergency Services has reviewed the evaluation with local officials and will take coordinated action should extensive monitoring equipment arrayed throughout the Parkfield region indicate that the anticipated earthquake is imminent.