Chronological Earthquake Index
TYPE OF FAULTING: right-lateral strike-slip
TIME: June 27, 1966 / 9:26 pm PDT
LOCATION: 35° 56' N, 120° 31' W 10 km (6 miles) NW of the the town of Parkfield
FAULT RUPTURED: San Andreas fault
DEPTH: 8.6 km
LENGTH OF RUPTURE: about 27 km (17 miles)
MAXIMUM SURFACE OFFSET: about 10 cm (4 inches)
Despite being a fairly strong earthquake, the 1966 Parkfield quake caused very little damage, even near the epicenter. This is mostly due to the epicentral region being sparsely inhabited.
What is more significant about the 1966 Parkfield earthquake is that the USGS used it to suggest a 21-to-22-year periodicity of magnitude 6 earthquakes at Parkfield. The previous five magnitude 6 events at Parkfield were in 1857 (uncertain, but likely), 1881, 1901, 1922, and 1934. In April 1985, the USGS issued a prediction that an earthquake of approximately magnitude 6 would occur before 1993 on the San Andreas fault near Parkfield, which gained this little town of 34 inhabitants national fame as being the site of the first officially recognized scientific prediction of an earthquake in the United States.
Since 1966, there has been no earthquake greater than 5.0 at Parkfield. The year 1993 came and went with no magnitude 6 earthquakes in the area, though Parkfield did experience a magnitude 4.7 quake on October 20, 1992, a magnitude 4.4 on April 4, 1993, a magnitude 5.0 on November 14, 1993, and a magnitude 4.9 on December 20, 1994.
It has been suggested that the 1983 Coalinga earthquake (magnitude 6.4, located outside of the clickable map area), 40 km northeast of Parkfield, could have altered the stress of the San Andreas fault enough at Parkfield to throw off the timing of the next magnitude 6 shock, or maybe "defuse" it somewhat, causing less energy to be released at Parkfield. It may also be that the recurrence interval at Parkfield is lengthening over time, somehow. When the next magnitude 6 quake is expected at Parkfield, no one can be sure, but it seems certain that it will be not far in the future, geologically speaking.
For a more detailed look at the Parkfield Earthquake Prediction Experiment, see Volume 20, Number 2, of Earthquakes and Volcanoes, 1988.