Moderate-size earthquakes have occurred on the Parkfield section of the San Andreas fault at fairly regular intervals - in 1857, 1881, 1901, 1922, 1934 and 1966. While little is known about the first three shocks, available data suggest that all six earthquakes may have been "characteristic" in the sense that they occurred with some regularity (mean repetition time of about 22 year) and may have repeatedly ruptured the same area on the fault. (see Bakun and Lindh, 1985 and Bakun and McEvilly, 1979)
The similarity of waveforms recorded in the 1922, 1934 and 1966 events, shown below, is possible only if the ruptured area of the fault is virtually the same for all three events.
