Main shocks of the earthquake sequences that occurred on the Parkfield section of the San Andreas fault in central California in 1922, 1934, and 1966 are characterized by southeast rupture expansion over the same 20- to 30-km-long section of the fault. Whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10%, the seismic moment for 1966 is 20% greater than for the earlier events to within a precision of 20%. The Parkfield ara seismicity, in general, seems well described by a recurring moderate size characteristic earthquake, repeating the same epicenter, magnitude, seismic moment, rupture area, and southeast direction of rupture expansion. An unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the Parkfield earthquakes in 1857, 1818, 1901, 1922, 1934, and 1966 represent a strictly periodic process. Assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 and 1934 period, the next characteristic Parkfield earthquake should occur between 1983 and 1993.