Seismicity on the San Andreas fault near Parkfield occurs in a tectonic section that differs markedly from neighboring sections along the San Andreas to the northwest and to the southeast. Northwest of the Parkfield section, small shocks (magnitudes of less than 4) do occur frequently, but San Andreas movement occurs predominantly as aseismic fault creep; shocks of magnitude 6 and larger are unknown, and little, if any, strain is accumulating. In contrast, very few small earthquakes and no aseismic slip have been observed on the adjacent section to the southeast, the Cholame section, which is considered to be locked, in as much as it apparently ruptures exclusively in large earthquakes (magnitudes greater than 7), most recently during the great Fort Tejon earthquake of 1857. The Parkfield section is thus a transition zone between two sections having different modes of fault failure. In fact, the regularity of significant earthquakes at Parkfield since 1857 may be due to the nearly constant slip rate pattern on the adjoining fault sections. Until the magnitude 6.7 Coalinga earthquake on May 2, 1983, 40 kilmoeters northeast of Parkfield, the Parkfield section had been relatively free of stress changes due to nearby shocks; the effect of the Coalinga shock on the timing of the next Parkfield shock is not known.
