Immediately following the 1966 Parkfield earthquake a continuing program of fault displacement measurements was undertaken, and several types of instruments were installed in the fault zone to monitor ground motion. In the year subsequent to the earthquake a maximum of at least 20 cm of displacement occurred on a 30 km section of the San Andreas fault, which far exceeded the surficial displacement at the time of the earthquake. The rate of displacement decreased logarithmically during this period in a manner similar to that of the decrease in aftershock activity. After the initial high rate of activity it could be seen that most of the displacement was occurring in 4–6 day epochs of rapid creep following local aftershocks. The variation of fault displacement along the surface trace was measured and shown to be consistent with a vertidal fault surface 44 km long and 14 km deep, along which a shear stress of 2.4 bars was relieved.

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