Earthquake deformation cycle on the San Andreas Fault near Parkfield, California

Abstract

Six moderate earthquakes are thought to have ruptured the Parkfield segment of the San Andreas fault since 1857. The similar characteristics of the three seismically recorded events, together with the quasi‐regular 22‐year recurrence interval, have led to the forecast of a similar event in 1988 ± 5 years. In this study the potential for the hypothesized earthquake is assessed by estimating the time required for interseismic straining to recover the strain energy released by the most recent, June 1966, Parkfield earthquake. For a simple model of the earthquake cycle this interval is proportional to the ratio of the 1966 seismic moment to the interseismic moment deficit rate. Baseline lengths measured before and after the 1966 earthquake are inverted to estimate the magnitude and distribution of the combined coseismic and transient postseismic slip. Although the geodetic data determine only broad spatial averages of the fault slip, they do require that slip at depth exceeded the surface offsets measured in 1966. Inversions with extreme prior models are calculated to assess the range of seismic moment and moment deficit rates consistent with the available geodetic data. Inversions assuming only that the slip distributions are to some degree smooth yield strain energy recovery intervals of 14–25 years. Extreme values range from as little as 6 years to as much as 29 years. In comparison, historic recurrence intervals of Parkfield earthquakes range from 12 to 32 years. These results are consistent with the expectation of a magnitude 5½–6 earthquake near Parkfield by 1993.