Geodetic measurements near Parkfield, California, 1959–1984

Abstract

Trilateration measurements have been made by various agencies since 1959 near Parkfield, California, in the transition zone between the central creeping section and the southern locked section of the San Andreas fault. Within the transition zone there are several creep meters and alignment arrays, four small‐aperture trilateration networks, and a broad trilateration network extending 70 km southwest from the San Andreas fault to the Pacific coast. Line length changes observed after the 1966 ML = 5.6 Parkfield earthquake indicate a geodetic moment of 4.5 ± 1.0 × 1025 dyn cm. The data are consistent with either 91 ± 8 cm slip on a rupture zone which is 30 km long and extends from 3 to 8 km below the surface or 59 ± 5 cm on a 30‐km‐long zone 2–10 km deep. This moment estimate includes some postseismic slip and is a factor of 2–4 greater than the moment determined from seismic surface waves. The post‐1966 data set is well suited to discriminate between the effects of surface and deep slip. Surface slip rates determined by creep meters, alinement arrays, and small‐aperture trilateration networks decrease from 29 mm/yr at Slack Canyon to zero south of Highway 46; a gradient of roughly −0.48 ± 0.01 mm/yr/km. Deformation of a subset of the trilateration network between Cholame and San Luis Obispo is consistent with shallow slip of 2.9 ± 1.2 mm/yr from the surface to a depth of 16 km and 33.4 ± 5.5 mm/yr below 16 km. The trilateration data alone do not uniquely constrain both the transition depth from shallow to deep slip and the deep slip rate. A transition depth near 16 km is preferred because it provides the best fit to the data and because the predicted slip rate is consistent with independent estimates of the total slip rate across the San Andreas fault. This simple two‐dimensional model neglects along‐strike variations in the deformation. The data show no evidence for significant strike‐slip deformation across faults between the San Andreas and the Pacific coast.