Geodolite measurements of deformation near Hollister, California, 1971–1978

Abstract

A 24‐station trilateration network spanning the San Andreas and Calaveras faults near Hollister, California, has been surveyed each year between 1971 and 1978, inclusive. Two moderate (ML = 5) earthquakes have occurred within the network during the interval. No convincing preseismic or coseismic anomalies associated with those earthquakes have been identified. The deformation of the network can be described roughly by rigid body motion of the three blocks bounded by the two faults with accommodation occurring by right‐lateral strike slip on the San Andreas (13±2 mm/a) and Calaveras (17±2 mm/a) faults. The required slip rates are within the range of the observed fault creep on those faults. A more detailed analysis of the deformation indicates appreciable strain accumulation (0.4 μstrain/a tensor shear) within the block lying between the San Andreas and Calaveras faults. Many of the features of the observed deformation can be produced by an elementary dislocation model, indicating that most of the deformation is associated directly with slip on the major faults. The network is not extensive enough to define uniquely the relative motion across the San Andreas fault system, but the data are consistent with a value of about 38 mm/a. The rate of deformation in 1971–1978 was not uniform but rather appears to have been higher than normal in 1973–1974 and lower than normal in 1975–1976.Supplement is available with entire article on microfiche. Order from American Geophysical Union', 2000 Florida Avenue, N.W., Washington, D.C. 20006. Document J79‐008;$01.00. Payment must accompany order.