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.

Precision of geodolite surveys: A Reply to Jackson and Cheng

“…[ 1982] and Jackson [ 1983] feel that the "corrected" dilatation (bottom plot in Figure 9) is a more reasonable representation of the dilatation at Hollister than is the actually observed dilatation, principally because the "corrected" dilatation shows an offset in mid-1979 at about the time of a magnitude 5.9 earthquake that occurred 10 km outside of the Hollister network [King et al, 1981 ]. However, the "corrected" dilatation does not show an offset in late 1974 at the time of a magnitude 5.2 earthquake within the network [Savage et al, 1979]. It is curious that the 1979 offset in the "corrected" dilatation is wholly an artifact of the postulated correction and also that the 1979 offset in the "corrected" dilatation would not have been significant had the systematic errors postulated in the preceding paragraph been employed (i.e., use the dashed rather than solid correction in the uppermost plot in Figure 9).…”

The relationship between the instantaneous velocity field and the rate of moment release in the lithosphere

“…In the first, the block model, it is supposed that the continental lithosphere consists of coherent, unfaulted elastic blocks moving with respect to one another (e.g. Savage & Burford 1973; Savage et al 1979; Hashimoto & Jackson 1993). Over very long time periods (averaged over many seismic cycles), relative motions among the blocks are accommodated by motions across the faults which divide them, and the blocks themselves behave rigidly.…”

“…Over very long time periods (averaged over many seismic cycles), relative motions among the blocks are accommodated by motions across the faults which divide them, and the blocks themselves behave rigidly. The block model accounts for the elastic effects associated with strain accumulation localized on the block boundaries, this strain being determined by a model of resistance to the relative motions above a locking depth (Savage et al 1979; Matsuúra et al 1986; Hashimoto & Jackson 1993). The second model is the ‘thin‐sheet’ model (England & McKenzie 1982; England & Molnar 1997; Flesch et al 2000).…”

Preseismic and coseismic deformation associated with the Coyote Lake, California, earthquake

“…ure 1, Gil-Canada and the lines farther south). The average length of the lines in this network is about 9 km, and the expected standard error in length measurement is about 3.5 min.Most of the lines in the network have been observed annually since 1971 Savage et al [1979]. have described the deformation of that network through mid-1978, and in this paper we will simply extend those results through September 1979.…”