GEODETIC MONITORING OF THE SOUTHERN SAN-ANDREAS FAULT, CALIFORNIA, 1980-1991

Five geodetic arrays (10 to 40 km aperture) located along the San Andreas fault have been surveyed frequently (several times in most years) over the 1980-1991 interval to detect possible fluctuations in the deformation rate. In each survey of an array the distances between the same four to seven pairs of geodetic monuments were measured. The distances measured (with corresponding standard deviation) range from 8.4 km (3.4 mm) to 38 km (8.2 mm). The data are displayed as plots of measured distance as a function of time. Linear fits in such plots furnish a satisfactory representation of the data. That is, the scatter of the data about the linear fits is within the range expected for the estimated standard deviations in measurement. However, there are coherent low-amplitude (within the observational error) fluctuations apparent in some of the measured distances. Those fluctuations need not be tectonic effects but rather may be either random patterns or artifacts introduced by systematic drift in instrument calibration or wander of the geodetic monuments. We conclude that the measurements are consistent with steady deformation of the arrays over the 1980-1991 interval, and we find no convincing evidence in the data for fluctuations in the rate of deformation.