| Abstract: | The distance between two points on opposite sides of the San Andreas Fault is being derived from laser tracking of near-earth satellites as part of an experiment to estimate the motion along the plate boundary. The two sites, at Otay Mountain near San Diego and at Quincy in northern California, are nearly 900 km apart and approximately 150 and 270 km, respectively, away from the main strike of the San Andreas Fault. The angle between the fault and the intersite vector is approximately 25°. In the fall of 1972 satellite laser tracking systems occupied these two sites, and from the data collected the relative location of the two sites was determined. The two sites were reoccupied in the fall of 1974 and again in the fall of 1976, and provided two further estimates of the relative positions of the two sites.The results of these first three measurements indicate a shortening of the intersite baseline between San Diego and Quincy at an average rate of 9 ± 3 cm/year, suggesting a much larger possible present-day motion across the fault system than expected. The main source of error in this analysis is the motion of the spacecraft which is significantly affected by unmodeled anomalies in the earth's gravity field. However, major advances in our knowledge of the gravity field are expected over the next few years and as these occur the accuracy of the present results will improve. |
