Annual measurements of the distance between station marks on either side of the San Andreas fault have yielded estimates of fault movement. Measurements have been made by the California Department of Water Resources on over 500 miles of the San Andreas and related faults with a Model 2A Geodimeter since 1959. Results have been reported by this author and others in California's Department of Water Resources Bulletin 116-6, Geodimeter Fault Movement Investigations in California, May 1968, and in Proceedings of the 2nd United States-Japan Conference on Research Related to Earthquake Prediction Problems, June 1966.
Earthquakes of Richter Magnitude 4.5 to 6 were often preceded by anomalous fault movements measured with the Geodimeter. These events have been analyzed during the past year to determine their effectiveness in providing earthquake warnings.