Geodetic networks versus time

Abstract

The great San Francisco earthquake of April 18, 1906 , presented an opportunity for geodesists to assist in the study of earthquake mechanisms. It was a rather elementary matter for residents of the area to report displacements of 3 to 5 meters by merely noting the offsets in fence lines, orchard rows, or other manaligned features. Geodetic surveys had been made in the San Franeiseo area as early as 1851, for the purpose of controlling nautical charts. By 1885, a more comprehensive firstorder network had been completed in the region. Many of these old points were recovered in a repeat survey following the 1906 earthquake. By noting the differences in the geographic positions, it was possible to determine very accurately the relative movement which occurred at the time of the earthquake. Quite unexpectedly, those studies introduced some evidence of earlier displacements which probably occurred at the time of the 1868 earthquake. There is little evidence to indicate that any significant geodetic crustal movement studies were made in the United States during the 15 years following the 1906 earthquake. In 1922, Dr. Arthur L. Day, who was the Director of the Geophysical Laboratory of the Carnegie Institution, and Chairman of the Committee on Seismology of that Institution, suggested that the Coast and Geodetic Survey reobserve the primary network of triangulation along the California Coast between San Francisco and Los Angeles. These resurveys were completed in 1924. The observational data verified that large changes had taken place, but it was quite difficult to obtain a satisfactory analysis because of the total length of the scheme with uncertainties due to the accumulation of errors. Inasmuch as this basic scheme straddles or is parallel to the San Andreas fault, these repeat surveys made in the 1920 ' s, are fundamental data for all later studies. The problem encountered in the analysis of that resurvey led to the recommendation that there be established a series of arcs of triangulation crossing the San Andreas fault at right angles. These arcs were generally 50 to 80 kin. in length, with a spacing of 3 to 5 kin. between points. Within these arcs, traverse lines, with very closely spaced points, were established at the crossing of the fault line or in the immediate area of the fault zone. Most of these surveys have been repeated at regular 10-year intervals. In earlier studies, the interest seemed to be in the ability to detect displacements, or rebound, in the fault zone. Because there had been no major earthquakes in some of the regions~ the first analyses seemed to be negative, but later analyses of these repeat surveys gave