1978 Southern California Releveling and its implications with respect to the sea slope problem

Abstract

Since the apparent sea slope along the Pacific coast between San Diego, California, and Neah Bay, Washington, was originally reported by Bowie in 1929, numerous points of view have been expressed in literature concerning this possibility. A recent interpretation proposes a complete agreement between geodetic and steric leveling. Another geodetic study shows a time variation of sea slope over a decade, suggesting a possible rise of San Pedro with respect to San Francisco (or San Francisco subsiding with respect to San Pedro) at an average rate of about 8 cm/yr. However, tidal records do not reveal large changes in the trend of mean sea level relative to land at the tide stations.

Our results from the 1978 Southern California Releveling Program (SCRP), completed in a period of less than six months indicate large unexplained misclosures, of the order of 150 mm, difficult to attribute to tectonic movements. Analysis of two routes from SCRP data between Avila Beach and San Diego, California, shows completely different sea slopes at the same epoch, hence eliminating the possibility of a time‐dependent phenomenon. Our investigation concludes that there are still unknown variables affecting geodetic leveling that must be understood, clarified, and accounted for before any definitive assertion can be made about the role of crustal motions in any analysis of geodetic leveling.