Results of four leveling surveys carried out by the National Geodetic Survey between Anchorage and Whittier, Alaska, combined with an analysis of sea level measurements at Anchorage, indicate as much as 0.55 m of land uplift in the decade following the 1964 Prince William Sound earthquake. The pattern of uplift is parabolic in shape, convex upward, and reaches a maximum approximately halfway between Anchorage and Whittier, or about 300 km northwest of the Aleutian trench axis. The data suggest that the position of maximum uplift is migrating away from Anchorage, i.e., toward the Aleutian trench. The observed uplift occurs in a region which subsided as much as 1.9 m during the earthquake. The rate of uplift has decreased exponentially since the time of the 1964 earthquake. These movements appear to represent postseismic deformation associated with the 1964 Alaska earthquake. The observations are most easily explained by creep along the downdip extension of the fault which ruptured during the 1964 earthquake, although viscoelastic rebound and long‐term elastic strain accumulation mechanisms may play a part. There is no evidence supporting magma intrusion or dilatancy mechanisms. These results provide new constraints for models of tectonic processes at convergent plate margins.

The inherent precision of spirit leveling has preserved its utility as a geodetic measurement system for over a century. While various instrumental and procedural modifications designed to enhance this precision have been introduced over the years, the basic measurement system has remained virtually unchanged since the mid‐nineteenth century. Possible systematic error has dictated the majority of the procedural and instrumental requirements associated with geodetic leveling; the physical source(s) of several of these errors remain poorly understood. Statistically independent random errors, which accumulate according to the square root of the survey distance, are generally controlled through redundancy and procedural randomization; they range from 0.5 mm L1/2 for the highest‐order modern leveling to about 6 mm L1/2 for the lowest‐order nineteenth‐century geodetic surveys, where L is the survey distance in kilometers. Height differences are conceptually distinct from observed or measured elevation differences in the sense that the former are uniquely defined, whereas the latter are path dependent, a distinction that arises from the nonparallelism of the equipotential surfaces of the earth’s gravity field. The number of possible height systems is virtually limitless. They include the systems of geopotential numbers and dynamic heights; although neither of these systems is geometrically informative, each provides perfectly valid height characterizations that may be especially useful in the solution of certain physical problems. The most generally used system of heights is the orthometric height system; the resulting heights are true geometric heights above the geoid. Normal height systems are referred to the quasi‐geoid rather than the geoid. Each of the various height systems meets the requirement of uniqueness, and none can be viewed as being conceptually superior. Conversion of the observed elevation differences obtained from leveling into uniquely defined height differences requires the application of a gravity‐dependent correction. Because gravity coverage in North America was generally sparse until recently, an approximation for this correction, which provides for the effects of the poleward covergence of the equipotential surfaces, has usually been used on this continent. Heights have been traditionally referred to mean sea level as a datum, a usage that implies coincidence between mean sea level and the geoid (or quasi‐geoid). Because the determination of mean sea level is dependent on the length of the observation period, because its definition makes no allowance for vertical crustal displacements or changes in eustatic sea level, and because its definition disregards the demonstrable existence of sea surface topography, local mean sea level generally departs from the geoid. This introduces errors in computed heights that probably equal or exceed those due to leveling. Repeated levelings continue to provide the best basis for determining terrestrial vertical displacements. These displacements are necessarily...

A down‐to‐the‐east crustal tilt rate across western Washington disclosed by precise leveling over a 70‐year period suggests that the Juan de Fuca plate is aseismically underthrusting the North American plate. According to this hypothesis, the frequent occurrence of large thrust earthquakes that ordinarily accompany plate convergence need not be expected along the Juan de Fuca subduction zone. This conclusion is consistent with the fact that there have been no great earthquakes in western Washington in historical time (the past 140 years).

No abstract

No abstract

Comparisons between 'free' adjusted normal orthometric leveling elevations and mean sea level at tide stations are given by Braaten and M½Combs (1963) for the United States coastlines. Their report indicated an apparent systematic difference between the 1963 free adjustment, which had a mean epoch of 1939, and mean sea level along the coasts. Since that time, new leveling surveys, performed primarily by the National Ocean Survey's National Geodetic Survey, have been accomp•hed between San Francisco and San Pedro tide stations for . The observed elevations at tide stations are compared to mean sea levels of the 1941-1959 epoch. Leveling surveys show very good consistency with a steady trend from negative to positive between 1968 and 1978. This trend indicates that San Pedro is rising with respect to San Francisco or that San Francisco is subsiding with respect to San Pedro at an average rate of about 70 mm/yr. However, an examination of the tidal records does not reveal large changes in the trend of mean sea level with respect to the land at the tide stations. Hicks and Crosby (1972) report that the mean sea level trend with respect to land (1940-1972 series) is -0.3 mm/yr at San Pedro and + 1.8 mm/yr at San Francisco. Thus the indicated relative movement rate (70 mm/yr) from leveling is about 30 times greater than the rate indicated by tidal observations. The reasons for the large discrepancy between relative movement rates from repeat levelings and tidal observations are presently unknown. ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß I c.q ß ß ß ß ß ß ß I ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß I ß ß ß ß

No abstract

No abstract