New vertical geodesy

Abstract

Vertical geodesy is undergoing a revolution because of two factors. First, new precise three-dimensional position measurement techniques used over very long distances and based on extraterrestrial reference systems provide a new class and precision of geometric data previously unavailable for geophysical investigations. Second, physical models in tectonic theory for large earthquakes predict crustal distortions that violate the conventional assumptions used to interpret gravity and leveling data. Leveling and geometric elevation measurements are not directly comparable because the interpretation of leveling data is density-model dependent. Estimates of pre-1971 San Fernando earthquake elevation changes based on leveling of about l0 cm may be as much as 3 cm, or 40%, too large. Pre-1964 Niigata earthquake leveling surveys, previously used as confirmation of the dilatancy model, do not require dilatancy as an explanation and easily allow an alternative model with a subsurface density increase. Gravity is also not a dependable estimator of elevation change. But a combination of gravity with either leveling, if the dimensions of the distorted body are known or small, or geometric elevation measurements is essential for the determination of crustal density and strain changes. The 1965-1967 Matsushiro earthquake swarm leveling and gravity data show a significant dilatant strain of 0.6-1.8 x l0 -4 if the proper model dimensions are used. This dilatant strain would be adequate to cause the observed drop in V,o/Vs, even if the crust were initially saturated prior to distortion. The combination of gravity, leveling, and the new geometric elevation measurements provides a useful parameter, gravitational potential, for the inversion of subsurface density distributions. Use of this parameter, defined as the free-air elevation anomaly, is illustrated for a nearly compensated mountain root structure and shows that this technique holds significant promise for the study of large, deep structures in the crust and upper mantle.

Local changes in gravity resulting from deformation

“…The change in the horizontal component is found to be sufficiently small that the error in measurements of uplift due to errors in level is negligible. Whitcomb [1976] found that error in level led to appreciable errors in uplift in his model. The lack of agreement in our conclusions apparently is due to the differences in the deformation fields.…”

“…As a consequence, simple models have been used to get approximate results. In these models [Whitcomb, 1976;Ruff et al, 1976], uplift is simulated by uniformly deforming a cylinder imbedded in the earth's surface; deformation in the cylinder is decoupled from the surrounding rock, which is unstrained. The deformation field in these studies is highly idealized, and so application to real tectonic events is uncertain until results from more realistic models are available for comparison.…”

Lower Paleozoic deep-water facies of the Medfra area, central Alaska: A section in <i>Geologic studies in Alaska by the U.S. Geological Survey, 1997</i>

“…Anchorage is near the zone of elastic expansion that provided the push for this movement. Whitcomb (1976) showed that density changes can significantly reduce the gravity change expected from elevation changes, and, a few years later, Jachens (1979) published a graph showing that, for elastic expansion and compression with a Poisson's ratio near 0.25, the resulting gravity change could be very small. The gravity data from the Anchorage area thus provides further support for Plafker's hypothesis, which is now a well-accepted aspect of plate tectonics.…”

“…However, the agreement was least satisfactory in the negative-elevation-change or subsidence portion of the graph, for which many of the points were acquired after the initial publication. Later studies have refined the analysis of gravity changes and have considered the gravity-change effects of density variation and specific dislocation models (Whitcomb, 1976;Walsh and Rice, 1979;Savage, 1984)…”

Geomorphological Processes on Terrestrial Planetary Surfaces

“…These newer systems include the use of modulated light waves (geodimeter), pulsed radio waves (tellurometer), laser ranging to satellites including Moon (Bender Silverberg 1975), radio doppler, and various types of radio and radar altimeters. Recently, radio interferometry employing extra-galactic radio sources, so distant that they behave as fixed points, has been used over very long baselines (Coates et al 1975, Whitcomb 1976.…”