Absolute Gravity Measurements: Global Sea Level Change

Abstract

Melting of the ice masses that covered large portions of North America and northern Europe during the last ice age have redistributed major loads on the crust and mantle. The process has caused an upward rebound that still exceeds 1 cm/year in Canada and Scandinavia and a few millimeters/year in more southerly areas. The formation of foldbelts and basins by global tectonic forces is another source for uplifts and subsidences of similar magnitude in many regions of the world. In the United States several areas are affected by vertical land motions, including the general area of the Great Lakes, large sections of the Gulf of Mexico, and the Atlantic and Pacific coasts. In order to detect eustatic sea level changes from tidal records, the vertical motions affecting coastal areas have to be determined accurately. A quick and relatively inexpensive way of monitoring vertical motions is through repeat gravity observations. Discounting changes of density redistribution, elevation changes on the surface of the Earth are reflected in changes in the value of gravity. For a 1 cm elevation change the gravity change is approximately 3 pGals. A change of this size in gravity now can be detected, using an absolute gravity instrument developed by the Joint Institute for Laboratory Astrophysics (JILA). During the past 2 years the National Geodetic Survey (NGS) has been using this instrument to test its performance, and to develop and refine observational, data reduction, and analysis procedures. Absolute gravity has been determined at more than 40 sites, and several more sites have been evaluated for suitability for future long-term observations. From repeat measurements at 10 of the observation points we estimate that at carefully selected sites the measurements have a precision of better than 1 to 2 pGals and an accuracy of perhaps 3 to 5 pGals. Based on these encouraging results, the NGS has begun the development of an absolute gravity reference network to monitor vertical motions in North America. The gravity reference network will be used to derive improved estimates of vertical motions caused by glacial rebound and tectonic forces at tide gauge stations. This will improve the estimates of the change in global sea level from historical tide gauge records, as well as current and future observations. We believe that the use of this new absolute gravity instrument may be the most cost effective and sensitive tool for monitoring and mapping vertical motions during the next decade.