| Abstract: | A transcontinental profile of vertical crustal movement from San Diego, California, to Meldrim (Savannah), Georgia, has been assembled from precise leveling data of the National Geodetic Survey. Assuming constant movement during the time interval between leveling and releveling surveys, rates of movement have been plotted. The overall rate of movement of the East Coast relative to the West Coast derived from leveling is opposite in sign to the corresponding rate from tide gauges at San Diego and Savannah. Though this discrepancy is within the acceptable limits of normal random error associated with leveling measurements, it may also result in part from unavoidable uncertainties at the connection points between the various segments. In the absence of deterministic evidence on the source of the discrepancy, a least-squares adjustment was performed to distribute this difference in the overall trend through the profile. Apart from this trend, the western end of the profile is dominated by pronounced subsidence (rates 10 cm/yr) owing to water withdrawal and associated consolidation of alluvial sediments. With the exception of these movements, rates of vertical movement of the eastern and western United States are similar, suggesting that if such measurements represent significant tectonic activity, this activity may not be confined to the western United States. Movements are correlated with topography only in the peninsular ranges of California. Although these apparent elevation changes may reflect actual ground movement, this correlation may also result from systematic leveling errors. Comparison with gravity, depth to basement and depth to Moho shows no correlation. |
