A map of the United States showing vertical movements of the Earth's crust indicates that the land in much of the country is slowly rising or falling and that very few really stable areas exist.
CRUSTAL MOVEMENT MAP OF USA
GEODETIC SURVEY MAP SHOWS CRUSTAL MOVEMENT
A map of the United States showing vertical movements of the Earth's crust indicates that the land in much of the country is slowly rising or falling and that very few really stable areas exist.
The first compilation of its kind for the United States, the map reveals probable annual rates of crustal movement over large regions.
The map is based on measurements made over the past 100 years by surveyors and geodesists of the National Geodetic Survey, an agency of the National Oceanic and Atmospheric Administration (NOAA). Only the larger areas of land subsidence and uplift are shown on the map; much of it is based on interpolation between widely spaced lines of elevation that have been measured by geodetic field parties.
Some of these movements are minute, detectable only by painstaking and repeated measurements over a period of years. Others are large enough to be of concern in the design and maintenance of engineering structures, and in some areas land subsidence is sufficiently rapid to cause alarm. Samuel P. Hand, Chief of the National Geodetic Survey's Vertical Network Division, which developed the map, cited as one example of this the Houston-Galveston area in Texas which has subsidence as much as 5 feet in 20 years. In the New Orleans area, subsidence of more than 1 foot has been detected in a 25-year period.
These cities lie in a large area of subsidence which extends along the entire coastal plain of Texas and Louisiana. Hand said this rapid subsidence, and subsidence of the Central Valley in California, are largely the result of the removal of underground resources.
The NOAA geodesist pointed to Terminal Island at Long Beach, Calif., as an outstanding example of large-scale subsidence. Subsidence there totaled more than 25 feet before it was checked by the injection of fresh water.
In such flat coastal areas, subsidence increases drainage problems and the danger of inundation during storms. Among other areas so affected are the eastern shores of Delaware. Maryland, and Virginia, and the vicinity of Atlantic City, New Jersey.
Whereas some subsidence is caused by man, widespread vertical movements are due to natural internal forces which are forcing the Earth's crust up or down.