Subsidence of a residence in Florida (USGS photo)
Tide gauges in the Chesapeake Bay indicate that sea level there is
rising at twice the global average. However since tide gauges are
a relative measure of sea level height (see
Variations in Sea Level),
it is impossible to discern a sea level rise from land subsidence.
To resolve this ambiguity, we are installing GPS receivers in the
Chesapeake Bay region. The GPS receivers will monitor the absolute
motion of the crust in this region (see
GPS Section of this document).
Land subsidence, in the Chesapeake Bay region, may result from
The Chesapeake Bay is a
depositional basin that is being filled by sediment
[Hobbs et al, 1990].
During the 100 year period ending in the mid-1950's net deposition in the
estuary was approximately 2,915 million metric tons. To get a feel for
whether the subsidence caused by this load would be significant, we assume
the load was deposited at a constant rate over the last 100 years and we
further assume that the sediment was all deposited in an area of
100 kmē. (This area limit is unrealistic, but it will help us bound the
upper limit of the subsidence). Using Farrell's Green's functions
for an elastic earth, we find that the subsidence would be on the
order of 0.6 mm/year which is too small to contribute to the
observed rate of sea level rise. We dismiss sedimentation as a
source of the subsidence.
- transported by rivers
- derived from the continental shelf
- and caused by shoreline erosion.
The Maryland Geologic Survey's Department of Hydrology has been monitoring
wells in the area surrounding the Chesapeake Bay since the 1940's
[Frederick Mack, personal communication]. Annual maps displaying the depth
of the water table (see Figure 2 for an example) of the Aquia Aquifer in
Southern Maryland, indicate that the surface of the water table has
dropped on average by approximately 10 meters [Curtin et al., 1980-1991].
Specific wells display drops of as much as two to five times that amount.
Ground subsidence is expected to result from the subsequent compaction of
the aquifer. We would expect to see a larger subsidence in the GPS data
near areas of maximum water withdrawal as opposed to areas distant from
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