Abstract

Leveling data released by the U. S. Geodetic Survey indicate a land rise anomaly centered near Wilmington-Southport, NC, which over the span of 1932–1963 has averaged 6.5 mm per year. Existing tide guage data are consistent with the elevation changes indicated by the leveling. Groundwater pressures are anomalously high in deep wells in the same vicinity of the land rise and reach heights of 33 meters above sea-level. Attempts to explain the potentiometric anomaly by simple hydraulic conditions, osmotic imbalances, or the presence of natural gas have failed. The most plausible explanation seems to be the contemporary existence of deep stresses in the basement. The pressures appear to be of relatively recent occurrence - years, or decades at the most. The historical seismicity of the area to date is indistinguishable from that of Charleston, SC, prior to 1886. Charleston is only 230 km away and in the same geologic province. Since Charleston had a major earthquake in 1886 it is concluded that a similar event is possible at Wilmington-Southport. At least history cannot be used to rule out such an event. Until the Charleston event has been proven unique to that area, the entire Atlantic Coastal Plain from Georgia to New Jersey could conceivably be capable of such activity. Estimates based upon areal extent and time of the duration of these phenomena suggest an earthquake of order of magnitude, 7.0. The data are not sufficient to suggest a plausible time into the future for such an event. An earthquake this large in this area would be one of the most intense in U.S. history. A new net of SP seismographs is currently being installed there to look for velocity anomalies and to detect possible microseisms. A new tide guage is also being reinstated at Southport and a continuously recording potentiometric monitor has been put on one of the deep artesian wells. It is recognized that this additional data may or may not yield definitive results. It will be at least late 1976 or early 1977 before significant results can be obtained. It is hoped that this additional data will suggest an alternate explanation and, thus, not confirm the earthquake hypothesis. However, if such data does indicate the imminence of an earthquake, it is hoped that a prediction of size, time, and location can be achieved in order to make appropriate preparations, including the securing of an operating nuclear power plant there.

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