Abstract

Understanding of the South Carolina seismic zone and the 1886 Charleston earthquake is much improved today as a result of seismological studies which began in 1973. A permanent seismic network was installed in 1974 in South Carolina between Charleston on the Atlantic Coast and Columbia in the central part of the state. Temporary field networks of portable instruments were established in the epicentral areas of earthquakes near reservoirs in the northwest part of the state. The seismic network data have provided precise hypocenter determinations in previously known zones of continuing activity near Middleton Place and Summerville, Bowman, and in a newly identified source of seismicity near Adams Run. Several new composite focal mechanisms were determined, and abundant aftershock data were acquired. Results of recent geophysical and geological studies, when combined with the seismological results, indicate two seismic regimes in South Carolina.

The first regime covers the buried basement structure of the middle and lower Coastal Plain province, which has been shown by geophysical studies to be quite unlike Piedmont province structures to the northwest. The Middleton Place-Summerville and Bowman seismic zones are associated with buried early Mesozoic basins and large, elongate mafic intrusives inferred from coincident positive aeromagnetic and gravity anomalies. The current seismicity is concentrated in clusters rather than distributed along a northwest-trending fault zone as suggested earlier by various workers.

The second regime covers the exposed Piedmont province and the upper Coastal Plain. Earthquake activity may be associated with strain release on or near mapped faults or contacts between metamorphic belts. In particular, studies of aftershock and microearthquake activity in the vicinity of reservoirs indicates that in areas of high ambient stress, the seismicity appears to be induced by fluctuations in the impounded water and pore pressure at depth.

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