Abstract

An improved velocity model for the meizoseismal area of the 1886 Charleston earthquake was used to relocate current seismicity, which showed marked separation into clusters. The relocated hypocenters and composite focal plane solutions were compared with available geophysical data to interpret their tectonic significance and possible association with the 1886 earthquakes. There is a distinct velocity discontinuity at a depth of about 10 km, where Vp increases from 5.9 to 6.45 km/s. The relocated hypocenters and composite focal plane solutions delineate two main source zones lying at different depths. The shallower zone, at 4 to 8 km depth and collinear with the Ashley River, is herein named the Ashley River seismogenic zone. The composite focal plane solution suggests reverse faulting on a steeply dipping northwest-striking fault with the southwest side upthrown. This zone is also associated with aeromagnetic and gravity anomalies. The deeper zone, at 9 to 13 km, suggests a right slip on a fault extending N26°E from east of Ravanel to Jedburg, a distance of more than 25 km, and dipping steeply to the west-northwest. Its location and extent are similar to the so-called Woodstock fault. Examination of geomorphic data suggests that there may be some ongoing tectonic uplift and subsidence in the area. The inferred P axes from fault-plane solutions are oriented S60°W. Firsthand accounts of the 1886 earthquakes suggest that two source areas were active in 1886 and the months that followed; I postulate that the two zones of current seismicity are coincident with the 1886 source areas.

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