Abstract

The Middleton Place Summerville Seismic Zone (MPSSZ), located about 20 km northwest of Charleston is the most active seismic zone in South Carolina. Between 1980 and 1991, 58 events with Md 0.8 to 3.3 were recorded in MPSSZ. They lie in a diffuse area of 11 km by 14 km of which over two-thirds are located in a narrow 5 km by 6-km zone. The hypocentral depths range from 2 to 11 km with over 90% deeper than 4 km.

Single fault plane solutions were obtained for 35 events. Based on the focal mechanisms the earthquakes were grouped into five subsets. The mean P-axis of all fault plane solutions is oriented N63°E, in general agreement with the direction of SHmax obtained from in situ stress measurements.

Of the 35 events, 18 are associated with reverse faulting on NW - SE striking and SW dipping fault planes. These events were inferred to be associated with the Ashley River fault zone, which is not a planar feature, but is composed of short segments of varying strikes (N20°W to N70°W) and dips (40° to 70°SW). Eleven events were associated with strike-slip motion on NNE - SSW striking vertical faults and with thrust faulting on N - S oriented faults dipping to the west, respectively. These two sets are identified as being parts of the Woodstock fault zone.

The concentrated zone of seismicity included events associated with both the ARF and WF zones suggesting that it is at the intersection of these two fault zones.

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