Fault plane solutions and relocations of recent earthquakes in Middleton Place Summerville Seismic Zone near Charleston, South Carolina

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.

Further evidence for the East Coast fault system and faults associated with the Summerville restraining bend and their possible relationship to the 1886 Charleston, South Carolina, earthquake, USA

“…Hypocentral depths of microearthquakes in the MPPSZ (Fig. 2) range from 1 to 12 km (Madabhushi and Talwani 1993;Chapman et al 2016). Studies of focal mechanisms from the MPSSZ show a wide variety of fault types and orientations (Madabhushi and Talwani 1993;Chapman et al 2016) that Marple and Hurd (2020, fig.…”

“…2) range from 1 to 12 km (Madabhushi and Talwani 1993;Chapman et al 2016). Studies of focal mechanisms from the MPSSZ show a wide variety of fault types and orientations (Madabhushi and Talwani 1993;Chapman et al 2016) that Marple and Hurd (2020, fig. 14) postulated is from small displacements along several faults east of the Summerville restraining bend.…”

“…Thus, the Mid-dleton Place-Summerville seismic zone (MPSSZ) may not accurately reflect the locations of the faults that ruptured August 31, 1886. Second, fault plane solutions from the MPSSZ show a wide variety of fault types and orientations (Shedlock 1988;Madabhushi and Talwani 1993;Chapman et al 2016) that are from displacements along several faults with different orientations. Third, faulting beneath the 700to 1100-m-thick, southeast-dipping wedge of Cretaceous and Cenozoic unlithified sediments and weakly lithified to indurated sedimentary rocks beneath the Charleston region (Gohn 1988) would likely fold rather than fracture the overlying strata (Stein and Yeats 1989).…”

Performance Assessment for the E-Area Low-Level Radioactive Waste Disposal Facility at the Savannah River Site: Chapter 2

“…Studies by Madabhushi and Talwani (1993) refined the earlier model by utilizing 58 additional well-recorded events in the Middleton Place-Summerville Seismic Zone from 1980 to 1991.…”

“…Results of this effort demonstrated that the Ashley River and the Woodstock faults are not simple planar features, but resemble zones composed of short segments of varying strike and dip. Madabhushi and Talwani (1993) concluded that the seismicity in the Middleton Place-Summerville Seismic Zone defines the intersection of two fault zones, which they inferred to be the Ashley River fault zone and the Woodstock fault zone. Not felt except by a very few under especially favorable circumstances (I Rossi-Forel Scale).…”