Comment on the Companion Articles "Finding Faults in the Charleston Area, South Carolina: 1. Seismological Data" by I. Dura-Gomez and P. Talwani and "Finding Faults in the Charleston Area, South Carolina: 2. Complementary Data" by P. Talwani and I. Dura-Gomez

Abstract

Two different models have been presented to describe the geometry of the Woodstock fault near Summerville, South Carolina. The fault bend model of Marple and Miller (2006) postulated that much of the seismicity southeast of Summerville is associated with a 12° restraining bend in the Woodstock fault and nearby secondary faults, such as the Ashley River fault (Figure 1A). The second model depicts the Woodstock fault as having a 3–6-km-wide, left-step offset southeast of Summerville (Garner 1998; Talwani 1999; Gangopadhyay and Talwani 2003, 2005; Talwani and Dura-Gomez 2009) (Figure 1B). Although the two most recent papers by Dura-Gomez and Talwani in 2009 (both published in Seismological Research Letters 80 (5); see Dura-Gomez and Talwani 2009 and Talwani and Dura-Gomez 2009) provide an excellent reevaluation of seismicity and ground effects near Summerville, several inconsistencies between their fault model and the Charleston area geology suggest that the Woodstock fault is not offset. Based on previous studies of restraining offsets along strike-slip faults (McClay and Bonora 2001), dextral motion on the Woodstock fault (WF) should produce uplift at Talwani and Dura-Gomez's (2009) 6-km-wide restraining offset (Figure 2). Yet, there is no evidence for such uplift from morphological studies of the Ashley River where it traverses the offset, or from studies of the local stratigraphy. The only evidence for Holocene uplift along the Ashley River occurs west of (the upthrown side) of the WF, as evidenced by its incised channel and low-lying terraces in this reach (Marple 1994; Marple and Talwani 1993, 2000) (Figure 3). This is a critical point because rivers are sensitive indicators of uplift (Ouchi 1985; Schumm et al. 2002). If such a restraining offset existed along the WF, then this section of the Ashley River should be incised and its valley terraced. Instead, …