Shallow Faulting and Folding in the Epicentral Area of the 1886 Charleston, South Carolina, Earthquake

Abstract

The moment magnitude (Mw) ∼7 earthquake that struck Charleston, South Carolina, on 31 August 1886 is the largest historical earthquake in the United States east of the Appalachian Mountains. The fault(s) that ruptured during this earthquake has never been conclusively identified, and conflicting fault models have been proposed. Here we interpret reprocessed seismic reflection profiles, reprocessed legacy aeromagnetic data, and newly collected ground penetrating radar (GPR) profiles to delineate faults deforming the Cretaceous and younger Atlantic Coastal Plain (ACP) strata in the epicentral area of the 1886 earthquake. The data show evidence for faults folding or vertically displacing ACP strata, including apparent displacements of near-surface strata (upper ∼20 m). Aeromagnetic data show several northeast (NE)-trending lineaments, two of which correlate with faults and folds with vertical displacements as great as 55 m on the seismic reflection and radar profiles. ACP strata show only minor thickness changes across these structures, indicating that much of the displacement postdates the shallowest well-imaged ACP strata of Eocene age. Faults imaged on the seismic reflection profiles appear on GPR profiles to displace the erosional surface at the top of the upper Eocene to Oligocene Cooper Group, including where railroad tracks were bent during the 1886 earthquake. Some faults coincide with changes in river trends, bifurcations of river channels, and unusual river meanders that could be related to recent fault motion. In contrast to our interpreted NE fault trends, earthquake locations and some focal mechanisms in the modern seismic zone have been interpreted as defining a nearly north-striking, west-dipping zone of aftershocks from the 1886 earthquake. The relationship between the modern seismicity and the faults we image is therefore enigmatic. However, multiple faults in the area clearly have been active since the Eocene and deform strata in the upper 20 m, providing potential targets for field-based geologic investigations.

Rift Basins and Intraplate Earthquakes: New High‐Resolution Aeromagnetic Data Provide Insights Into Buried Structures of the Charleston, South Carolina Seismic Zone

“…This includes some of the NE‐striking faults delineated by Pratt et al. (2022) based on deformation in Cretaceous and younger sediments such as the NE‐striking Middleton and Cypress Swamp faults; slight changes to the Gants and Cooke faults are suggested, but the general trends are similar (Figures 2 and 9a).…”

“…(1983), extended by Pratt et al. (2022), and discussed more below. The southeastern ends of the ESE‐trending anomalies also terminate against other NE‐trending lineaments.…”

“…Most disturbances impact the sediments evenly, leading Pratt et al. (2022) to deduce that the disturbances are likely to be Eocene or younger. Furthermore, two magnetic lineaments coincide with surficial landforms visible in lidar data (Figure 12), suggesting very recent slip.…”

“…Pratt et al. (2022) took the stacked profiles from the Chapman and Beale (2008, 2010) papers and migrated them using a frequency‐wavenumber (f‐k) algorithm and a velocity profile derived from the analyses by Yantis et al. (1983) and Schilt et al.…”

“…We used the new aeromagnetic data to image and interpret igneous and metamorphic rocks and structure buried beneath Atlantic Coastal Plain sediments with previous borehole data (Horton et al., 2022) for ground truth. We next used reprocessed seismic reflection data collected in the 1970s and 1980s (Chapman & Beale, 2008, 2010; Pratt et al., 2022) to verify fault locations suggested by the magnetic data and to examine corresponding disturbances in Cretaceous and younger sedimentary cover. Data from seismic monitoring efforts (Chapman et al., 2016) and lidar analyses (Marple & Hurd, 2020) were also used to further evaluate fault activity.…”