EVIDENCE OF POSSIBLE SURFACE-FAULTING AND SAND-BLOWS IN THE COLONIAL TOWN OF DORCHESTER, SC, DURING THE 31 AUGUST 1886 CHARLESTON EARTHQUAKE

Colonial Dorchester, abandoned for the 3rd time about 1820, was near the epicenter of the 31 August 1886 Charleston earthquake. Dutton (1889) described the church’s toppled bell tower and cracks formed in Fort Dorchester’s walls, but ignored the foundations of other structures in the abandoned town. Because of many changes in the area during the 124 years since the earthquake, colonial Dorchester actually represents a unique intact research site for the 1886 earthquake. As part of our ongoing archaeological/geological work there, we conducted preliminary gradiometer surveys of the town to look at selected features which might be relicts of the 1886 earthquake. Several large depressions (0.5-m-deep; 2-3-m diameter) are located within the old 12 x 60-m market place. A survey over one depression showed a segmented, N30-35E-trending, linear band of locally reduced magnetism at depth which is consistent with a clastic dike beneath a sand-blow vent. Other evidence indicates that older sand-blows also occurred prior to construction of the fort in 1758/60. Another survey showed a 9 x 9-m-square (house-size) structure, at depth, with a 0.5-m right-lateral offset along a N10-20W-trend. This is sub parallel to the N21W-trending, steeply SE-dipping fault with a right-lateral component to the slip-vector determined for the causative fault of the 1886 earthquake from the published analysis of joints and faults (observed by Dutton, 1889) in the walls of the old fort. It is consistent with geomorphic evidence, including combined shoreline and river deflections, that indicate E-side up across this fault and it is consistent with the recent reinterpretation of data from the Ashley River seismic zone confirming the NW-striking, SE-dipping fault-model for the causative fault of the 1888 Charleston earthquake. This possible offset of a Dorchester structure may be the first identified surface fault associated with the 1886 Charleston earthquake. As such, verification of its actual orientation and slip-vector is crucial to a resolution of the causative fault of the 1886 Charleston earthquake (NW-striking, SE-dipping reverse fault versus NE-striking, right-lateral strike-slip fault).