Title

Paleoseismites which formed prior to and during the 31 august 1886 Charleston earthquake in Colonial Dorchester, South Carolina

Abstract

The causative fault of the 1886 Charleston earthquake is unknown and different types of studies (modern seismicity; moment magnitude; geomorphic analysis; fracture analysis) have indicated that it might have different orientations: NW-trending fault, NE-side up; NW-trending fault, SW-side up; NE-trending fault, NW-side up; or NE-trending strike-slip fault. One area exists where many features which were formed near the epicenter of the 1886 earthquake are still preserved essentially undisturbed by either modern development or agricultural use. This area, within a South Carolina State Park, is Colonial Dorchester and the adjacent Fort Dorchester. As part of ongoing archaeological/geological work there, we reviewed earlier archaeological excavation-records which showed that sand-blows occurred prior to construction of the fort in 1758/60. Our preliminary gradiometer surveys of the town also indicate possible relict features of the 31 August 1886 Charleston or earlier earthquakes. Several large depressions (0.5-m-deep; 2-3-m diameter) are located within the old 12 × 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 sandblow vent Another survey showed a 7 × 9-ms-quare structure (kiln?), at depth, with a 0.5-m component of left-lateral reverse offset along a ∼N45W-trend. This is consistent with the recent reinterpretation of the Ashley River seismic zone which confirmed the NW-striking, SE-dipping fault-model for the causative fault of the 1886 Charleston earthquake as determined from the published analysis of joints and faults first observed by Dutton (1889) in walls of the old fort. It is consistent with geomorphic evidence, including combined shoreline and river deflections, which indicate E-side-up across the causative fault This offset of a Dorchester kiln(?) may be the first identified surface fault, which can be verified by excavation, 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, NE-dipping reverse fault versus NE-striking, right-lateral strike-slip fault).

Publication Title

Southeastern Geology

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