THE 1886 CHARLESTON, SOUTH CAROLINA EARTHQUAKE: FAULT REACTIVATION IN A MESOZOIC EXTENSIONAL TERRANE

The 1886 Charleston, South Carolina earthquake was perhaps the best-documented earthquake of the 19th century. However, remaining questions about the source of this enigmatic shock have a major impact upon seismic hazard assessment throughout the eastern United States.

Seismic reflection data collected during 1975-1983 in the epicentral area near Summerville were reprocessed. The results resolve previously unrecognized Mesozoic-Cenozoic faults and an early Mesozoic extensional basin, approximately 20 km in width, between Summerville and Charleston. The basin is delineated by the geometry of reflections from early Mesozoic rocks and by positive magnetic and gravity anomalies. Cenozoic reactivation of Mesozoic extensional faults is imaged in the interior of the basin. The northwestern boundary is marked by a sharp gradient in the magnetic field. Folded Cretaceous and Tertiary coastal plain sediments associated with diffractions and truncated reflections from the Mesozoic section at four locations along this magnetic gradient indicate that the northwestern basin boundary is faulted.

Instrumentally located earthquake epicenters are clustered at the location of the faults imaged in the interior of the basin and in proximity to the northwestern margin. Modeling of magnetic and gravity data indicates that the upper crust beneath the structural basin is composed largely of mafic rocks to a depth of at least 4 km. Cretaceous coastal plain sediments unconformably overlie an early Jurassic basalt throughout the study area. The Jurassic basalt extends beyond the border of the basin, and shows minor tectonic deformation compared to the deeper early Mesozoic units imaged within the basin.

We interpret the deeper reflections beneath the shallow Jurassic basalt within the basin to be a sequence of Triassic volcanic and sedimentary rocks, and propose that the Charleston earthquake occurred due to compressional reactivation of a Mesozoic fault in a localized zone of intense early Mesozoic continental rifting.