Abstract

Multichannel seismic-reflection profiles in the meizoseismal area of the 1886 Charleston earthquake and nearby offshore area show evidence of Cenozoic faulting. Deformation observed on two land profiles defines a northeast-striking reverse(?) fault zone named the Cooke fault. A Jurassic-age basalt layer at 750 m depth has 50 m vertical displacement, southeast side down. Overlying Upper Cretaceous and Cenozoic beds have decreasing displacement with decreasing depth. These displacements, coupled with larger (∼ 190 m) displacements of reflectors below the basalt, indicate continuing Cenozoic movement of a post–basalt-flow, pre–Late Cretaceous fault into Eocene time or later. The fault extends into a cluster of 1973–1978 epicenters; this suggests a possible causal relationship with seismicity. Marine multichannel seismic-reflection data combined with single-channel, high-resolution reflection profiles reveal Cenozoic faulting beneath the continental shelf near Charleston. A reverse fault 12 km offshore, named the Helena Banks fault, extends for at least 30 km and is observed as shallow as 10 m below the sea bottom, with the most recent movement in post-Miocene or Pliocene time. There is no known seismicity in the area of this fault. The reverse movement on the Helena Banks and Cooke faults is consistent with a northwest-southeast compressional stress regime. Elucidation of the relation, if any, between reactivated Triassic or older structures and/or a decollement (defined by diffractions at 11.4 ± 1.5 km depth) on the one hand and seismicity on the other will be important in assessing earthquake risk in many areas of similar tectonic setting outside of the Charleston region in the Atlantic coastal and continental margin area.

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