Studies related to the Charleston, South Carolina, earthquake of 1886: A preliminary report

Abstract

PART A: The seismic history of the southeastern United States is dominated by the 1886 earthquake near Charleston, S.C. An understanding of the specific source and the uniqueness of the neotectonic setting of this large earthquake is essential in order to properly assess seismic hazards in the southeastern United States. Such knowledge will also contribute to the fundamental understanding of intraplate earthquakes and will aid indirectly in deciphering the evolution of Atlantic-type continental margins. The 15 chapters in this volume report on the first stage of an ongoing multidisciplinary study of the Charleston earthquake of 1886. The Modified Mercalli intensity for the 1886 earthquake was X in the meizoseismal area, an elliptical area 35 by 50 km, the center of which was Middleton Place. Seismic activity is continuing today in the Middleton Place-Summerville area at a higher level than prior to 1886. The present seismicity is originating at depths of 1 to 8 km, mostly in the crystalline basement beneath sedimentary rocks of the Coastal Plain. The crystalline basement beneath the Charleston-Summerville area is not simply a seaward extension of crystalline rocks of the Appalachian orogen that are exposed in the Piedmont to the northwest, but has a distinctive magnetic signature that does not reflect Appalachian orogenic trends. The area underlain by this distinctive geophysical basement, the Charleston block, may represent a broad zone of Triassic and (or) Jurassic crustal extension formed during the early stages of the opening of the Atlantic Ocean. The Charleston block is characterized in part by prominent, roughly circular magnetic and gravity highs that are thought to reflect maflc or ultramafic plutons. A continuously cored borehole put down over the shallowest (about 1.5 km deep) of these magnetic anomalies on the edge of the meizoseismal area bottomed at 792 m in amygdaloidal basalt. Although the K-Ar ages of about 100 m.y. for the basalt are consistent with the Late Cretaceous (Cenomanian) age of the overlying Cape Fear Formation, this must be a minimum age as a result of chemical alteration. The interpreted magmatic composition of the basalt most closely resembles the high-Ti quartz-normative tholeiites of Late Triassic and Early Jurassic age from eastern North America; age of the basalt is probably similar. Various geophysical surveys suggest that Coastal Plain sedimentary rocks do not simply dip homoclinally to the southeast on a gently dipping basement surface but are disturbed by structures not yet clearly deciphered. The present stress regime of the Charleston-Summerville area appears to be one of NE-SW. compression rather than of extension as it presumably was in the Mesozoic. The present stress regime seems similar to that of much of the eastern United States. Comparison of several seismic source areas in eastern North America shows that epicenters are typically near the periphery of positive gravity features interpreted to represent mafic or ultramafic bodies. Earthquakes may be caused by the concentration of regional stress around the peripheries of these inhomogeneities in an otherwise more homogeneous plate. Whether the inhomogeneities are more or less rigid than the. surrounding material is uncertain. PART B: In 1889, C. E. Dutton published all his basic intensity data for the 1886 Charleston, S.C., shock but did not list what intensity values he assigned to each report, nor did he show the distribution of the locations of these data reports on his isoseismal map. The writer and two other seismologists have each independently evaluated Dutton's 1,300 intensity reports (at least two of the. three interpreters agreed on intensity values for 90 percent of the reports), and the consensus values were plotted and contoured. One map was prepared on which contours emphasized the broad regional pattern of effects (with results similar to Dutton's) ; another map was contoured to depict the more