abstract

A study of the macroseismic effects associated with the two largest of Virginia's historic earthquakes, based primarily on journals and archival newspaper accounts, reveals that exceptionally large felt areas can be associated with moderate earthquakes in the middle Atlantic region. Occurring at opposite ends of the state, the two shocks exhibit similar effects to the north, which appear related to the surficial geology, but differing intensity distributions to the southwest, which may reflect the differences between the two areas that exist in the geological structure. Differences in focal depth and focal mechanism may also be contributing factors. The Giles County earthquake of May 31, 1897, in the folded Appalachian Province of western Virginia, is assigned a maximum intensity (MM) of VIII and a felt area of at least 280,000 square miles. The Chesterfield County earthquake of December 22, 1875, in the crystalline Piedmont Province near Richmond, had a maximum intensity (MM) of VII and a 50,000-square-mile felt region. In both cases the previously reported intensities and felt areas were found adequate. For the moderate seismicity of the Appalachians, an extremely long period, May 3 to June 6, of seismic and acoustic phenomena occurred in conjunction with the May 31, 1897 shock.

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