Reevaluation of the Magnitudes of Three Destructive Aftershocks of the 1886 Charleston Earthquake

A review of newspaper reports of the three largest aftershocks of the August 31, 1886 Charleston earthquake, which occurred on October 22, 1886 (5:20 am and 2:45 pm local time) and on November 5, 1886 (12:20 pm local time), documents that they were felt over larger areas than those listed in current catalogs. We estimated felt areas and area within various isoseismal levels. Using recursion relationships of Johnston (1996) between intensity areas and their seismic moments, we have calculated their moment magnitudes. Our results give Mw of 5.1, 5.7, and 5.3 for the three events, significantly greater than the mb 4.4, 4.7, and 4.4, respectively, listed in current catalogs. These new magnitude estimates are probably more reasonable values for the largest aftershocks of the M 7.3 mainshock.