We estimate locations and moment magnitudes M and their uncertainties for the three largest events in the 1811–1812 sequence near New Madrid, Missouri, and for the 1 September 1886 event near Charleston, South Carolina. The intensity magnitude MI, our preferred estimate of M, is 7.6 for the 16 December 1811 event that occurred in the New Madrid seismic zone (NMSZ) on the Bootheel lineament or on the Blytheville seismic zone. MI is 7.5 for the 23 January 1812 event for a location on the New Madrid north zone of the NMSZ and 7.8 for the 7 February 1812 event that occurred on the Reelfoot blind thrust of the NMSZ. Our preferred locations for these events are located on those NMSZ segments preferred by Johnston and Schweig (1996). Our estimates of M are 0.1–0.4 M units less than those of Johnston (1996b) and 0.3–0.5 M units greater than those of Hough et al. (2000). MI is 6.9 for the 1 September 1886 event for a location at the Summerville–Middleton Place cluster of recent small earthquakes located about 30 km northwest of Charleston.