The magnitude of these series of earthquakes, usually named the New Madrid, Missouri, earthquakes, vary considerably between the mb and Ms values estimated by Nuttli. The mb was estimated from isoseismal maps, and the MS was estimated from a spectral scaling relation by Nuttli for mid-plate earthquakes. The value of MS magnitude has a functional relationship to the mb. The authors have chosen to include the Mfa magnitude because it was estimated from isoseismal maps, as were most of the historical earthquakes.
The first and second earthquakes occurred in Arkansas (December 16, 1811 - two shocks - Mfa 7.2, MSn 8.5 and Mfa 7.0, MSn 8.0) and the third and fourth in Missouri (January 23, 1812, Mfa 7.1, MSn 8.4; and February 7, 1812, Mfa 7.4, MSn 8.8). Otto Nuttli, however, has postulated another strong earthquake in Arkansas on December 16 at 18:00 UTC (MSn 8.0). This would make a total of five earthquakes of magnitude MSn 8.0 or higher occurring in the period December 16, 1811 through February 7, 1812.
http://earthquake.usgs.gov/regional/states/events/1811-1812.php#december_16
More recent work has been done by Susan Hough:
http://pasadena.wr.usgs.gov/office/hough/NMintro.html
The 1811-1812 New Madrid Earthquakes
The earthquake sequence that struck the New Madrid region of the Mississippi Valley in 1811-1812 had remarkably far-reaching effects. Ground motions from the three principal events were felt in places as far away as Canada, New England, and the coast of South Carolina. Contemporary accounts document three principal events: approximately 02:15 (local time) on December 16, 1811; around 08:00 am on January 23, 1812, and approximately 03:45 am on February 7, 1812 (henceforth NM1, NM2, and NM3, respectively). All three events were felt throughout much of the central and eastern United States. Additionally, a large aftershock to NM1 occurred near dawn on December 16, 1811. Substantial aftershock activity following all events was also documented, with felt events persisting through the following decade.
Paleoseismic investigations suggest a repeat time of the order of 400-500 years for the New Madrid events; they also suggest that the New Madrid Seismic Zone tends to produce prolonged sequences with multiple, distinct mainshocks, the magnitudes of which are comparable to those of the 1811-1812 events. Thus the magnitude of the earthquakes becomes a critical issue for an understanding of intraplate earthquake processes.
The New Madrid earthquakes have considerable societal as well as scientific importance. A repeat of the 1811-1812 sequence would clearly have a tremendous impact on the present-day midcontinent region. The New Madrid Seismic Zone contributes a nontrival component of seismic hazard in relatively distant midwestern cities such as Saint Louis, Missouri, and Cincinnati, Ohio. Yet the 1811-1812 sequence has remained vexingly enigmatic because of several factors: (1) the lack of instrumental data; (2) our generally limited knowledge of intraplate earthquakes, and (3) the geology of the New Madrid/Mississippi Embayment region, which effectively obscures most surface expression of faulting.
Because an evaluation of the magnitudes of the 1811-1812 events is so critical to a determination of long-term seismic hazard in the region, tremendous effort and ingenuity have been invested in gleaning quantitative information from the limited available data. Available data include (1) paleoliquefaction features preserved by the sediments within the Mississippi embayment, (2) the present-day distribution of seismicity in the New Madrid Seismic Zone, which is assumed to illuminate the principal fault zones, (3) modern estimates of the long-term deformation/strain accumulation in the region, (4) first-hand reports ("felt reports") of the shaking and/or damage caused by the events over the central/eastern United States.
Determination of magnitudes for the 1811-1812 mainshocks hinges on the felt reports and their interpretation for modified Mercalli intensity (MMI) values, which provide the only direct constraint on magnitude. In 1973, Otto Nuttli drew isoseismal contours based on his compilation of approximately 40 felt reports. He determined mb values of 7.2, 7.1, and 7.4 for NM1, NM2, and NM3, respectively, based on a relationship between ground motion and intensities from smaller and more recent instrumentally recorded earthquakes in the central United States.
Subsequent efforts have been made to improve the magnitude estimates of the New Madrid mainshocks. Values as high as M8.75 were obtained for the largest shocks. In the 1990s Arch Johnston developed a method with which the overall shaking intensity from historical events could be compared with that from more recent, instrumentally-recorded earthquakes. This improved method yield magnitude values closer to M8.0 for the three principal events.
The Hough et al. (2000) study further refines the magnitude estimates downward, to M7.0-7.5 for the three principal mainshocks, based on a reinterpretation of the historical accounts. These values are more consistent with magnitude estimates derived from available indirect evidence. However, it bears mention that, even with these "reduced" magnitudes, the New Madrid earthquakes were still enormously powerful events. The February event in particular was as large as the devastating 1999 (Izmit) Turkey earthquake, and not much small than the 1999 Taiwan (Chi-Chi) earthquake.
I live, I learn. Actually, I remember the 64 Alaska quake quite well. Must have had a senior moment when I repeated the statement I read on another site. (sigh)