Five more were 7.7's. Ten were 6.7's. Thirty five were in the range of 5.9. Some researchers claim the big ones were no more than 7's or even 6's on the Richter scale. There were no seismographs then.
Researchers estimate the strength by studying damage accounts.
Knox and Stewart say the tectonic plate under the New Madrid area was almost pulled apart 600 million years ago, and again 200 million years ago. If it had been, New Madrid would be on the Atlantic coast, and the southeastern US might be attached to Africa. The New Madrid Fault now rattles its area 150 or more times a year. Most of the shakes are less than 3.0 on the Richter scale, cause no damage, and may be felt only for a few miles. A six reading on the Richter scale is serious within 50-100 miles.