The earthquakes of 1811 and 1812 provide convincing evidence of the differences in ground motion to be expected for large-magnitude earthquakes in the Mississippi Valley seismic region, as compared to those which occur in the western United States. The combination of poor soil conditions in the epicentral area and of low attenuation of surface-wave energy produced damage and felt areas about 100 times greater than those of western North America earthquakes of the same magnitude.

Fortunately, the frequency of recurrence of earthquakes of the size of those that occurred in 1811 and 1812 evidently is low. None of that magnitude have occurred since, although there is continuing minor to moderate seismic activity in the area, which indicates that large-magnitude earthquakes can be expected there some time in the future. We have begun a study of the recurrence rate for the area, using the techniques described in this paper to assign magnitudes to the intermediate and smaller earthquakes of the past 150 years. Fuller (1912) found topographic and geological evidence of large-magnitude earthquakes predating the 1811-1812 sequence, e.g., cracks as large as any caused by the earthquakes of that sequence in which trees fully 200 years old grew from the bottoms and slopes. He also found recent faults and sandstone dikes filling old earthquake cracks.

The absence of large-magnitude earthquakes in eastern North America since the Charleston, S.C. earthquake in 1886 has resulted in complacency, or perhaps unawareness on the part of the general populace of the existence of any earthquake threat to them. When such earthquakes of the size of the 1811-1812 sequence recur, the emotional problems which will result for large numbers of people in the widely affected area will likely be severe, particularly if the earthquake energy is released over a long period of time, in the manner of the 1811 and 1812 earthquakes.