Seismicity patterns and the `characteristicʹ earthquake in the Central Mississippi Valley, USA

The magnitude-frequency distribution of the Armbruster–Seeber (1992) catalog of the Central Mississippi valley earthquakes from 1811 to 1985 can be well described as a linear combination of three earthquakes populations each of which fits a normal distribution with respect to magnitude. The mean magnitudes of the lower magnitude populations are consistent with previous work based on a 1974–1991 catalog of the same area. The presence of the high magnitude population, mean M=7.2, accounts for the development of a characteristic earthquake pattern. This pattern is very dependent on the assumed activity of the 7.2 population. The operating basis earthquake (144 years) of interest to structural engineers occurs in the region of greatest sensitivity to those assumptions about activity, thus complicating matters for design.