The New Madrid seismic zone is the most important regional seismic hazard in middle America, having produced several very large earthquakes in historic and geologic time. In this study we simulate time histories in Memphis, Tennessee, and St. Louis, Missouri, for M 7.5 and M 8.0 earthquakes along two faults in the New Madrid seismic zone. Simulations are based on a well-established and calibrated finite-fault simulation program (FINSIM), which has been shown to reproduce ground motions for earthquakes of M 4-8 in eastern and western North America (Beresnev and Atkinson, 2001a). Simulations are made for representative soil profiles for each city, as well as for reference bedrock conditions (at the base of the soil profile); the effect of nonlinearity on soil amplification is considered for the soil sites. Uncertainty in the results is considered by modeling a wide range of alternative scenarios, representing the major uncertainties in input parameters. The total uncertainty in ground motions (response spectra) is about a factor of 2. Results are most sensitive to magnitude, hypocenter location (directivity effects), and maximum slip velocity. The resulting time histories and spectra for all modeled scenarios are available as an electronic supplement to this article.
Results are validated to the extent possible using the modified Mercalli intensity (MMI) observations from the 1811-1812 earthquakes. The simulated motions are consistent with MMI observations at St. Louis and Memphis. It is not possible to distinguish, based on the MMI observations, which of the M 7.5 or M 8.0 scenarios are most likely. However, we conclude that the most extreme scenarios, such as M 8.0 with high slip velocity or large directivity effects, are not likely because they would produce higher MMI values than those that were observed.
Online material: time histories and spectra for modeled scenarios.