ASSESSING THE IMPORTANCE OF NEWLY DISCOVERED CENTRAL US FAULTS ON SEISMIC HAZARD

Recent geologic work near Memphis, Tennessee and in southeastern Arkansas suggests the presence of three Holocene faults, albeit with poorly constrained recurrence intervals. For one Memphis-area proposed fault, geologic evidence suggests that it may have ruptured about 400 AD. Paleoliquefaction data from southeastern Arkansas suggests magnitude (M) 5.5 to 6.0 earthquakes occur about every 1500 years.

We evaluate the significance of these proposed active-faults using two alternative hazard modeling approaches. In the first approach we include the proposed Meeman Shelby Forest, Ellendale, and SE Saline faults (the first two north and northeast of Memphis, and the latter in southeastern Arkansas) as characteristic ruptures on strike-slip faults in 2%-in-50y probabilistic seismic hazard maps. We examine the effect of 1,500 y, 3,000 y, 5,000 y, and 10,000 y recurrence intervals for these proposed faults on the USGS National Seismic Hazard Maps. In the second approach, we generated scenario seismic hazard maps to see the impact of reasonable magnitude earthquakes for these proposed faults on Memphis, Tennessee.

Unless it is included with other sources in a probabilistic assessment, a fault that generates an M 6 earthquake every 2500 years or more has no effect on 2%-in-50y probabilistic seismic hazard estimates. Including all three proposed faults as characteristic sources in the Frankel et al. (1996) seismic hazard model has minimal impact if recurrence intervals near 5,000 y are chosen. We conclude that slip rates exceeding 0.1 mm/y on any of these suggested faults are needed to affect probabilistic seismic hazard maps for the New Madrid area. On the other hand, scenario (deterministic) seismic hazard maps show that a M 6.5 earthquake on either of the two proposed faults near Memphis can generate peak ground accelerations in excess of 0.5 g over much of Shelby County as well as in downtown Memphis.