The 2003 Memphis, Shelby County, Tennessee, Seismic Hazard Maps


Memphis, Shelby County, Tennessee is built on a 1-kilometer-thick sequence of sediments deposited in a trough known as the Mississippi embayment. This thick pile of sediments significantly affects earthquake ground motions.  New ground-motion (seismic hazard) maps for the area account for these effects. These new maps complement the USGS national seismic hazard maps, which do not include the effects of local geologic structure. Otherwise both sets of maps use the same information and methodology. We emphasize that the Memphis maps are still regional in nature and should not be used for site-specific analyses. 

 

Probabilistic maps express the hazard in terms of the levels of horizontal ground shaking that have a specified chance of being exceeded in a given time period. Our maps display the hazard for 2%,  5%, or 10% probabilities of exceedance in 50 years and characterize the ground shaking in terms of the maximum or peak acceleration (PGA) and the spectral acceleration (Sa) at 0.2 second and 1.0 second periods.


Because the strongest ground-motions are deamplified by sediments at high frequencies, the 2% in 50 year PGA and 0.2 second Sa maps show an overall decrease in ground-motion hazard across the study area relative to the national seismic hazard maps.   A thickening of the sediments under Shelby County toward the west causes more deamplification of high frequencies than to the east. As a result, the PGA and 0.2 second Sa ground motions are more uniform across the six-quadrangle study area than those in the national maps.


At lower frequencies the thick sediments under Memphis amplify motions in the 2% in 50 year 1.0 s Sa map.   For 1.0 s Sa, thicker sediments in the west tend to amplify long-period ground motions more than in the east and the hazard maps retain a decreasing gradient to the southeast, away from the largest earthquake sources.