Soil Effects on Earthquake Ground Motions in the Memphis Area

Keywords: Lifelines, Infrastructure, Dams, Bridges, Cohesionless Soils, Memphis, Tennessee, Shelby County, Tennessee, Computer Programs, New Madrid Seismic Zone, New Madrid Type Earthquakes, Soil Properties, Cohesive Soils, MASH, Soil Models, Amplification, Sands, Clays, and Silt.

Abstract: This report presents a thorough microzonation study of the Memphis area using state-of-practice methods. The authors have processed 424 soil logs out of 8,500 existing boring logs using the MASH computer program. A dynamic soil model is established for each soil log and then excited by an acceleration time history at the bedrock level resulting from a moment magnitude 7.5 New Madrid earthquake. The low-strain site period estimated from average shear wave velocity of a soil profile and the dynamic site period, at which the maximum spectral accelerations ratio occurs, are determined and shown in contour maps. The results of the site response analysis indicate that the soils have significant effects on ground motions in Memphis and Shelby County. The soil deposit acts as a filter when the bedrock earthquake motions are transmitted through it. The soil deposit filters out a significant portion of high frequency contents of the bedrock accelerations. On the other hand, it strongly amplifies the bedrock spectral accelerations between 0.15 and 1.4 seconds. This amplification is important in engineering applications since most structures have fundamental periods in this range.