Regional variations in near surface shear wave velocity in the Greater Memphis area

Abstract

Site-specific and regional analyses were conducted for an area that includes southwestern Tennessee, eastern Arkansas, and northwestern Mississippi to establish characteristic shear wave velocity (Vs) profiles based on measured profiles compiled from several researchers. Site-specific studies were conducted at two locations near Memphis, Tennessee to determine the uncertainty and randomness in measuring Vs due to test-related factors and local variability. The result of this analysis was subsequently used as a criterion for establishing the range of variability for the characteristic Vs profiles. Measured Vs profiles were aggregated based on the age and type of geologic deposits in the Memphis area to obtain characteristic profiles. Holocene alluvial deposits were found to have a fairly uniform shear wave velocity profile in the Mississippi River floodplain. Pleistocene deposits characterized by a surficial layer of loess in the terrace regions of Mississippi and Tennessee were found to be more variable.The average shear wave velocity in the upper 30m was calculated for each measured profile based on the recommendations of the 1997 National Earthquake Hazards Reduction Program (NEHRP) provisions to assess the susceptibility of a deposit to ground motion amplification. The lowest and highest average near-surface shear wave velocities were identified in the Mississippi River alluvium and the Pleistocene deposits, respectively. Based on the NEHRP site classes, all deposits may be classified as D (stiff soil). However, due to the thickness of soft soil deposits in the alluvial plain, these deposits may be classified as E (more than 3m of soft clay) based on the site class description rather than the shear wave velocity. Therefore, a re-evaluation of the NEHRP site classes is proposed for the Central US to more accurately represent the actual site conditions.