NEHRP soil classifications for estimating site-dependent seismic coefficients in the Upper Mississippi Embayment

Abstract

Local soil conditions have a profound influence on the characteristics of ground shaking during an earthquake. Exceptionally deep soil deposits, on the order of 100–1000m deep, are found in the Upper Mississippi Embayment of the central United States. Shear waves (SH) from earthquakes in the New Madrid seismic zone are expected to be strongly affected by the sharp impedance contrasts at the bedrock/sediment interface, attenuation of seismic waves in the soil column, and the SH-wave velocities of the more poorly consolidated near-surface (≤50m) soils.SH-wave velocities of the near-surface soils at nearly 400 sites in the Upper Mississippi Embayment were determined using conventional seismic SH-wave refraction and reflection techniques. Based on the average SH-wave velocities of the upper 30m of the soils, sites in the Mississippi River floodplain portion of the study area are predominantly classified as Site Class D (180–360m/s) in accordance with the 1997 NEHRP provisions. Sites away from the active floodplains in western Kentucky and western Tennessee, the SH-wave velocities of the upper 30m of soils typically ranged from mid-200 to mid-300m/s. Several sites in western Kentucky had averaged SH-wave velocities greater than 360m/s, thereby qualifying them as Site Class C (360–760m/s) in accordance with the 1997 NEHRP provisions.One-dimensional site effects, including amplification and dynamic site period, were calculated for a representative suite of sites across the Upper Mississippi Embayment at latitude ∼38.5°. Although seismic attenuation is greater in the Mississippi River floodplain (i.e. thicker, lower velocity material), the site effects tend to be greater than in the uplands of western Tennessee because of larger impedance contrasts within the near-surface soils.