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

Local soil conditions have a profound influence on the characteristics of g round shaking during an earthquake. Exceptionally deep soil deposits, on th e order of 100-1000 m deep, are found in the Upper Mississippi Embayment of the central United States. Shear waves (SH) from earthquakes in the New Ma drid seismic zone are expected to be strongly affected by the sharp impedan ce contrasts at the bedrock/sediment interface, attenuation of seismic wave s in the soil column, and the SH-wave velocities of the more poorly consoli dated near-surface (less than or equal to 50 m) soils. SH-wave velocities of the near-surface soils at nearly 400 sites in the Upp er Mississippi Embayment were determined using conventional seismic SH-wave refraction and reflection techniques. Based on the average SH-wave velocit ies of the upper 30 m of the soils, sites in the Mississippi River floodpla in portion of the study area are predominantly classified as Site Class D ( 180-360 m/s) in accordance with the 1997 NEHRP provisions. Sites away from the active floodplains in western Kentucky and western Tennessee, the SH-wa ve velocities of the upper 30 m of soils typically ranged from mid-200 to m id-300 m/s. Several sites in western Kentucky had averaged SH-wave velociti es greater than 360 m/s, thereby qualifying them as Site Class C (360-760 m /s) in accordance with the 1997 NEHRP provisions. One-dimensional site effects, including amplification and dynamic site peri od, were calculated for a representative suite of sites across the Upper Mi ssissippi Embayment at latitude similar to 38.5 degrees. Although seismic a ttenuation is greater in the Mississippi River floodplain (i.e. thicker, lo wer velocity material), the site effects tend to be greater than in the upl ands of western Tennessee because of larger impedance contrasts within the near-surface soils. (C) 2001 Elsevier Science B.V. All rights reserved.