USGS hazard map compatible depth-dependent seismic site coefficients for the Upper Mississippi Embayment

Integrated probabilistic seismic hazard and site response analysis procedure, denoted as 2014 PSHA-NL, are performed to derive probabilistic site coefficients of the Upper Mississippi Embayment (UME). The site coefficients are determined for upland and lowland profiles, and NEHRP site classes C, D, and E. The probabilistically derived coefficients are compatible with 2008 USGS hazard maps and account for the thickness of the embayment. Randomized soil profiles and four sets of pressure-dependent dynamic soil curves are used to account for the uncertainties in the dynamic soil properties. It is shown that NEHRP coefficients are only applicable for site class D soil columns less than 50 m in thickness. NEHRP coefficients greatly underestimate short-period site coefficient (F_a) for site class C profiles and overestimate F_a for site classes D and E exceeding 100 m in thickness. Long-period coefficient (F_v) is shown to have a higher depth-dependency compared to Fa. For site classes C and D, the calculated site coefficients are larger than NEHRP values for all soil columns exceeding 50 m. The pronounced depth-dependency and discrepancy with the NEHRP coefficients highlight the need to use UME specific site coefficients in assessing the seismic hazard in the embayment.