Abstract

Measured and calculated values of the effective quality factor Qef and the site attenuation parameter κ0 for unconsolidated and semiconsolidated sediments in eastern North America (ENA) indicate that the latter is strongly dependent on sediment thickness. Estimates of κ0 for National Earthquake Hazard Reduction Program (NEHRP) BC site profiles (sediment plus hard rock) in the Mississippi Embayment and the Atlantic Coastal Plain were found to increase from about 9 to 31 msec for sediment thicknesses ranging from 116 to 600 m. Stochastic simulations using the 175 m thick hypothetical NEHRP BC site profile used to estimate ENA ground motions in the national seismic hazard maps by the U.S. Geological Survey (USGS) indicate that forumla provides a smaller estimate of amplification that agrees more closely with the low-strain short-period site coefficients in the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (NEHRP Provisions) than the 10 msec value used by the USGS. A linear regression of the κ0 estimates compiled in this study indicates that forumla corresponds to a relatively thick BC sediment thickness of forumla. These same stochastic simulations indicate that the relatively shallow USGS site profile provides estimates of amplification that are smaller than the low-strain long-period site coefficients in the NEHRP Provisions. The dependence of both site attenuation and site amplification on sediment thickness suggests that the use of a single reference site condition for hazard mapping might not be appropriate. Instead, these results imply that either a regional set of reference site profiles should be developed or that a more uniform site condition such as hard rock should be used to define a more stable reference site condition in ENA.

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