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Estimates of Shear-Wave Q and ₀ for Unconsolidated and Semiconsolidated Sediments in Eastern North America

EQECAT, Inc., 1030 NW 161st Place, Beaverton, Oregon 97006 KCampbell{at}eqecat.com

Measured and calculated values of the effective quality factor Q_ef and the site attenuation parameter ₀ for unconsolidated and semiconsolidated sediments in eastern North America (ENA) indicate that the latter is strongly dependent on sediment thickness. Estimates of ₀ 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 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 ₀ estimates compiled in this study indicates that corresponds to a relatively thick BC sediment thickness of . 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.