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

Abstract

Measured and calculated values of the effective quality factor Q ef and the site attenuation parameter κ 0 for unconsolidated and semiconsolidated sediments in eastern North America (ENA) indicate that the latter is strongly dependent on sediment thickness. Estimates of κ 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 κ 0 20 msec 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 κ 0 estimates compiled in this study indicates that κ 0 20 msec corresponds to a relatively thick BC sediment thickness of 460 116 m. 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. * Note: Values in italics represent the BC section of the sedimentary column. The values for Q ef were taken from data provided by Cramer et al. (2004) and Boore and Joyner (1991); all other data were taken from Gomberg et al. (2003) and Cramer et al. (2004). Intermediate Q ef values are intermediate to those of Cramer and Boore and Joyner. The values of κ 0 for each layer were calculated from equation (10).

A Study on the Variability of Kappa ( ) in a Borehole: Implications of the Computation Process

“…This site effect is comparable to what was found in other studies that compared downhole to surface κ estimates: Oth et al (2011) andVan Houtte et al (2011) both found a difference of about 50% on average between surface records and downhole rock at depths of 100 or 200 m. In any case, the fact that even at rock level (178 m) we still find a considerable κ 0 value is an indication of the deeper origin of κ. Indeed, both the seminal paper of Anderson and Hough (1984) as well as much more recent works (Campbell, 2009;Van Houtte et al, 2011) suggest that κ 0 may be affected by underlying formations not only down to a few hundreds of meters but as deep as a few kilometers.…”

“…After revisiting the ANZA network where the notion of κ started, and comparing four definitions of κ, Kilb et al (2012) find it to be site dependent with scatter that is source dependent. Because of its empirical nature, the debate as to the origins and physical meaning of κ is still ongoing, as is the debate for its correlation to more rigorously defined parameters such as Q (see the comprehensive paper of Campbell, 2009, for a fuller background review).…”

Reference Rock Site Condition for Central and Eastern North America

“…One might expect site attenuation in this region to be greater than that for hard rock (e.g., Chandler et al, 2006). Figure 21 shows mean values of hard-rock κ 0 in CENA from various sources in the literature, which range from 0.002 to 0.009 s (Atkinson, 1984(Atkinson, , 1996Toro and McGuire, 1987;Silva et al, 1988;Silva and Darragh, 1995;Chapman et al, 2003;Atkinson and Boore, 2006;Campbell, 2009). Also shown are mean κ 0 values derived from WNA recordings on hard rock, which range from 0.0003 to 0.01 s (Hough et al, 1988;Anderson, 1991;Silva and Darragh, 1995;Atkinson, 1996;Campbell, 2009).…”

“…The site attenuation governs the high-frequency characteristics of the ground motion at site-source distances lower than about 50 km, where crustal damping effects are small. More background on κ 0 , the procedures used to derive it from data, and recommended values for various site conditions are given in Campbell (2009) and Campbell et al (2013).…”

Taxonomy of : A Review of Definitions and Estimation Approaches Targeted to Applications

“…Its adequacy as a proxy for site attenuation, which is tied to the first few km of the profile (Anderson and Hough, 1984;Campbell, 2009), is questionable. From a physical viewpoint, we would also expect other parameters to correlate to κ 0 , possibly tied to the deeper structure.…”