Reference rock site condition for central and eastern North America
23 August 2013
The reference rock site condition is defined by the shear (S)- and compression (P) -wave velocities, as well as a site attenuation parameter. The significance of the reference rock is that it represents the site condition for which ground motions will be estimated using semi-empirical ground motion prediction equations (GMPEs). Moreover, it is the site condition where site amplification are referenced to (i.e., site amplification is unity for reference rock). This paper presents the recommendation of a reference S-wave velocity, a reference P-wave velocity, and S-wave site attenuation parameter, associated with a reference rock site condition in Central and Eastern North America (CENA), supported by results of a comprehensive literature search and additional studies.
The work is part of the Next Generation Attenuation Relationships for Central and Eastern North America (NGA-East) Project. The recommended S- and P-wave velocities are 3,000 and 5,500 m/sec, respectively. Considering a +/- 5% impact on the site amplification, ranges for the reference S- and P-wave velocities of 2,700 to 3,300 m/sec and 5,000 to 6,100 m/sec, respectively, are recommended. The recommended reference rock site kappa is 0.006 sec. The uncertainty in the site kappa is described using a log-normal distribution with an aleatory standard deviation of 0.43, and epistemic standard deviations of 0.12, when uncertainty in source, path, and site-amplification effects are included in simulations used to develop ground motion prediction equations, and 0.20, when they are not.
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