PEER has just published Report No. 2014/11 titled “Reference-Rock Site Conditions for Central and Eastern North America: Part I – Velocity Definition” as a new addition to the PEER Report Series. It was authored by Youssef M.A. Hashash, Albert R. Kottke, Jonathan P. Stewart, Kenneth W. Campbell, Byungmin Kim, Ellen M. Rathje, and Walter J. Silva.
As part of the Next Generation Attenuation Relationships for Central and Eastern North America (NGA-East) Project, we have gathered data to define an appropriate reference-rock site condition for CENA. The reference-rock site condition is defined by the S- and P-wave velocities, as well as the local crustal damping (K0). This report deals solely with the definition of the reference P- and S-wave velocities. The significance of the reference-rock definition is that it represents the site condition for which ground motions will be predicted using semi-empirical ground motion prediction equations (GMPEs). Moreover, it represents the site condition to which site amplification factors are referenced (i.e., site amplification is unity for reference rock). There are significant differences in the reference-rock site conditions between active tectonic regions, such as Western North America (WNA), and mid-plate regions, such as Central and Eastern North America (CENA).
Using velocity measurements reported in the license applications at nuclear power plants as well as published data, we developed criteria to assess the presence of the reference-rock site condition that is based on the seismic velocities and their gradient with respect to depth. We apply the criteria to the available profiles, which are included with this report, from which we recommend that seismic velocities for reference rock in CENA be defined as follows:
• Vs,ref = 3000 m/sec or 9800ft/sec (2700 to 3300 m/sec or 8900 to 10,800 ft/sec)
• Vp,ref =5500 m/sec or 18,000 ft/sec. (5000 to 6100 m/sec or 16,400 to 20,000 ft/sec)
The range given for seismic velocities is based on a ±5% change in amplification using quarter wavelength theory.
We do not find evidence for regional dependence of the reference velocities, which are derived principally from three general geographic regions: (1) the Atlantic Coast; (2) continental interior; and (3) Appalachian Mountains. Our data do not provide reference velocities for the Gulf Coast region. In this region the depth to the CENA reference-rock condition is expected to be much greater than other CENA regions due to several kilometers of overlying sediments. We do not provide a reference-rock condition for the Gulf Coast. Our recommendation is to adopt a consistent reference-rock condition for the entire CENA region, as given above, and then estimate transfer functions to a softer reference condition (such as 760 m/sec) for application of the NGA-East GMPEs.