ALBERT

Description: The reference rock site condition has two important applications for ground-motion prediction in the stable continental region of central and eastern North America (CENA). (1) It represents the site condition for which ground motions are computed using semiempirical ground-motion prediction equations. In addition, (2) it represents the site condition to which site amplification factors, which are used to modify ground-motion intensity measures for softer site condition, are referenced (i.e., site amplification is unity for reference rock). We define reference rock by its shear ( S )- and compression ( P )-wave velocities, as well as a site attenuation parameter ( 0 ), which is used in stochastic ground-motion simulation methods. Prior definitions of reference rock conditions in CENA were based mostly on indirect large-scale crustal velocity inversions and judgment. We compile and interpret a unique database of direct velocity measurements to develop criteria for assessing the presence of reference rock site condition based on measured seismic velocities and their gradient with respect to depth. We apply the criteria to available profiles and perform rigorous statistical analysis from which we recommend S - and P -wave velocities of 3000 and 5500 m/s, respectively, for the reference rock condition. We recommend that, for practical applications, use ranges of reference S - and P -wave velocities of 2700–3300 m/s and 5000–6100 m/s, respectively. The ranges are 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) Atlantic coast, (2) continental interior, and (3) Appalachian Mountains. Our data do not provide reference velocities for the Gulf Coast region. The recommended velocity-compatible reference rock site kappa is 0.006 s.