ALBERT

Description: We determined the stress parameter, Delta sigma , for the eight earthquakes studied by Atkinson and Boore (2006), using an updated dataset and a revised point-source stochastic model that captures the effect of a finite fault. We consider four geometrical-spreading functions, ranging from 1/R at all distances to two- or three-part functions. The Delta sigma values are sensitive to the rate of geometrical spreading at close distances, with 1/R (super 1.3) spreading implying much higher Delta sigma than models with 1/R spreading. The important difference in ground motions of most engineering concern, however, arises not from whether the geometrical spreading is 1/R (super 1.3) or 1/R at close distances, but from whether a region of flat or increasing geometrical spreading at intermediate distances is present, as long as Delta sigma is constrained by data that are largely at distances of 100 km-800 km. The simple 1/R model fits the sparse data for the eight events as well as do more complex models determined from larger datasets (where the larger datasets were used in our previous ground-motion prediction equations); this suggests that uncertainty in attenuation rates is an important component of epistemic uncertainty in ground-motion modeling. For the attenuation model used by Atkinson and Boore (2006), the average value of Delta sigma from the point-source model ranges from 180 bars to 250 bars, depending on whether or not the stress parameter from the 1988 Saguenay earthquake is included in the average. We also find that Delta sigma for a given earthquake is sensitive to its moment magnitude M, with a change of 0.1 magnitude units producing a factor of 1.3 change in the derived Delta sigma .

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 (kappa (sub 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 + or -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.

Description: Ground-motion models (GMMs) and ground-motion adjustment factors developed using the hybrid empirical method (HEM) are used in seismic-hazard analyses throughout the world as an alternative to GMMs developed from the more traditional empirical and simulation methods. The HEM uses the ratio of stochastic ground-motion simulations between a target and host region to adjust empirical GMMs from the host region to use in the target region. The HEM is used primarily in regions where strong-motion data are sparse or exist only for small-magnitude earthquakes. The most common application of the HEM has been in the development of GMMs for eastern North America (ENA), two of which were used in the 2008 U.S. national seismic-hazard maps, but the method also has been used to develop or adjust GMMs in many other regions of the world. A comparison of four ENA GMMs developed using the HEM and a fifth developed using the closely related referenced empirical approach show that they fall into three distinct groups based on differences in the methods, models, and parameters used to calculate the host-to-target adjustment factors, and on differences in the selection of the host empirical ground-motion models. A different set of groups are implied from the aleatory variability models. General guidance is provided to aid the user in the selection and weighting of the five GMMs for application in seismic-hazard analysis.