Results and Implications of the Next Generation Attenuation (NGA) Ground Motion Project (Abstract)

by Norman Abrahamson

Abrahamson, Norman (2007). Results and Implications of the Next Generation Attenuation (NGA) Ground Motion Project (Abstract). SMIP07 Seminar on Utilization of Strong-Motion Data, p. 107 - 108.


The NGA project has developed give new ground motion attenuation models for application to California. The models are applicable to all relevant shallow crustal earthquakes in California: M5-8.5 for strike-slip earthquakes and M5-8.0 for reverse earthquakes; distance of 0-200 km; strike-slip, reverse, and normal mechanisms. The models are also defined for spectral periods up to 10 seconds. As a result, the user of the models does not need to extrapolate them for most applications.

Key changes from the previous models include the use of VS30 for the site condition, inclusion of a depth of rupture factor, inclusion of hanging wall factors, and inclusion of depth of soil factors. All of the models include non-linear site response effects. Three of the models include the effects of the soil non-linearity on the standard deviation.

Comparisons of the differences between the five NGA models and between the NGA models the previous models that they are replacing are shown. The new models show a reduction in the median ground motion close to large earthquakes and an increase in the standard deviation for large earthquakes. There is an increase in the short period median ground motion for sites over the hanging wall of thrust earthquakes. Overall, the NGA models lead to reduced design ground motions based on IBC procedures. About half of the reduction is due to use of VS30, consistent with the building code, rather than using generic rock models.