NGA Model for Average Horizontal Component of
Peak Ground Motion and Response Spectra
Brian S.-J. Chiou
California Department of Transportation
and
Robert R. Youngs
AMEC Geomatrix

PEER 2008/09
NOVEMBER 2008

ABSTRACT
We present a model for estimating horizontal ground motion amplitudes caused by shallow
crustal earthquakes occurring in active tectonic environments. The model provides predictive
relationships for the orientation-independent average horizontal component of ground motions.
Relationships are provided for peak acceleration, peak velocity, and 5-percent damped pseudospectral
acceleration for spectral periods of 0.01–10 sec. The model represents an update of the
relationships developed by Sadigh et al. (1997) and includes improved magnitude and distance
scaling forms as well as hanging-wall effects. Site effects are represented by smooth functions of
average shear-wave velocity of the upper 30 m (VS30) and sediment depth. The new model
predicts median ground motion that is similar to Sadigh et al. (1997) at short spectral periods, but
lower ground motions at longer periods. The new model produces slightly lower ground motions
in the distance range of 10–50 km and larger ground motions at larger distances. The aleatory
variability in ground motion amplitude was found to depend on earthquake magnitude and on the
degree of nonlinear soil response. For large-magnitude earthquakes, the aleatory variability is
larger than found by Sadigh et al. (1997).