Chiou and Youngs PEER-NGA Empirical Ground Motion Model for ...

ecorded within 70 km of earthquake ruptures, the region of primary concern for application
of the model. We also believe that the model properly represents the attenuation of ground
motion at larger distances in California. The issue we have raised points out the need to
systematically collect and process all recordings from large earthquakes if there is interest in
correctly modeling the attenuation of ground motions at large distances.
Equation (19) provides a relationship for γ for pga. The relationships for other spectral
periods was constructed by using the period-dependence of γ computed from analysis of the
broadband data from the three small southern California earthquakes and scaling the values
by the relative difference in γ at pga, that is:
γ ( pga)
California
γ ( T ) California = γ ( T ) Anza−
YL−BBC
×
(20)
γ ( pga)
Anza−YL−BBC
Figure 30 compares the model developed for γ with values obtained from direct regression of
the PEER-NGA data without imposing any distance truncation and without using truncated
regression. The variation of γ with spectral period T is similar.
Figure 30: Comparison of the model developed for γ (Equations 19 and 20) versus values of γ
obtained from regression without truncation (black points) of the PEER-NGA data set.
MODELING STEPS
The parameters of the ground motion model were developed through an iterative process that
involved performing regressions for the entire spectral period range with some parts of the
model fixed, examining the trend of remaining parameters with spectral period, developing
smoothing models for these parameters with period, and then repeating the analysis to
examine the variation of the remaining parameters. The major steps in the development of
the parameters are summarized below. All analyses were performed using the PEER-NGA
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