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

thrust earthquakes. The effect was attributed to the inability of the RRUP distance measure to
capture the general proximity of a hanging wall site to rupture on the fault plane dipping
beneath it. Chiou et al. (2000) performed extensive analyses of empirical and numerical
modeling data for dipping fault ruptures and reached the same conclusion. They were able to
remove the effect by using a root-mean-squared distance measure, RRMS.
The hanging wall effect is seen in the PEER-NGA data. Figure 14 shows intra-event
residuals for data from dip-slip earthquakes (both reverse/thrust and normal) plotted versus
horizontal distance from a point above the top of the fault rupture. The dashed line is a
locally-weighted least-squares fit to the residual. It shows an upward warp in the region
where the hanging wall effect is expected to be present. The effect is also seen in the 1-D
rock ground motion simulations conducted for the PEER-NGA project (Somerville et al.,
2006).
Figure 14: Intra-event residual for sites in the hanging wall (positive fault tip distance) and footwall
(negative fault tip distance) from dip-slip earthquakes. Black dashed line is a locally-weighted leastsquares
(lowess) fit to the residuals.
Abrahamson and Silva (1997) included the hanging wall effect in their empirical ground
motion model as a distance-dependent term with an abrupt switch from no effect to full effect
as one crosses into the hanging wall region, defined as sites with a source-site angle, θSITE,
between 60º and 120º. Boore et al. (1997) conclude that their use of RJB implicitly accounts
for the hanging wall effect in that all site directly above the rupture have RJB = 0. Campbell
and Bozorgnia (2003) introduces a smooth variation in the hanging wall effect by tapering
the effect from a maximum at RJB = 0 to zero for RJB > 5 km while using RSEIS as the primary
distance measure for assessing ground motion amplitudes.
We have extended the concept proposed by Campbell and Bozorgnia (2003) by defining the
hanging wall effect to be proportional to the factor [ 1−
RJB
/ RRUP
] . Figures 15 through 17
illustrate the behavior of this function. Figure 15 shows a map view of a fault rupture with
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