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

Figure 13: Coefficients resulting from fits of Equations (8), (9), and (10) to the broad band data for
earthquakes 0163, 0167, and 0170. T is spectral period in seconds.
The form of Equation (10) has a sharp break in the slope of the attenuation curve. This form
appears to work well for individual earthquakes. However, the ground motion model
developed in this study is intended to apply over all of California and includes data from a
number of active tectonic regions. Therefore, Equation (10) was modified to introduce a
smooth transition from near-source to far-source attenuation rates. The selected form for the
distance attenuation function for the ground motion model developed in this study is:
2 2
[ R + C cosh{
C max( M − 3,
0)
} ] + ( −0.
5 − C ) ln R + R + γ ( M)
ln( y ) ∝ C ln RUP
RUP B (11)
4
5
6
This form incorporates magnitude-dependent extended source effects, potentially magnitudedependent
wave propagation effects on responds spectra at large distances and a smooth
transition from dominance of ground motions by direct waves at small distances to
dominance by Lg waves at large distances.
Source-Site Geometry Effects: Analyses of ground motions from thrust earthquakes by
Somerville and Abrahamson (1995) and Abrahamson and Somerville (1996) proposed the so
called “hanging wall” effect in which ground motions are enhanced in the hanging wall of
C&Y2006 Page 22
4