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

data are plotted as a function of station elevation in Figure C-2. Except for a few outliers,
the correlation of Vs30 with elevation is clearly shown, particularly within Geomatrix-D
category. The bimodal distribution observed in Figures 1 can be adequately modeled by
the station elevation.
Mapping Vs30 by C3 and Elevation
Based on the trend noted in Figure C-2, the following functional form is selected to
model Vs30 as a function of station elevation (Elv, in meters),
ln( Vs
30
) = ln( φ1
) +
1+
e
ln( φ2
) − ln( φ1
)
.
(ln( φ ) − ln( Elv )) / φ
3
Interpretations of the unknown coefficients are as followed: φ 1 and φ 2 are the asymptotic
Vs30 as the elevation approaches 0 and ∞, respectively; at the elevation of φ3 meters, Vs30
is the (geometric) average of the two asymptotes.
The above model was used to fit Vs30 data in each of the five Geomatrix categories. The
estimated coefficients (φi, i=1, 4) are listed in Table C-1 and the fitted curves are shown
on the left hand plot in Figure C-3.
To show the difference between this mapping scheme and that used in California, Vs30
estimates of the non-measurement sites are plotted in the right panel of Figure C-3. The
solid lines are Vs30 mapped by the new model; circles are Vs30 mapped by California’s
model (Silva, 2004; see Note #34 in the site file NGA_Site_V017.XLS). In most cases
the difference is not large. The larger difference occurs in Geomatrix-D category at
elevation higher than 70m, as expected.
REFERENCES:
4
Lee, C.T., Cheng, C.T., Liao, C.W., and Tsai, T.B., 2001, Site classfication of Taiwan
free-field strong-motion stations: Bulletin of the Seismological Society of
America, v. 91, no. 5, p 1283-1297.
Wills, C.J., Petersen, M.D., Bryant, W.A., Reichle, M.S., Saucedo, G.J., Tan, S.S,
Taylor, G.C, and Treiman J.A., 2000, A site conditions map for California
based on geology and shear wave velocity: Bulletin of the Seismological
Society of America, v. 90, no. 6b, p S187-S208.
Wills, C.J. and Silva, W., 1998, Shear wave velocity characteristics of geologic units
in California: Earthquake Spectra, v. 14, p. 533-556.
Wills C.J. and Clahan K., 2005, NGA site condition metadata from geology, report to
PEER-LL, 2005.
C&Y2006, Appendix C C-2