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

Appendix C
Estimation of Vs30 at CWB’s Free-Field Sites
Brian Chiou and K.L. Wen
(June 08, 2006)
Introduction
When Vs30 measurement is missing, an estimate is typically calculated using an empirical
relationship between Vs30 and other site data, such as surface geology or Geomatrix’s 3 rd
letter (C3). This approach is used in the PEER-NGA project and has also been widely
used in California (Wills and Silva, 1998; Wills and others, 2000; Wills and Clahan, 2004)
and other regions. This short note reports our efforts to establish empirical Vs30 - C3
relationships for strong-motion sites in Taiwan. The Taiwan-specific relationship uses C3
as the main predictor. It also uses station elevation to capture the within-category
variation of Vs30. The use of elevation as a predictor leads to a significant improvement of
mapping accuracy within a site category, particularly within Geomatrix’s category D.
Data
Vs30 data used in this study are derived from the P-S logging provided to the PEER-NGA
project by NCREE, who manage the site investigation of strong-motion stations operated
by CWB in Taiwan 1 . During the period of 2000 - 2004 a total of 231 holes were drilled
and logged. The site investigation project is still on going and more P-S logging data will
become available in the next several years.
Dr. Walt Silva (PE&A) reviewed the P-S logging data from each of the 231 holes,
smoothed the velocity profile, and computed Vs30 from the smoothed profile. We
selected 165 sites for use in our study. The remaining 66 strong-motion sites were not
used because either the drill hole is less than 20 m deep or a Geomatrix C3 classification
has not been assigned yet.
Other site data are also collected. They include: (1). Geomatrix’s 2 nd and 3 rd letters (D.
Wells, pers. comm., 2004); (2) The site classification (PSC) by Lee et al. (2001); and (3)
Station elevation (Lee et al., 2001).
Variation of Vs30 within a Site Category
Histogram of the 165 Vs30 data, grouped by C3, is shown in Figure C-1. This figure
highlights two difficulties in using C3 to map Vs30 in Taiwan. First, categories A, B, and
C are not distinguishable by Vs30. Secondly, the most populous category (D) is bimodal
with a large Vs30 variation within that category. The active tectonic in Taiwan leads to a
strong correlation between elevation and geology (Lee et al., 2001). The higher the
elevation the stiffer the surface material tends to be. This unique feature prompts us to
explore if elevation could be used to explain the large Vs30 variation in category D. Vs30
1 Plots of the drilling logs can be viewed on CWB website at http://www.cwb.gov.tw/V4/index.htm.
C&Y2006, Appendix C C-1