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▲▲Figure 7. SASW setup at a site in south Kaiapoi. Photo by B. Cox on 12 Sept 2010.and its environs after the Darfield and Christchurch earthquakes.Figure 8 shows the locations of the SASW test sites.Spectral analysis was used to separate the measured surfacewaves by frequency and wavelength to determine theexperimental (“field”) dispersion curve for the sites via phaseunwrapping. An effective/superposed-mode inversion thattakes into account ground motions induced by fundamentaland higher-mode surface waves as well as body waves (i.e., a fullwavefield solution) was then used to match theoretically thefield dispersion curve with a one-dimensional (1D) layered systemof varying layer stiffnesses and thicknesses (Roesset et al.1991; Joh 1996). The 1D V S profile that generated a dispersioncurve that best matched the field dispersion curve was selectedas the site profile. Per Youd et al. (2001), the V S profiles werethen normalized for effective overburden stress using the followingrelationship:V S1 = V⎛ P aS⎝ ′σ vo0.25⎞ , (2)⎠where V S1 is the shear wave velocity normalized to 1 atm effectivestress, P a is atmospheric pressure (i.e., 101.3 kPa), andσ′ vo is initial vertical effective stress (in the same units as P a ).Figure 9 shows a plot of V S and V S1 for a test site in the easternChristchurch neighborhood of Bexley, which experiencedsevere liquefaction during both the Darfield and Christchurchearthquakes. Also plotted in this figure is the empirically determinedupper-bound V S1 for liquefiable soils (i.e., soils havingV S1 > V* S1 will not liquefy regardless of the intensity of shakingimposed on them).ESTIMATION OF PGAs AT DCP AND SASW TESTSITESAs discussed in the next section, the in-situ test data describedabove correlates to the ability of the soil to resist liquefaction(i.e., capacity). However, to evaluate liquefaction potential,both the soil’s ability to resist liquefaction and the demandimposed on the soil by the earthquake needs to be known. Forthe approach used herein to evaluate liquefaction potential(i.e., stress-based simplified procedure), the amplitude of cyclicSeismological Research Letters Volume 82, Number 6 November/December 2011 933