Assessment of Seismic Ground Response Analysis Model Uncertainty at Christchurch Cathedral College, Christchurch Central Business District, New Zealand-Paper

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The study evaluates uncertainty in one-dimensional seismic ground response analysis (GRA) for Christchurch’s central business district, where a deep, relatively soft silty clay layer creates a shear-wave-velocity (Vs) inversion. During strong shaking, this layer can develop high shear strains and damp short- to moderate-period motions, causing analyses to underestimate surface response.<br /><br />Using subsurface data near the Christchurch Cathedral College strong-motion station, the authors modeled four Vs profiles and ran nonlinear analyses with motions recorded during the 2010 Darfield and 2011 Christchurch earthquakes. The profile that closely followed CPT-derived Vs data underestimated observed surface spectra. Smoothing shallow Vs inversions had little effect, while increasing the silty clay’s Vs improved agreement with observations at periods below roughly 1–2 seconds. This effect was stronger for the more intense 2011 motions.<br /><br />The authors attribute the need for a higher modeled Vs partly to significant local variability in the clay’s depth, thickness, and consistency. A higher value may represent the spatially averaged or “smeared” properties of the variable soil layers, rather than the properties at a single sounding.<br /><br />For a nearby project, the recommended approach was to smooth local Vs inversions and use a minimum silty-clay Vs of 190 m/s. The study also recommends adding a modeling-uncertainty standard deviation of 0.3 to amplification factors used in seismic hazard analyses. The authors note that further work—particularly on soil dynamic properties and reference-rock input motions—could improve accuracy.

Keywords

seismic ground response analysis

Christchurch

shear-wave velocity inversion

silty clay

nonlinear analysis

surface spectra

CPT-derived Vs

strong-motion station

modeling uncertainty

seismic hazard amplification