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

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James Dismuke’s study assessed how seismic ground-response analysis (GRA) modeling choices affect amplification factors for Christchurch Cathedral College (CCCC) in central Christchurch. It used ground motions recorded during the 2010–2011 Canterbury Earthquake Sequence to evaluate modeling uncertainties, focusing on the local conditions near the CCCC strong-motion station.<br /><br />The study conducted 1D GRA using a typical Christchurch CPT profile (CPT_484). Reference-rock motions for the Darfield and Christchurch earthquakes were estimated using records from 19 strong-motion stations; nearby records were adjusted systematically for rupture distance. The CPT shear-wave velocity profile was represented using four approaches: Strict, Smooth, “Fast” silt, and Design. Site investigations indicated greater geological variability at depth, and results showed that the assumed silt shear-wave velocity strongly influenced both calculated surface response spectra and their residuals relative to observed motions.<br /><br />Residual standard deviations across periods of 0.01–2.0 seconds ranged from 0.22 to 0.24 for Darfield, 0.23 to 0.27 for Christchurch, and 0.28 to 0.35 for the combined earthquakes, depending on the profile approach. These findings were used to incorporate additional modeling uncertainty into amplification-factor models.<br /><br />The resulting amplification models were combined probabilistically with reference-rock hazard curves to develop design ground motions. The work demonstrates that site-profile interpretation—especially the assumed properties of silt—can materially affect predicted ground motions and should be accounted for in seismic hazard design.

Keywords

seismic ground-response analysis

Christchurch Cathedral College

ground motion amplification

Canterbury Earthquake Sequence

1D GRA

shear-wave velocity profile

CPT_484

silt shear-wave velocity

modeling uncertainty

design ground motions