Abstract:
Seismic ground response analysis for the central business district of Christchurch, New Zealand must consider a firm silty clay layer at depth. Modeling the silty clay layer creates a shear wave velocity (Vs) inversion, which attenuates short-to-moderate-period ground motions due to elevated shear strains in the slower layer, which can underestimate surface response spectra at short-to-moderate periods. Nonlinear seismic ground response analyses were conducted for conditions at the Christchurch Cathedral College strong motion station to assess modeling uncertainty and the impact of the silty clay Vs on one-dimensional ground response analyses. Input motions were selected to represent the M7.1, 4 September 2010 Darfield and M6.2, 22 February 2011 Christchurch earthquakes. Modeling the silty clay layer with slower Vs underestimated surface motions at short-to-moderate periods. Increasing the Vs of the silty clay layer reduced the residuals between the observed and calculated spectra. Physically, increasing Vs of the silty clay is postulated to represent the impact of lateral and vertical variability of Vs. The results of the analyses were used to establish an approach for preparing ground response analysis models for a nearby project. The modeling uncertainty was evaluated for seismic hazard convolution analyses for the project.