Hard-Site κ_0 (Kappa) Calculations for Christchurch, New Zealand, and Comparison with Local Ground-Motion Prediction Models

摘要

The 2010–2012 Canterbury earthquake sequence generated a large number of near-source earthquake recordings, with the vast majority of large events occurring within 30 km of Christchurch, New Zealand’s second largest city. We utilize the dataset to estimate the site attenuation parameter, κ_0, at seven rock and stiff-soil stations in New Zealand’s GeoNet seismic network. As part of this study, an orientationindependent definition of κ is proposed to minimize the influence of observed highfrequency 2Dsite effects. Minimum magnitude limits for the traditional high-frequency fitting method are proposed, based on the effect of the source corner frequency. A dependence of κ_0 on ground-shaking level is also observed, in which events with large peak ground accelerations (PGAs) have lower κ_0 values than events with small PGAs. This observation is not fully understood, but if such a trend holds in future investigations, it may influence how κ_0 is used in hazard assessments for critical facilities. κ_0 values calculated from Fourier amplitude spectra of acceleration (κ_(0,AS)) are compared with the native κ_0 of local, empirical, ground-motion prediction equations (GMPEs), calculated using the inverse random vibration theory method (κ_0,IRVT)). κ_(0,IRVT) is found to be independent of magnitude and distance and agrees with the average κ_(0,AS) for the region. κ_(0,IRVT) does not scale strongly with V_(S30), indicating that current GMPEsmay be capturing the average kappa effect through the V_(S30) scaling. The results from this study are of particular interest for site-specific ground-motion prediction studies as well as for GMPE adjustments between different regions or rock types.