Hard-Site  0 (Kappa) Calculations for Christchurch, New Zealand, and Comparison with Local Ground-Motion Prediction Models

Abstract

The 2010 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 2D site 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 GMPEs may 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.

Robustness of Kappa (κ) Measurement in Low‐to‐Moderate Seismicity Areas: Insight from a Site‐Specific Study in Provence, France

“…variable, depending on the frequency band. Moreover, in agreement with recent studies(Van Houtte et al 2014;Edwards et al 2015;Laurendeau et al, 2017), the site amplification can explain a part of the observed variability, as it is frequency dependent and each individual is measured for different frequency windows. Thus, should be considered carefully, as a site amplification component cannot be excluded even for rock sites, especially for low-seismicity context when are estimated from limited spectral windows.…”

“…Indeed, the spectral modulations induced by site effects can change the slope of the decay and thus modify the estimates, depending on the selected frequency windows. Moreover, the modification of the spectrum shape can hide the frequency interval where the decay should be linear in the absence of site amplification, and thus alter the identification of the "true" frequency band where should be measured (Hough et al 1999;Parolai and Bindi 2004;Van Houtte et al 2014;Edwards et al 2015). The smaller the , the greater the influence of the site amplification on should be.…”

Understanding the physics of kappa (κ): insights from a downhole array

“…Moreover, the attenuation in the uppermost layers might be affected by non-linear behaviour under high-level excitations. In geotechnical engineering terms, this could be expected to lead to an increase in hysteretic damping (ξ = (2Q) −1 ) with shear strain, but there are still few and contradicting observations of non-linearity on κ 0 (Dimitriu et al 2001;Van Houtte et al 2014). Our proposed division of κ 0 (or t * ) into source/path and local site components is schematically illustrated in Fig.…”

Hybrid broadband ground motion simulation validation of small magnitude earthquakes in Canterbury, New Zealand

“…Table C.1 in Electronic Supplement C provides the list of stations along with their geographic coordinates, 30-m time-averaged shear wave velocity ( V s 30 ), and number of earthquakes observed ( N E s ). V s 30 values are taken from Wood et al (2011), Van Houtte et al (2014), and Wotherspoon et al (2014, 2015, 2016) who collectively characterized the sites through various geotechnical and geophysical techniques. The Canterbury region has a complex geology which results in the strong motion stations being located on a diverse range of site conditions, ranging from rock conditions on Banks Peninsula and the Canterbury Foothills, to gravels and marine fines in the Christchurch City and Canterbury Plains (west of Christchurch City) areas.…”

“…The GODS prediction exhibits a broad peak in δ S 2 S s at roughly 1.0 Hz. Van Houtte et al (2014) determined that the fundamental frequency of the GODS station was around 1 Hz through horizontal to vertical spectral ratios. Although the fundamental period is long for a rock site, Kaiser et al (2014) investigated the site and attributed this feature to topographic effects from being located on the crest of a basaltic lava flow ridge.…”