Influence Of Site Effects On Observed Ground Motions In The Wellington Region From The M-W 7.8 Kaikoura, New Zealand, Earthquake

This article presents ground-motion and site-effect observations in the Wellington region of New Zealand from the 14 November 2016 M-w 7.8 Kaikoura earthquake. Despite being similar to 60 km from the northern extent of the causative earthquake rupture, amplification of long-period ground motions due to site and basin-edge effects resulted in appreciable ground motions and subsequent damage to the built environment in this major urban area and capital city of New Zealand. The largest long-period ground motions were observed in the Thorndon and Te Aro basins in central Wellington, where similar site amplification effects were also observed during the 2013 Cook Strait earthquake sequence. Comparisons of pseudoacceleration response spectra with current estimates of fundamental site period across central Wellington indicate that this long-period amplification, relative to nearby rock stations, cannot be explained by 1D site effects alone (i.e., layered impedance), and thus it is inferred that there is a significant contribution from basin-edge-generated surface waves. In contrast, in the Lower Hutt-Petone area, north of central Wellington, ground motions from the GeoNet strong-motion station array across a deep alluvial valley clearly demonstrate the influence of 1D site effects. The 5%-95% significant duration of ground motions in central Wellington was on the order of 30 s, consistent with empirical models for this earthquake magnitude and source-to-site distance. The observations from the 2016 Kaikoura earthquake and comparison with the ground-motion characteristics of recent earthquakes has highlighted the need to improve characterization of the regional basin structures, particularly in regard to quantifying the contribution of basin resonance and basin-edge-generated surface waves.