Ground motion and site effect observations in the wellington region from the 2016 Mw7.8 Kaikōura, New Zealand earthquake

This paper presents ground motion and site effect observations in the greater Wellington region from the 14 November 2016 Mw7.8 Kaikōura earthquake. The region was the principal urban area to be affected by the earthquake-induced ground motions from this event. Despite being approximately 60km from the northern extent of the causative earthquake rupture, the ground motions in Wellington exhibited long period (specifically T = 1 - 3s) ground motion amplitudes that were similar to, and in some locations exceeded, the current 500 year return period design ground motion levels. Several ground motion observations on rock provide significant constraint to understand the role of surficial site effects in the recorded ground motions. The largest long period ground motions were observed in the Thorndon and Te Aro basins in Wellington City, inferred as a result of 1D impedance contrasts and also basin-edge-generated waves. Observed site amplifications, based on response spectral ratios with reference rock sites, are seen to significantly exceed the site class factors in NZS1170.5:2004 for site class C, D, and E sites at approximately T=0.3-3.0s. The 5-95% Significant Duration, Ds595, of ground motions was on the order of 30 seconds, consistent with empirical models for this earthquake magnitude and source-to-site distance. Such durations are slightly longer than the corresponding Ds595 = 10s and 25s in central Christchurch during the 22 February 2011 Mw6.2 and 4 September 2010 Mw7.1 earthquakes, but significantly shorter than what might be expected for large subduction zone earthquakes that pose a hazard to the region. In summary, the observations highlight the need to better understand and quantify basin and near-surface site response effects through more comprehensive models, and better account for such effects through site amplification factors in design standards.