Seismic Site Effects in Owhiro Bay Quarry, Wellington: Field Observations and Numerical Modelling

Local topography and geology are well known factors affecting the intensity of seismic shaking. We have investigated the seismic site effects at a site of the Owhiro Bay Quarry in Wellington, New Zealand, using instrumental records and numerical models. Both common methods, Standard Spectral Sations (SSR) and Horizontal-to-Vertical Spectral Ratios (HVSR), were used to analyse the site response during more than 200 earthquakes. The observations were then compared with the results of 2D numerical models of the site. The combination of the three methods illustrates some well-known phenomena, such as distortion of the observed SSR due to the site effects at the reference station and its inability to detect the fundamental resonance frequency of a ridgeline. However, a good match between the results of SSR and HVSR was found across most of the investigated frequency range. This is somewhat surprising given the both methods have inherent but unrelated biases. The results also show that when comparing the results of the numerical models with the field observation, the most consistent comparison was achieved when the “field SSR” are calculated in the models by dividing the calculated spectra by the spectrum from the simulated reference station.