Site effects in Parkway Basin: comparison between observations and 3-D modelling

This paper is a contribution towards filling the large gap that currently exists between theoretical and observational studies of site effects. We present results of numerical modelling of ground motion in Parkway Valley, a small alluvial valley in New Zealand. We use a 3-D, finite-difference code, and compute ground motion for the vertical incidence of plane waves, P and S. The results are compared with a thorough analysis of site effects in this basin presented previously, based on the records obtained using a dense temporary seismic array. The results of the simulations confirm the observation that ground motion in Parkway Basin is dominated by locally generated surface waves. Diffracted surface waves are generated mainly towards the south. This direction does not depend on the polarization of the incident waves. We obtain a very good agreement between observed and predicted transfer functions for the two horizontal components of stations located towards the centre of the valley. Our model clearly performs less well towards the edges of the basin, where the details of the real structure are missing in our model. Our results suggest that the 3-D shape of the basin may be more important than a detailed knowledge of the subsoil mechanical properties in predicting ground response. A correct estimate of the absolute amplification level, however, requires a correct estimate of the impedance contrast, but this also applies to any modelling of site response.