3D Site Effects: A Thorough Analysis of a High-Quality Dataset

Abstract

This article is a contribution toward filling the large gap that currently exists between theoretical and observational studies of site effects. We present a detailed analysis of 10 events recorded by a dense temporary seismograph network that operated in the small (roughly 1.3 by 0.4 km) alluvial valley of Parkway, New Zealand. We computed f - k spectra for the three recorded components of those 10 events as a function of frequency and time window along the seismograms. Our results show that a major part of the energy in the records propagates as surface waves. The small size of the basin explains the observation that small phase velocity waves appear shortly after direct P - and S -wave arrivals. Both Love and Rayleigh waves are present. It was difficult to isolate individual wavetrains, but directions of propagation are dominated by southerly directions, especially at lower frequencies and earlier times along the seismograms. The inversion of observed phase velocity dispersion is in very good agreement with available information on the subsoil structure for this valley. A comparison with a different interpretation of one event from our dataset shows clearly how misleading it can be to analyze partial observations. The large variability of ground motion in the time domain contrasts with the very stable results obtained for this dataset in the frequency domain and indicates that we still have a long way to go before we are able to incorporate locally generated surface waves into the frame of modern building codes.