Observation and Synthesis of Seismic Wavefields in Basin Structures

Abstract

An experiment was performed in a small basin near Alfredton, North Wairarapa, New Zealand to study the fine scale amplification pattern of seismic ground motion by soft surface sediments. Amplification patterns in the basin were calculated from 15 earthquakes recorded by a 2D array. Amplification for peak amplitudes was about 3. The fundamental resonant frequencies and amplification factors were 3-4 Hz and <6 respectively. Waves from deep distant events were significantly less amplified than local events. As well, waves from the deep distant events were also less spatially coherent than local events. Coherence distance of the incident waves was no more than a half wavelength. An approach for modelling spatially and temporally random full wavefields in arbitrary heterogeneous media in a 3D basin was developed for the 3D modelling of random waves in the Alfredton Basin. 24,840 harmonic response wavefields in the basin due to incident SH, SV P, Love and Rayleigh waves, 100 harmonic response wavefields due to spatially random incident waves, and 20 full response wavefields with a time history of 10 seconds and maximum frequency 12Hz due to random earthquakes were modelled. The observed amplification patterns are better matched by the modelling with random incident waves than that with pure plane waves. Spatially incoherent incident waves with an outstanding pulse in the time history tend to be amplified more than those with relatively even waveforms. This is the cause for the amplification difference between the observed local and distant events. It follows that (a) the amplification pattern in a 3D basin is affected not only by the local geology of the basin, but also by the source mechanism and the path scattering; (b) data from events as local as possible should be included in data sets for site response evaluation so as to avoid possible significant underestimation of the amplification at a site. In addition, a new technique of envelope analysis has been developed for either quick estimation of seismic site response by using underground parameters, or estimation of the underground properties such as shear wave velocity and Q by using seismic spectral ratios. The technique has been used to analyse data from two sites in Porirua, New Zealand. The soil layer velocities of these sites obtained through this analysis are close to that obtained by seismic cone penetrometry. The range of possible values of Q has been determined as functions of basement velocity. By using velocity information of similar rocks in the region, the Q values have been estimated to be between 13-25 with a mean at about 19.