Dense array analysis of ground motions in a sedimentary basin

Abstract

The ground shaking experienced on soft sediments during an earthquake is amplified when compared to nearby sites on solid rock. However the level of amplification varies from earthquake to earthquake and the cause of this variation has not been adequately explained. In this project, seismic data from Parkway, Wainuiomata, New Zealand, were used to study the relationship between the variability of seismic amplification by soft soils in a basin and the nature of the seismic waves arriving in the basin. The seismic waves arriving in the basin have been characterised by their coherence, which is a measure of the uniformity of the incoming wave across the basin.

Ten earthquakes that were recorded at the same six soil sites inside the basin and three rock sites surrounding the basin were selected for the study. The resonant frequency at the soil sites was 1.7Hz and the average amplification of the soil sites relative to the rock sites for all the earthquakes was 8.5. The average amplification of individual earthquakes in the basin was determined to show the variation between earthquakes and this was compared to the coherence of the seismic waves for each earthquake. The results demonstrate that there is a correlation between the amplification and the coherence of the seismic waves at the resonant frequency.

To test the interpretation of the recorded data, three-dimensional computer modelling of the ground motion in the basin was conducted, using different types of seismic waves to approximate different levels of coherence. The modelling supported the correlation found from the observed data and suggests that the more coherent the incoming seismic waves, the greater the ground motion amplification in a sedimentary basin.

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Technical Abstract

Seismic data from Parkway, Wainuiomata, New Zealand, were used to study the relationship between the variability of seismic amplification by soft soils in a basin and the spatial coherence of seismic waves. The basin is a 400m wide alluvial valley with a depth to basement of up to 65m. Ten earthquakes that were recorded at the same six soil sites inside the basin and three rock sites surrounding the basin were selected for the study. Using the average seismic spectrum from the three rock sites as a reference, a spectral ratio of each soil site to the rock sites was calculated for each event. The fundamental resonant frequency at the soil sites was 1.7 +/- 0.1 Hz. The fundamental amplification was 8.5 +/- 1.5.

The relative strength of amplification of an earthquake in the basin, defined as the amplification of the event normalised by the average over all the events, was used as an index to show how much the seismic waves were amplified in the basin compared with other earthquakes. The strength of amplification at the fundamental resonant frequency for the ten events ranged from 0.82 to 1.14. Spatial coherence functions of seismic waves recorded at the six soil sites were also calculated for each event. The coherence values at the resonant frequency varied from 0.68 to 0.92 for the ten events. The results demonstrate that there is a correlation between the strength of the amplification and the coherence of the seismic waves at the fundamental resonant frequency.

To test the interpretation of the recorded data, three-dimensional computer modelling of the ground motion in the basin was conducted for three types of incident waves. The 3D model of the basin was developed in a two step process. First, the available geotechnical data were used to create three 1D models in different parts of the basin which were adjusted until they approximately matched the resonant frequency and amplification levels of recorded data from nearby sites. Then the 3D model for the basin was built by extrapolating the parameters of the 1D models across the basin.

The seismic waves introduced into the 3D model included normal incidence SH waves, normal incidence SV waves and spatially randomly incident SH and SV waves. The first two represent coherent wavefields and the last represents a randomly incident wavefield. The modelling shows that the pure SH or SV incident waves were amplified significantly more than the random waves. This supports the correlation found from the observed data and suggests that the more coherent the incoming wave field, the greater the ground motion amplification in a sedimentary basin.