Observation and synthesis of spatially-incoherentweak-motion wavefields at Alfredton basin, New Zealand
DOI:
https://doi.org/10.5459/bnzsee.30.1.14-31Abstract
To observe and model the detailed pattern of ground motion amplification in a small soft-soil basin an experiment was conducted at Alfredton, New Zealand. 19 seismometers were deployed for 5 weeks at closely spaced sites in and around a 400-500 m diameter, sediment-filled depression in soft, sandstone basement. During this period 112 earthquakes, with "weak" ground motions, were detected by at least some of the instruments, and 15 well-recorded events were selected for detailed analysis. Geotechnical data obtained to provide the parameters for the 3-dimensional modelling included measurements of the shear-wave velocity. Across the basin this is 60 m/s at the surface, increasing steadily to 300+ m/s at the bottom of the basin, and the shear-wave velocity in the basement is 850 m/s. Thus, there are no boundaries where the contrast in shear-wave impedance is especially large.
In contrast to situations where there are large contrasts in shear-wave impedance to trap seismic energy in soft-soil layers, the amplifications observed in the basin at Alfredton were small. The small amplifications are confirmed by the 3-dimensional modelling. Another feature of the observed wavefields is that in all cases the incident motions, recorded at the basement sites around the basin, were spatially incoherent. In other words, the wavefields arriving at the basin were of a complex, seemingly random nature. This is the first occasion that the spatial coherency of wavefields has been measured in a fine-scale experiment in New Zealand. Apart from the small amplications and the observed lack of coherency between the basement sites, the most striking result, which was obtained from both the observations and the modelling of similarly incoherent wavefields, is that for short-duration events in which the main motions last for no more than a second, the amplifications in the basin are larger than for events in which the motions are of longer duration; that is, the extent to which differently propagating incoherent wave packets interfere destructively inside the basin increases with the duration of the wavefields.
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