| dc.creator | Taber, J. John | |
| dc.creator | Smith, Euan G. C. | |
| dc.date | 1992-12-31 | |
| dc.identifier | https://bulletin.nzsee.org.nz/index.php/bnzsee/article/view/695 | |
| dc.identifier | 10.5459/bnzsee.25.4.303-331 | |
| dc.description | The relative ground response due to microearthquakes has been examined at a total of 36 sites in the Porirua and Lower Hutt regions of New Zealand, as part of a multi-disciplinary microzoning project conducted with the Wellington Regional Council. The sites were studied in two separate experiments and were chosen to sample a variety of soil types and depths ranging from strong rock to thick sections of alluvial gravels and sands to soft water-saturated fine-grained deposits. The amplitude response of each site relative to a bedrock reference site has been determined as a function of frequency. Fourier spectral ratios (Fsr) were calculated for each earthquake and then between three and twenty-six earthquakes were averaged together at each of the sites. Spectral ratios of individual earthquakes varied significantly from the average spectral ratio.
In the Hutt Valley there is a gradual down-valley increase in shaking in a similar pattern to the down-valley increase of the depth to bedrock and thickness of near-surface soft sediment. The response at the upper-most valley sites, underlain by less than 50 m of alluvial gravel and silty sand, is similar to the response at the rock sites on the side of the valley (Fsr = 2.4) while the Fourier spectral ratios reach 14 at the lower-most valley sites, which are underlain by greater than 20 m of soft sediment. The highest amplifications were recorded at two sites on soft flexible sediments (10 to 35 m thick) in an enclosed valley (Fsr = 16 to 18) and a site on an apparently drained and filled swamp (Fsr = 15). A spectral ratio of 18 corresponds to an increase in peak ground velocity by a factor of 5.
The amplification at most Lower Hutt sites occurred over a broad frequency band from 0.5 Hz to up to 5 Hz, with the high frequency limit of the band decreasing as the spectral ratio in the band increased. Two of the flexible sediment sites exhibited a very narrow frequency response with a peak in the 1-2 Hz range, similar to three flexible sediment sites in the Porirua basin where the amplification was in the 1-3 Hz frequency band. These flexible sediment sites had Fourier spectral ratios of up to 18 relative to a hard rock site. Three other Porirua sites had spectral ratios greater than 5 at some frequency. Two of these sites were on fan alluvium and fine grained sediment, while the third was on siltly sand on a topographic ridge. The remaining five sites were on weathered gravels and showed little amplification. | en-US |
| dc.format | application/pdf | |
| dc.language | eng | |
| dc.publisher | New Zealand Society for Earthquake Engineering | en-US |
| dc.relation | https://bulletin.nzsee.org.nz/index.php/bnzsee/article/view/695/670 | |
| dc.rights | Copyright (c) 1992 J. John Taber, Euan G. C. Smith | en-US |
| dc.rights | https://creativecommons.org/licenses/by/4.0 | en-US |
| dc.source | Bulletin of the New Zealand Society for Earthquake Engineering; Vol 25 No 4 (1992); 303-331 | en-US |
| dc.source | 2324-1543 | |
| dc.source | 1174-9857 | |
| dc.title | Frequency dependent amplification of weak ground motions in Porirua and Lower Hutt, New Zealand | en-US |
| dc.type | info:eu-repo/semantics/article | |
| dc.type | info:eu-repo/semantics/publishedVersion | |
| dc.type | Article | en-US |
