Frequency dependent amplification of weak ground motions in Porirua and Lower Hutt, New Zealand
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.
Borcherdt, R.D. 1970. "Effects of local geology on ground motion near San Francisco Bay", Bull. Seism. Soc. Am. 60: 29-61.
Borcherdt, R.D. 1991. "On the observation, characterization, and predictive GIS mapping of strong ground shaking for seismic zonation (A case study in the San Francisco Bay region, California)", Pacific Conference on Earthquake Engineering Proceedings, 1-24.
Borcherdt, R., Glassmoyer, G., Andrews, M., Cranswick, E. 1989. "Effects of site conditions on ground motion and damage", Earthquake Spectra, special supplement, August, 1989, 23-42.
Dellow, G.D., Read, S.A.L, Begg, J.G., Van Dissen, R.J., Perrin, N.D., 1992, "Distribution of geological materials in Lower Hutt and Porirua, New Zealand: a component of a ground shaking hazard assessment", Bull. New Zealand National Society for Earthquake Engineering, this volume. DOI: https://doi.org/10.5459/bnzsee.25.4.332-344
Gledhill, K.R., Randall, M.J., Chadwick, M.P. 1991. "The EARSS seismograph: system description and field trials", Bull. Seism. Soc. Am. 81: 1380-1390.
Haines, A.J. 1990. "Intensities felt in Wellington from (13 May) Dannevirke earthquake (abstract)", New Zealand Geophysical Society meeting, August, 1990.
Jarpe, S.P., Cramer, C.H., Tucker, B.E., Shakal, A.F. 1988. "A comparison of observations of ground response to weak and strong ground motion at Coalinga", California, Bull. Seism. Soc. Am. 78: 421-435.
Jarpe, S.P., Hutchings, L.J., Hauk, T.F., Shakal, A.F. 1989. "Selected strong- and weak-motion data from the Loma Prieta earthquake sequence", Seismological Research Letters 60: 167-176. DOI: https://doi.org/10.1785/gssrl.60.4.167
King, J.L., Tucker, B.E. 1984. "Observed variations of earthquake motion across a sediment-filled valley", Bull. Seism. Soc. Am. 74: 137-151.
Sritharan, S., McVerry, G.H. 1992. "Microzone effects in the Hutt Valley in records from a strong motion accelerograph array", Bull. New Zealand National Society for Earthquake Engineering, this volume. DOI: https://doi.org/10.5459/bnzsee.25.4.246-264
Stephenson, W.R. 1989. "Normal modes of a 'cylindrical valley' of alluvium", Bull. New Zealand National Society for Earthquake Engineering 22: 76-80. DOI: https://doi.org/10.5459/bnzsee.22.2.76-80
Stephenson, W.R. 1989. "Observation of a directed response in soil driven by transverse rock motion", Bull. New Zealand National Society for Earthquake Engineering 22: 81-89.
Stephenson, W.R., Barker, P.R., "Evaluation of sediment properties in the Lower Hutt and Porirua areas by means of cone and seismic cone penetration tests", Bull. New Zealand National Society for Earthquake Engineering, this volume.
Tucker, B.E., King, J.L. 1984. "Dependence of sediment-filled valley response on input amplitude and valley properties", Bull. Seism. Soc. Am. 74: 153-165.
Tucker, B.E., King, J.L., Hatzfeld, D., Nersesov, I. L. 1984. "Observations of hard-rock site effects", Bull. Seism. Soc. Am. 74: 121-136.
Van Dissen, R.J., Taber, J.J., Stephenson, W.R., Sritharan, S., Read, S.A.L., McVerry, G.H., Dellow, G.D., Barker, P.R., 1992, "Earthquake ground shaking hazard assessment for the Lower Hutt and Porirua areas", Bull. New Zealand National Society for Earthquake Engineering, this volume. DOI: https://doi.org/10.5459/bnzsee.25.4.286-302
Copyright (c) 1992 J. John Taber, Euan G. C. Smith
This work is licensed under a Creative Commons Attribution 4.0 International License.