Earthquake ground shaking hazard assessment for the Lower Hutt and Porirua areas, New Zealand

  • R. J. Van Dissen GNS Science, Lower Hutt, New Zealand
  • J. J. Taber Victoria University of Wellington, Wellington, NZ
  • W. R. Stephenson GNS Science, Lower Hutt, New Zealand
  • S. Sritheran GNS Science, Lower Hutt, New Zealand
  • S. A. L. Read GNS Science, Lower Hutt, New Zealand
  • G. H. McVerry GNS Science, Lower Hutt, New Zealand
  • G. D. Dellow GNS Science, Lower Hutt, New Zealand
  • P. R. Barker Landcare Research Ltd, Lincoln, New Zealand

Abstract

Geographic variations in strong ground shaking expected during damaging earthquakes impacting on the Lower Hutt and Porirua areas are identified and quantified. Four ground shaking hazard zones have been mapped in the Lower Hutt area, and three in Porirua, based on geological, weak motion, and strong motion inputs. These hazard zones are graded from 1 to 5. In general, Zone 5 areas are subject to the greatest hazard, and Zone 1 areas the least. In Lower Hutt, zones 3 and 4 are not differentiated and are referred to as Zone 3-4. The five-fold classification is used to indicate the range of relative response.

Zone 1 areas are underlain by bedrock. Zone 2 areas are typically underlain by compact alluvial and fan gravel. Zone 3-4 is underlain, to a depth of 20 m, by interfingered layers of flexible (soft) sediment (fine sand, silt, clay, peat), and compact gravel and sand. Zone 5 is directly underlain by more than 10 m of flexible sediment with shear wave velocities in the order of 200 m/s or less.

The response of each zone is assessed for two earthquake scenarios. Scenario 1 is for a moderate to large, shallow, distant earthquake that results in regional Modified Mercalli intensity V-VI shaking on bedrock. Scenario 2 is for a large, local, but rarer, Wellington fault earthquake. The response characterisation for each zone comprises: expected Modified Mercalli intensity; peak horizontal ground acceleration; duration of strong shaking; and amplification of ground motion with respect to bedrock, expressed as a Fourier spectral ratio, including the frequency range over which the most pronounced amplification occurs. In brief, high to very high ground motion amplifications are expected in Zone 5, relative to Zone 1, during a scenario 1 earthquake. Peak Fourier spectral ratios of 10-20 are expected in Zone 5, relative to Zone 1, and a difference of up to three, possibly four, MM intensity units is expected between the two zones. During a scenario 2 event, it is anticipated that the level of shaking throughout the Lower Hutt and Porirua region will increase markedly, relative to scenario 1, and the average difference in shaking between each zone will decrease.

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Published
1992-12-31
How to Cite
Van Dissen, R. J., Taber, J. J., Stephenson, W. R., Sritheran, 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, New Zealand. Bulletin of the New Zealand Society for Earthquake Engineering, 25(4), 286-302. https://doi.org/10.5459/bnzsee.25.4.286-302
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Articles

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