Microzoning project Parkway Basin subsurface investigations, Wainuiomata

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Beetham, R.D. 1999 Microzoning project Parkway Basin subsurface investigations, Wainuiomata. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 99/14 1 v.

Abstract: IGNS and other researchers associated with the FRST funded Microzoning Project have been investigating and modelling the response of ''sedimentary basin'' areas to earthquake shaking. As sedimentary basins of weak soils have been known to strongly amplify some earthquake bedrock ground motions, they are regarded as potential locations for microzoning research into the varied response of basins to earthquake shaking. Where a basin is small it can be well instrumented for a period of a few months with a coverage of sensitive seismograph ground motion recorders to record small magnitude, generally non-felt earthquakes that occur regularly in NZ. Once the earthquake records and corresponding relevant ground materials data gave been collected, sophisticated modelling and analysis can be carried out to compare derived ground motions with actual. Part of this process involves having realistic properties of ground materials available for input into the modelling and analytical work. Subsurface investigations have been carried out at Parkway to determine the ground materials and their properties. The research at Parkway follows on from earlier work at Alfredton Basin in the Wairarapa (Stephenson 1996). A small, confined, ''ideal'' basin of soft, recent sediments that would be likely to respond in a whole-basin mode was sought in the Wellington area - close to our IGNS base at Gracefield. After consideration of various options, Parkway was chosen because of ease of access, security for recording instruments, and a prior knowledge of some conditions in the basin, including a resonant response at one site. Additional ground frequency measurements using the Nakamura technique were carried out and identified the experimental area at Parkway as possibly an area having a whole-basin mode response. As well, Parkway came highly recommended as it is the home of several distinguished IGNS staff. This report describes the subsurface investigations that were carried out at Parkway for the research project and interprets these in the context of the known geological background of Parkway and Wainuiomata. (auth)