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Subsoil class map of central Wellington

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Subsoil class map of central Wellington with bold letters denoting specific subsoil classes defined in the New Zealand Structural Design Standard (B = rock; C = shallow soils; D = deep or soft soil)

Subsoil class map of central Wellington with bold letters denoting specific subsoil classes defined in the New Zealand Structural Design Standard (B = rock; C = shallow soils; D = deep or soft soil) (Semmens & Others 2011; Semmens & Others 2010; Perrin & Others 2010). The influence of local geologic (site) conditions on the intensity of ground shaking and earthquake damage is well documented. Structural design in New Zealand takes conditions into consideration largely through the incorporation of the New Zealand Structural Design Standard (NZS 1170). The standard prescribes structural design actions on the basis of site subsoil class to accommodate likely increased earthquake loadings due to shaking modification. The standard sets out five site subsoil class categories, based on geological and geotechnical properties, which must be used in the calculation of horizontal and vertical loading. As part of It’s Our Fault, the distribution of NZS 1170 site subsoil classes were defined throughout central Wellington. This work was based on geological and geotechnical data from over 1000 borehole logs, existing geological mapping and derived geophysical parameterisation of these materials. This is the first time that a detailed earthquake ground shaking hazard map has been prepared for Wellington, expressed in terms of the specific subsoil classes used in the Structural Design Standard. This has greatly facilitated the uptake of this information which is already being used by structural and geotechnical engineers in Wellington, and the insurance industry too. A similar study has been completed for Lower Hutt, and complementary investigations are planned for other urban areas throughout the region.