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Geographic area covered: The study focuses on the Porirua Basin in Wellington Region, including suburbs such as Tawa, Linden, Titahi Bay, Whitby, Paremata, Pauatahanui, Mana, and Plimmerton. 

This report presents the findings of a liquefaction hazard assessment conducted in 1993 for the Porirua Basin. Commissioned by the Wellington Regional Council and prepared by Works Consultancy Services Ltd, the study forms part of a broader regional seismic risk strategy. The assessment evaluates the potential for soil liquefaction and associated ground damage during earthquake events. 
 
The study area includes Porirua City and surrounding suburbs, characterized by underlying Wellington Greywacke rock and overlying Holocene sediments such as alluvium, estuarine deposits, dune sands, and reclaimed land. Investigations included two boreholes and five cone penetration tests (CPTs), along with laboratory testing of soil samples and groundwater level measurements. 
 
Two earthquake scenarios were considered: a distant magnitude 7 event similar to the 1848 Marlborough earthquake, and a local magnitude 7.5 event on the Wellington Fault with a recurrence interval of approximately 600 years. The latter scenario poses a significantly higher risk of liquefaction. 
 
Liquefaction potential was assessed using standard methods based on soil properties and penetration test data. Areas identified with high susceptibility include Plimmerton, Mana, Paremata, Pauatahanui, and parts of Titahi Bay and Porirua City. Tawa and Linden were found to have low susceptibility due to gravelly soils. 
 
Estimated ground subsidence ranges from 25 mm to 400 mm, with the greatest impacts expected in Porirua City and Pauatahanui. Potential ground damage includes subsidence, lateral spreading, and slope failures. Maps included in the report illustrate liquefaction susceptibility, potential, and expected ground damage. 
 
The report emphasizes that its findings are indicative and not a substitute for site-specific investigations. Data limitations exist in some areas, and boundaries shown on maps are approximate. The study provides valuable insights for planning and risk mitigation but should be used with caution for detailed site assessments.