Liquefaction Hazard in the Wellington region 2018
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Purpose of the report to identify areas in the Wellington Region where liquefaction hazard may occur and guide further investigation for planning and mitigation purposes.
Geographic Area Covered
The report covers the Greater Wellington Region, including Wellington City, Porirua City, Hutt Valley, Kāpiti Coast, and the Wairarapa Districts.
Summary
Liquefaction is a process where saturated, loose, fine-grained soils lose strength during strong earthquake shaking, causing ground deformation and damage to infrastructure. This report by GNS Science assesses the liquefaction hazard across the Wellington Region using historical earthquake data, geological mapping, and limited subsurface investigations. Liquefaction has been observed in the region during several historical earthquakes, notably in 1848, 1855, 1942, and 2013, with severity and extent correlating to the intensity of ground shaking. The most susceptible areas are those with young (Holocene), saturated, non-cohesive sediments, typically found in estuaries, lagoons, and floodplains. The report identifies areas with very high, high, moderate, low, or no susceptibility to liquefaction. Areas with very high susceptibility include parts of Wellington Harbour reclamation and southern Hutt Valley. Moderate to high susceptibility is found in parts of Porirua, Wainuiomata, and the Wairarapa plains near rivers and lakes. The report includes maps showing liquefaction susceptibility and areas where damaging liquefaction may occur. These maps are based on available geological and geotechnical data, which vary in quality and quantity across the region. The authors recommend that these maps be used to identify areas for further investigation rather than for site-specific planning decisions. Further work should include detailed geomorphic mapping, cone penetration testing, and groundwater monitoring to quantify liquefaction hazard. The report concludes that while the Wellington Region is less vulnerable to liquefaction than areas like Christchurch, damaging liquefaction can still occur and disrupt infrastructure. Improved data collection and hazard assessment are essential for informed land use planning and resilience building.