Liquefaction is a natural process where earthquake ground shaking increases the groundwater pressure in some types of unconsolidated sediments and soils resulting in temporary loss of soil strength. There are three key factors which influence whether liquefaction may occur and its severity:
Liquefaction can result in the ejection of liquefied material to the surface (sand boils), subsidence and lateral spreading and loss of bearing strength (i.e. ability to support building foundations). This in turn can cause significant damage to land, buildings, infrastructure (particularly underground services) and the environment, as well as economic and social disruption.
We have started work on a natural hazards plan change(external link) to the Tasman Resource Management Plan (TRMP). As part of the review process, we are improving our knowledge about local hazards and sharing that information with affected landowners and communities.
We have sought expert technical advice to classify areas in the District according to the following three liquefaction vulnerability categories:
Explore our map viewer which illustrates seismic liquefaction vulnerability in our District.
The methodology used to develop the liquefaction information shown on the liquefaction map viewer is described in the following report:
Tasman Regional Liquefaction Assessment 2021 (pdf 24 MB)
The mapping was undertaken at a regional level (‘Level A’ criteria) in accordance with MBIE’s Planning and Engineering Guidance for Potentially Liquefaction-prone Land (2017). It is based on published geological information as well as LiDAR ground elevation data and available groundwater level data. The mapping does not include site specific subsurface investigations which are part of more detailed assessments (Level B-D mapping criteria).
My property is located in an area of ‘Liquefaction Damage is Possible’, what does this mean?
The areas mapped as ‘liquefaction is possible’ do not necessarily mean liquefaction will occur across all of these areas. In fact, the gravelly nature of Tasman’s alluvial plains is such that incidences of liquefaction may be much more limited in extent, but there could be “pockets” of liquefiable fine grained sands and silts within these areas (such as an old infilled and buried river channel). The uncertainty of where such areas are located results in them being mapped as ‘liquefaction is possible’. Low lying land at the estuary and coastal margins are often vulnerable to liquefaction.
Landowners may hold site specific information, such as a geotechnical assessment, which will provide more accurate detail than what is shown in our liquefaction map viewer. This is because our liquefaction mapping is a desk top assessment (Level A mapping) based on MBIE’s Planning and Engineering Guidance for Potentially Liquefaction-prone Land (2017). It is based on published geological information as well as LiDAR ground elevation data and groundwater level data.
Can I subdivide or build if my property is located in an area of ‘Liquefaction Damage is Possible’?
Subdivision requires a resource consent. All natural hazards, including liquefaction, are taken into account when considering an application to subdivide land under the requirements of the Resource Management Act 1991.
There may be individual lots where existing information on liquefaction hazard is not available. When applying for a building consent, use of the regional liquefaction map will help applicants to understand liquefaction vulnerability in the District and if there is a need for a more detailed geotechnical assessment to confirm ground conditions as part of their building consent application. Designers must specify foundations with a resilience that reflects the importance level of the structure and the liquefaction hazard if present.
The mapping acts as a flag to pay extra attention when subdividing a site or undertaking new building work and requires a geotechnical engineer to assess the ground conditions and provide mitigation measures as necessary.
Read more about new buildings and liquefaction effects.
My property is located in an area of ‘Liquefaction Damage is Unlikely’, what does this mean?
The areas mapped as ‘liquefaction damage is unlikely’ are considered unlikely to be susceptible to liquefaction based on the underlying geology. However, given that it is difficult to precisely and accurately identify liquefaction prone land, there remains an element of uncertainty. This is because our liquefaction mapping is a desk top assessment (Level A mapping) based on MBIE’s Planning and Engineering Guidance for Potentially Liquefaction-prone Land (2017). It is based on published geological information as well as LiDAR ground elevation data and groundwater level data. Landowners may hold site specific information, such as a geotechnical assessment, which will provide more accurate detail than what is shown in our liquefaction map viewer.
If you are thinking of building in an area mapped as ‘liquefaction damage is unlikely’, you can read more about new buildings and liquefaction effects.
My property is located in an area of ‘Very Low Liquefaction Vulnerability’, what does this mean?
The areas mapped as ‘very low liquefaction vulnerability’ are considered to have a very low likelihood of liquefaction susceptibility based on the underlying geology, such as mountainous areas.
If you are thinking of building in an area mapped as ‘very low liquefaction vulnerability’, you can read more about new buildings and liquefaction effects.
How is the Council using this liquefaction information?
Following the 2011 Canterbury earthquake, nationally there has been more emphasis on understanding liquefaction and ensuring our communities are resilient to earthquakes and liquefaction hazards. Liquefaction information is used to inform:
Next Steps
The next steps for the TRMP natural hazards plan change will be to engage with the community on the ‘issues and options’ for managing our natural hazards, including liquefaction. Engagement is proposed to begin early to mid 2025 and will seek the community’s views on the options for updating the TRMP provisions that relate to natural hazards.
Landowners, iwi, stakeholders and the wider community will have opportunities to input into the natural hazards TRMP plan change as we progress through the plan drafting stages.
Where do I find information about what to do following a natural hazard event?
Information, advice on getting prepared and what to do during a natural hazard event can be found on the Nelson Tasman Emergency Management Group website.
The Natural Hazards Commission Toka Tū Ake also has information and advice for home owners, tenants and home buyers to ensure your home is safer for natural hazards.
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