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New Zealand is divided into four earthquake risk zones for the purpose of determining the bracing requirements of buildings to resist earthquakes.
Earthquakes can bring rapid, violent shaking both sideways and/or up and down, or slower rolling movements.
In some areas – especially hillsides – unstable ground may slide and rocks may fall.
On flat ground where there is a high water table and the soil is low-density sand or silt, liquefaction can take place. Liquid is forced to the surface, carrying sand and silt with it; land can slump; surface soil close to sloping ground (such as stream banks) can spread, with cracks opening up. Many local councils are investigating areas at risk of liquefaction and creating maps of these, including online maps that allow searches to be done for the risk to particular properties.
Depending on the location, earthquakes can bring other hazards such as tsunamis.
There are strong rules around house construction to help houses better cope with earthquakes, and some of these rules were strengthened following the Canterbury earthquakes of 2010/2011. (In the last decade, 187 people have died in New Zealand as a result of earthquake activity – 185 in Christchurch in 2011 and two in Kaikoura in 2016.)
Under NZS 3604:2011 Timber-framed buildings, bracing must be provided for all buildings, with greater bracing being required for buildings:
- in a higher earthquake zone
- where heavy roof and/or wall claddings are specified.
Earthquake bracing demand is covered in section 5.3 of NZS 3604:2011. The section includes maps showing earthquake zones 1–4.
Earthquake bracing is provided by lengths of wall where the cladding and/or lining works with the timber frame to form a bracing panel. NZS 3604 ensures an even distribution of bracing elements, so the building is not expected to twist significantly in an earthquake.
Nowadays, many light timber-framed buildings are not designed completely to NZS 3604 because homeowners want large windows on one side of the house to enjoy a view. This often requires special bracing elements designed by a structural engineer. The rest of the building can still be designed and constructed to the standard.
Simple houses constructed to NZS 3604 performed well in the Canterbury earthquakes, but houses with a mixed bracing system often had significant damage.
BRANZ conducted a research project funded by the Earthquake Commission around houses with mixed bracing systems. Based on the study, a simplified approach has been proposed for engineers to reduce the likelihood of significant earthquake damage where there is mixed bracing. You can read more about the BRANZ study here.
Another key standard regarding earthquakes is NZS 1170.5:2004 Structural design actions – Part 5: Earthquake Actions – New Zealand.
There are limits in how Building Code Acceptable Solutions can be used in some higher-risk locations. At the end of November 2019, for example, the regulations around liquefaction-prone ground that were already in place in Canterbury were extended to all New Zealand. Acceptable Solution B1/AS1 cannot be used on ground prone to liquefaction or lateral spreading.
Depending on when and where a house was built, and what construction methods were used, there may be steps that can be taken to make it more resilient to earthquakes.
Houses with simple shapes and with timber frames and timber cladding tend to withstand earthquake shaking better, while unreinforced brick buildings are more likely to suffer damage.
Work that can be done to earthquake strengthen a house includes:
- Fixing or replacing damaged piles
- Making sure that piles are firmly fixed to bearers
- Adding timber braces between piles and bearers or joists, or between piles and corner foundations
- Adding new plasterboard lining over old sarking on interior walls
- Removing or replacing unreinforced masonry chimneys
- Making sure clay or concrete tiles are properly fixed
Removal of a building element such as an unsound chimney does not require a building consent (but the work must still meet Building Code requirements). This exemption is limited to any building up to 3 storeys high as long as the removal does not aﬀect the primary structure, any speciﬁed system or any ﬁre separation (which includes ﬁrewalls protecting other property).
Any repair work that is necessary – for example, making good the gaps left in a roof after chimney removal – can also be done without a consent.
Consult a chartered professional engineer over what may be possible with unreinforced masonry walls.
EQC cover for earthquakes
New Zealand residential buildings that have a current house insurance policy that includes fire insurance are covered against natural disasters with EQ Cover. EQ Cover is provided by a government body, the Earthquake Commission, and is government guaranteed. Within certain limitations, it insures against physical loss or damage resulting from a natural disaster.
The natural disasters that are covered are:
- earthquakes, natural landslips, volcanic eruptions, hydrothermal activity or tsunami
- fires caused by a natural disaster
- for residential land, storms or floods.
In 2019, some of the details around EQ cover changed. Change include:
- There is no longer cover for contents.
- The time limit on lodging a claim has been extended from 3 months to 2 years.
- The cap on EQC residential building cover is increasing from $100,000 (+GST) to $150,000 (+GST). (Homeowners can get top-up insurance from private insurers to cover loss above this limit.)
- The EQC has more flexibility around sharing property-related information necessary to settle insurance claims.
Updated: 11 December 2019