Canterbury sits across the boundary of two large slabs (plates) of the Earth’s crust – the Pacific Plate and the Australian Plate. The forces involved in plate movement are huge, and they cause the rock in the Earth’s crust to buckle (fold) and fracture (fault), particularly near the plate boundary.
The plate boundary in the South Island is marked at the ground surface by a series of major faults through Marlborough and North Canterbury – the Wairau, Awatere, Clarence and Hope faults. These merge near Otira to form the Alpine Fault, which runs along the western edge of the Southern Alps. Away from the plate boundary there are other, smaller faults that also take up some of the plate movement.
The plates are always moving, but the Earth’s crust can take up a lot of bending before letting go and breaking suddenly (rupturing) along a fault, causing an earthquake. If a fault is big enough, the break can come all the way up to the ground surface, causing sudden offset and breakage (faulting), or buckling (folding) of the ground surface, along with the strong ground shaking.
In general, an earthquake on a fault needs to be bigger than about magnitude 6 to 6.5 for the break on the fault to come all the way up to the ground surface. So where we can see faults at the ground surface, we know that those faults are can create significant earthquakes – the longer the fault line (trace) on the ground, the larger the earthquake. Smaller earthquakes happen on faults that are completely under the ground. We only know they are there because these smaller earthquakes happen, and sometimes we can see them in seismic reflection soundings of the crust. These soundings are extremely expensive to do and are usually done as part of oil exploration.
A fault is called “active” if it has moved within the last 125,000 years and is likely to move again at some point in the future, creating an earthquake. Not all faults are created equal – some move far more often than others, anywhere from every few hundred years (like the Hope Fault and the Alpine Fault) and others every several thousand years (like the Greendale Fault).
All available information on known active faults in the Canterbury Region was collated in a report, with accompanying maps, in 1998. This report Earthquake Hazard and Risk Assessment Study: Earthquake Source Identification and Characterisation (ECan report number U98/10) compiled by the University of Canterbury, GNS Science and Geotech Consulting Ltd, is available from Environment Canterbury customer services.
The report was updated in 2008 with Earthquake Hazard and Risk Assessment Study: Canterbury Region Earthquake Source Identification and Characterisation (ECan report number U08/41). This is a large report and it is available from Environment Canterbury customer services.
If a fault breaks to the ground surface during an earthquake, aside from the ground shaking that is produced, the break in the ground can damage buildings and infrastructure across it. Identifying earthquake faults and managing development on and near them is one way of reducing the risk of damage from big earthquakes on those faults. The Ministry for the Environment guidelines Planning for Development of Land on or Close to Active Faults recommend a risk-based approach where restrictions on development vary depending on the long-term activity of the fault, and the type of building proposed for the site. Managing development near active faults does not address the ground shaking hazard from earthquakes – this is dealt with when a building is constructed, through the Building Act 2004.
Fault trace reports for each district in Canterbury with faults at the ground surface are being compiled for Environment Canterbury and district councils by GNS Science. Each report provides information on the location and character of active geological faults and folds in the district. The faults are mapped at a regional scale, and are not precise enough for site-specific assessments. The information is intended to highlight areas where there is a risk of fault movement, and where more detailed investigations should be done if development is proposed in that area.
Most of the faults and folds identified at the ground surface in Canterbury are in rural or very sparsely populated areas. In addition, most of the faults have relatively long recurrence intervals (long term average time between fault movements), in the order of several thousand years. Following the Ministry for the Environment Active Fault Guidelines, normal residential development would be allowed on or near faults with recurrence intervals this long.
In some areas, where faults that are most likely to move (with recurrence intervals of a few hundred to a few thousand years) are near populated areas, more detailed reports have been produced to enable development to be managed on or near those faults.
Active faults and folds in Hurunui district report (2.97 MB)
Active faults and folds in Waimakariri district report (3.1 MB)
In addition to the district report, more detailed mapping is being done for the Ashley and Loburn faults near Rangiora because of the higher likelihood of movement on these faults compared to others in the district, and the potential for future development across the faults. This report will be available in early 2014.
Active faults and folds in Selwyn district report (4 MB)
A separate report on the Greendale Fault was produced following the September 2010 Darfield (Canterbury) earthquake. There is more information about this report below.
Active faults and folds in Ashburton district report (2.8MB)
Active faults and folds in Mackenzie district report (1.5MB)
In addition to the district report, more detailed mapping was done for the Ostler Fault Zone near Twizel because of the higher likelihood of movement on that fault compared to others in the district, and the potential for future development across the fault.
Active fault and fold hazards in the Twizel area report (1.8MB)
Other fault trace reports are planned as as follows:
This work was reprioritised in December 2011 in consultation with the district councils as part of Long Term Plan discussions.
There is no report planned for Christchurch City, as there are no known faults at the ground surface within the Christchurch City Council area. The Port Hills Fault, and other faults discovered under Christchurch since February 2011 are completely underground and do not reach the ground surface.
The previously unknown Greendale Fault ruptured (moved) to the ground surface, causing up to 5 metres horizontal and 1 metre vertical offset of the ground, during the September 2010 Darfield (Canterbury) earthquake.
Environment Canterbury commissioned GNS Science, with help from the University of Canterbury, to define a planning 'fault avoidance zone' based on the mapping they had done already, and to estimate how often the fault moves (the recurrence interval).
There is little evidence for past movement on the fault in the last 16,000 years. However, because of the uncertainties involved, a conservative approach was taken and the fault has been categorised as a Recurrence Interval Class IV fault (a recurrence interval of between 5,000 and 10,000 years). A PhD study by a University of Canterbury student will work towards refining the recurrence interval class over the next three years.
Taking a risk-based approach, the Ministry for the Environment Active Fault Guidelines recommend that normal residential development be allowed within the fault avoidance zone for faults of this recurrence interval class, but recommends restrictions for larger community buildings or facilities with post-disaster functions.
The report will assist Selwyn District Council in granting consents for rebuilding houses on or near the Greendale Fault that were damaged in the September 2010 earthquake. It will also help inform future development on or near the Greendale Fault and the information will be available for Land Information Memoranda.
Report - Greendale fault (2.3MB)
Names are usually only given to faults that can been seen at the ground surface. However, because of the impact of the February 2011 Christchurch earthquake and the ability to "see" the fault deep underground through the aftershock pattern and seismic sounding investigations, the fault has been given the name the Port Hills Fault.
The movement on the Port Hills Fault during the February 2011 earthquake stopped somewhere around 1-2 km below the ground surface - it didn't break the ground surface. Because of this we are not commissioning a report like the Greendale Fault report to advise on managing fault rupture hazard at the ground surface.
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