Canterbury sits across the boundary of two large plates of the Earth’s crust that are moving towards each other – the Pacific Plate and the Australian Plate.
The forces involved in plate movement are huge. They cause the rock in the Earth’s crust to buckle (fold) and fracture (fault), particularly near the plate boundary.
Key facts about earthquake faults across the region
The boundary between the Australian and Pacific tectonic plates cuts through the South Island. The two plates meet at the Alpine Fault, which runs up the western edge of the Southern Alps, before splitting into four main faults through North Canterbury and Marlborough – the Wairau, Awatere, Clarence and Hope faults. Away from the main plate boundary faults there are many smaller faults throughout Canterbury.
Earthquakes happen when a fault suddenly breaks. Most of the time the rocks either side of a fault are ‘stuck’ together, and they can stay stuck together for a long time while the land around the fault is pushed and squeezed by the movement of the plates. But at some point there is too much pushing and squeezing and something has to give – the rock breaks along the fault and the shock waves are felt as an earthquake.
Some faults move more often than others, anywhere from every few hundred years at and near the plate boundary (like the Alpine and Hope faults) to every several thousand years further away (like the Greendale Fault).
This map shows the relative likelihood of earthquake shaking across Canterbury – the warmer the colour the more likely you are, over a lifetime, to experience strong earthquake shaking. The map is created by looking at where the known earthquake faults are and working out how big an earthquake they could create, and how often.
The 2016 Kaikoura earthquake happened in one of the most active areas of New Zealand – right in the plate boundary zone. While the complexity of the earthquake was surprising, the fact that we had a big earthquake in that area was not.
The 2010/11 Canterbury earthquakes were a long way (around 100 km) from the plate boundary.
Faults this far from the plate boundary move very infrequently, every several thousand years, so these earthquakes were very rare events, but they remind us big earthquakes can happen anywhere in Canterbury.
The strength (or intensity) of an earthquake is not the same as magnitude (sometimes called the Richter Scale), which measures how big an earthquake is where it starts, or how much energy was released.
The intensity of shaking that you feel during an earthquake depends on the magnitude, how deep it is, how far away from it you are, and what sort of ground you are on, for example, rock, gravels or sand. In general, earthquake intensity decreases the further away you are from the earthquake.
Think of a stereo speaker - the number on the volume dial is like the magnitude and how loud the music is where you are standing is like the intensity. How loud the music sounds (the intensity) depends on how high you have the volume dial turned up (the magnitude) and how far away from the speaker you are standing, and what’s in between you and the speaker.
When we talk about the effects of an earthquake at a particular location, it is more useful to talk about the shaking intensity, rather than the magnitude.
Surface fault rupture usually only affects a narrow corridor of land a few tens of metres wide where the fault meets the ground surface – a much smaller area than that affected by shaking from the earthquake – but it can seriously damage buildings or infrastructure like roads or pipes that cross the fault.
Surface fault rupture is relatively uncommon – there are only four known historical examples in Canterbury:
- The 1888 North Canterbury earthquake on the Hope Fault west of Hanmer Springs.
- The 1929 Arthur’s Pass earthquake on the Poulter Fault.
- The 2010 Darfield (Canterbury) earthquake on the Greendale Fault.
- The 2016 Kaikoura earthquake, which ruptured parts of The Humps, the Hundalee, Hope, Jordan Thurst, Papatea, Kekerengu and Needles faults.
Surface fault rupture usually produces distinctive landforms like scarps or steps in the landscape. If we map these features, we can tell where surface fault rupture could happen in future, and we can then set buildings and infrastructure back from the fault trace to reduce the likelihood of damage, or engineer them to withstand ground movement.
Read in-depth about Canterbury's faults on our reports page.
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 near faults vary depending on how active the fault is and the type of building proposed for the site.
Most of Canterbury’s faults are in rural areas or the mountains, and have a long time between earthquakes, so following the Ministry guidelines, normal residential development would be allowed on or near most faults in Canterbury.
The faults in our district earthquake fault reports are mapped at 1:250,000, which is not precise enough to say exactly where a fault is on the ground.
The information is intended to highlight where there is a possible risk of surface fault rupture and where more detailed investigations should be done for some types of development. Buildings and infrastructure can then be set back from the fault or engineered to withstand fault movement.
In some areas, where faults that are most likely to move are near populated areas, more detailed reports have been produced so that development to be managed around those faults. These include the Hanmer Fault at Hanmer Springs, the Hope Fault at Mt Lyford, the Ashley Fault near Rangiora and the Ostler Fault near Twizel.
Managing development near faults does not address earthquake shaking from earthquakes – this is dealt with when a building is constructed, through the Building Act 2004.
Environment Canterbury commissioned GNS Science, with help from the University of Canterbury, to define a 'fault avoidance zone' based on the mapping they had done of the fault, and to estimate how often the fault moves on average (the recurrence interval). Read the report here.
A PhD study by a University of Canterbury student has since shown that the last movement on the Greendale Fault before 2010 was in the order of 20,000 to 30,000 years ago.
The Ministry for the Environment Active Fault Guidelines recommend that development is allowed within the fault avoidance zone for faults with this level of activity, except for important facilities with post-disaster functions.
All of them, apart from the Papatea Fault that appeared to not be active any more, are in our district earthquake fault reports for Hurunui and Kaikoura. Being right on the plate boundary this is a really active area of New Zealand and is where we expect big earthquakes to happen.
Scientists also knew that faults could rupture together in an earthquake, but the number of faults involved in this earthquake was surprising, as is the amount they moved.
Land on one side of the Kekerengu Fault moved sideways by up to 10 metres and there was several metres of vertical movement recorded on many of the faults. This is how our mountains are built and our landscape formed: in earthquakes over millions of years.
Earthquake geologists from many different organisations are now studying how these faults moved – you can follow their latest work on GeoNet.