Geological hazards
Communities and infrastructure in the Canterbury/Waitaha region are subject to risks from geological hazards such as earthquakes, landslides and tsunamis.
Here are the current issues faced around geological hazards in Waitaha.
Issue: Surface fault rupture and related risks to communities and assets in Waitaha
Surface fault rupture is the permanent ripping and warping of the ground surface along a fault as the ground on one side moves sideways and/or up relative to ground on the other side.
This only affects narrow areas of land along the fault. It is a different hazard to earthquake shaking which can affect widespread areas a long way from the fault.
While fault rupture may only affect relatively small areas of the region, the impact in affected areas can be severe and difficult to mitigate.
Fault rupture can cause substantial damage to buildings and infrastructure (such as roads, railways, pipes and so on) that cross active faults.
Many faults ruptured to the ground surface in Waitaha during the 2016 Hurunui - Kaikōura earthquake, causing damage to houses, roads, and the railway line. The previously unmapped Greendale Fault also ruptured to the ground surface in the 2010 Darfield (Canterbury/Waitaha) earthquake, which damaged farmland, buildings and infrastructure.
Papatea fault rupture during the 2016 Kaikōura earthquake
Current actions or approach
In connection with surface fault rupture hazards, the Canterbury Regional Policy Statement (the CRPS) identifies that we are the responsible local authority for the control of the use of land within the beds of rivers and lakes, and within the coastal marine area (CMA).
City and district councils are responsible for the control of the use of land outside of those areas. District Plans are the primary resource management tool for the management of surface rupture natural hazards.
The Canterbury Regional Policy Statement directs that new subdivision, use and development of land on, or close to, an active earthquake fault trace is managed in order to avoid or mitigate the adverse effects of fault rupture.
Earthquake shaking is a different hazard to surface fault rupture. The Building Code aims to minimise impacts associated with earthquake shaking.
The map below indicates known and suspected faults in Waitaha.
Known and suspected faults
Issue: Liquefaction and related impacts on communities and assets in Waitaha
Liquefaction is a process that temporarily turns firm ground into a thick liquid during strong earthquakes.
Liquefaction happens in loose, silty and sandy soils that are below the water table. During strong earthquakes, the water pressure in the soil increases until it forces the soil particles apart so that they ‘float’ in the water.
Liquefaction can make buildings sink into the ground surface, cause buried objects (such as pipes and tanks) to float, silty water to erupt from the ground surface, and the ground level to drop.
Lateral spreading which is the horizontal movement of ground towards a ‘free-face’ in the landform (such as a riverbank) or downslope, can occur as a result of liquefaction. Lateral spreading can cause the ground level to drop and the ground surface to stretch, which can also impact buildings, assets and infrastructure.
Liquefaction, and to a lesser extent lateral spreading, can affect widespread areas and cause significant damage to buildings and infrastructure, as occurred in parts of Christchurch/Ōtautahi during the 2010/2011 earthquake sequence and parts of Kaikōura during the 2016 Hurunui-Kaikōura earthquake.
Current actions or approach
In connection with liquefaction hazards, the Canterbury Regional Policy Statement (CRPS) identifies that we are the local authority responsible for the control of land-use within the bed of rivers and lakes, land in the riparian margin, and land within the coastal marine area (CMA).
City and district councils are responsible for the control of the use of land outside those areas. District Plans are the primary resource management tool to manage the effects of liquefaction and lateral spreading.
The CRPS directs that new subdivision, use and development of land on or close to areas susceptible to liquefaction and lateral spreading is managed in order to avoid or mitigate the adverse effects. The Building Code also aims to minimise impacts associated with liquefaction.
The map below indicates where liquefaction is possible during strong earthquake shaking
Liquefaction susceptibility
Issue: Landslides and related impacts on communities and assets in Waitaha
Landslides are the gravitational movement of rock and soil down a slope and are typically triggered by earthquakes or significant rainfall events.
There are two main hazards associated with landslides:
- slippage of land (i.e. the movement or loss of land from beneath a slope)
- debris inundation where material from upslope covers downslope areas.
Debris flows (wet, rainfall-triggered landslides) can inundate areas a lot further from the source areas than dry earthquake-triggered landslides, as rock and soil become entrained in the water runoff.
Landslides can cause significant damage to buildings and pose a significant threat to life.
Landslides can also produce secondary hazards such as landslide dams, which can in turn lead to severe flooding if the dams breach catastrophically.
Landslide north of Kaikōura triggered by 2016 Kaikōura earthquake. Credit: Waka Kotahi
Many urban areas in Waitaha are located on the plains and are not subject to landslide hazards. However, parts of Christchurch/Ōtautahi situated on and immediately below the Port Hills, and other hill and high country areas of Waitaha can be subject to landslide hazards.
Parts of Christchurch/Ōtautahi were affected by rockfall (a type of landslide) during the 2010/2011 earthquake sequence. Many landslides were triggered in the Hurunui and Kaikōura districts by the 2016 Hurunui-Kaikōura earthquake, and by intense rainfall associated with ex-tropical cyclone Gita in 2018.
Climate change is likely to increase landslide risk in some areas due to changes in rainfall patterns and an increase in the intensity of storm events.
Debris flow north of Kaikōura triggered by rainfall associated with ex-tropical cyclone Gita 2018
Current actions or approach
There is no specific direction for managing the risk from landslides in the Canterbury Regional Policy Statement. The risk from landslides is addressed through a general hazard management approach where subdivision, use, or development of land is to be avoided if the risk from natural hazards is unacceptable.
This approach is implemented through district plans which are required to ensure that natural hazards are assessed taking into account the likelihood of the natural hazard event and its potential consequence. The Building Code also aims to minimise impacts associated with landslides.
Issue: Tsunami and related risks to communities and assets in Waitaha
A tsunami is a series of powerful waves that move through the ocean caused by a sudden movement to the sea floor or by a large mass of material into the ocean. Tsunamis can also occur in lakes.
They are most often caused by underwater earthquakes but can also be caused by landslides, volcanic eruptions and the calving of glacial ice.
Most tsunamis generated in the ocean are relatively small by the time they reach the New Zealand/Aotearoa coast and don't flood land, but even small tsunamis can cause strong currents and surges around the coast, which can be dangerous for people in the water or on the beach.
Occasionally, a tsunami will be large enough to flood land and very rarely could be large enough to cause significant loss of life and damage to property.
Waitaha is exposed to different tsunami sources. A tsunami can be local, regional, or distant, depending on where it originates and how long it takes to travel to areas where it could have an impact.
It is important to know the potential tsunami source, travel-time, size, and area likely to be inundated so we can plan and respond accordingly to minimise any potential loss of life or damage.
Damage to a house in Pigeon Bay from a tsunami triggered by 2016 Kaikōura earthquake. Credit: GNS
Tsunami inundation modelling
We have commissioned tsunami inundation modelling for much of the Waitaha coast. This modelling has helped us identify locations along the coast that could be affected by different-sized tsunamis from different sources, together with how far inland and how deep the associated flooding might be. The modelling also lets us know how much warning time we might have for a particular tsunami scenario.
Current actions or approach
There is no specific direction for managing the risk from tsunamis in the Canterbury Regional Policy Statement. The risk from tsunamis is addressed through a general hazard management approach where subdivision, use, or development of land is to be avoided if the risk from natural hazards is unacceptable.
This approach is implemented through district plans which are required to ensure that natural hazards are assessed taking into account the likelihood of the natural hazard event and its potential consequence.
We have worked with the district councils to develop tsunami evacuation zones using the tsunami modelling. These are areas that we recommend people evacuate from as a precaution after they feel a long or strong earthquake, or in the event of an official tsunami warning.
The tsunami evacuation zones are fundamentally about life safety and are primarily for evacuation planning and public education. The map below indicates where the tsunami evacuation zones in Waitaha.