To determine the likelihood of strong ground shaking in different parts of Canterbury, Environment Canterbury commissioned GNS Science to undertake a probabilistic seismic hazard assessment. (This information was later incorporated into the national seismic hazard model, which determines building standards under the Building Code.)
The GNS Science report can be downloaded below. The report gives ground shaking intensity maps in Modified Mercalli intensity, Peak Ground Acceleration (PGA) and spectral accelerations for the 50, 150, 475 and 1000 year return periods. It also gives these values for specific towns in Canterbury. An addendum report includes further towns. Earthquake scenarios are also provided for major towns.
Canterbury probabilistic seismic hazard assessment report (2.5MB)
Canterbury probabilistic seismic hazard assessment addendum report (1.09MB)
The seismic hazard assessment model looks at the location and level of activity of all known earthquake faults in and near Canterbury, and the potential size of earthquakes that could be generated on them. It puts all this information into a model along with all historic earthquakes, as well as some additional possible earthquakes that could be generated on as-yet unknown faults. Using all this information the model then calculates what intensity of ground shaking could be expected in different areas of Canterbury over different time periods – the longer the time period, the more likely a particular location is to experience strong ground shaking. The values for all these different locations are then contoured to give a graded map, like the one below.
This map shows the Modified Mercalli intensity expected in Canterbury in any 500 year period. This does not represent shaking from one particular earthquake, but rather the maximum ground shaking you would be likely to experience in one particular location during a 500 year period from a range of earthquakes on different faults. More simply put – the hotter the colour, the higher the earthquake hazard and the more likely you are, over the long term, to experience strong ground shaking.
The map shows that the long-term earthquake hazard is highest at and near the boundary between the Australian and Pacific tectonic plates, which runs along the Alpine Fault on the West Coast, and along the Hope Fault through North Canterbury. This is because these faults move, causing big earthquakes, much more frequently (every few hundred years) than other faults in and near Canterbury. The earthquake hazard decreases the further you are from the plate boundary.
Different ground types – hard rock, gravel soils, sandy soils – influence the intensity of ground shaking at a particular location during an earthquake, and hence the ground shaking hazard of a location. The model developed here does not take into account variations in ground type - it assumes shallow soil (Class C) site conditions for all locations.
While the earthquake hazard is highest in the Southern Alps and North Canterbury, anywhere in Canterbury could be affected by a big earthquake – it’s just less likely the further you are away from the plate boundary. The seismic hazard model did take into account the possibility of a large earthquake on an unknown fault somewhere near Christchurch. However, the faults this far from the plate boundary move very infrequently – every several thousand years – so do not contribute much to the overall hazard in Canterbury, particularly over the 500 year time period, which is what Building Code regulations are based on. The 2010 and 2011 earthquakes are very rare events.
The earthquake hazard for most of Canterbury is still the same as it was when the GNS Science report was produced in 2008, based on available knowledge. The earthquake hazard in the area affected by the 2010 and 2011 earthquakes (a band from Rakaia Gorge to Banks Peninsula) will be higher than shown in the map, and in the reports below, over the next few years and possibly decades because of aftershock activity and possible stress changes in the crust in the aftershock area which may cause further earthquakes. The seismic hazard values used for reconstructing buildings in Christchurch have been adjusted to take this possible increase in earthquake hazard.
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