Long-term forecasts

The expected probabilities (below) of further earthquakes cover the Canterbury region aftershock zone as indicated in the plot below. The numbers are based on the behaviour of aftershock sequences worldwide and the specific knowledge that scientists have of the Canterbury aftershock sequence since September 2010. The figures are generated from computer models that are updated as the aftershock sequence continues.

Canterbury region long-term probabilities

Issued on 3 September 2018 for the coming year.

Aftershock probabilities read from the table for the coming year:

• Within the next year, there is a 43% probability of one or more earthquakes of magnitude 5.0 to 5.9 occurring in the region shown in the box in the map. It is expected that there will be between 0 and 2 events of this magnitude during the coming year.
• It is very unlikely (5%) that there will be a magnitude 6.0-6.9 earthquake.
• It is extremely unlikely (less than 1%) that there will be an earthquake of magnitude 7 or greater.

This table shows an updated forecast based on a model from international expert elicitation. The forecast starts from 1 September 2018 and is for the region from 171.6-173.2 degrees east and 43.3-43.9 degrees south (see map).

As time passes the expected probabilities of earthquakes become smaller, but any further significant earthquakes that do occur will cause these probabilities to change. The magnitude categories illustrate clearly how the probability falls away as magnitude increases. The probability for an aftershock to occur decreases as magnitude increases, and a magnitude increase of one means a probability decrease of roughly 10 times. This means that a magnitude 7.9 earthquake is roughly 100 times less likely than a magnitude 6.0 earthquake and is therefore very unlikely. With every month that passes without a major aftershock, probabilities will continue falling. However, if another large aftershock occurs it can re-energise the system and spark a resurgence of earthquake activity for a month or so; this was seen with both the February and June 2011 magnitude 6.3 earthquakes. The maximum magnitude of an earthquake is also bounded by what scientists know about the size of faults in Canterbury. Scientists are currently not aware of any faults in Canterbury that are long enough to be able to produce a magnitude 7.9 earthquake. However, they cannot rule out this possibility with 100 percent certainty.

These figures are for the entire aftershock zone, not just for Christchurch city (see map for location of model).

How many have there been?

Numbers of Canterbury region earthquakes from 4 September 2010 to 3 September 2015

• This table was last updated on 4 September 2015
• Aftershock numbers/magnitudes may change as our analysts continue to fine-tune the aftershock sequence data.
• Magnitudes used are local magnitude calculations.

Peak ground accelerations (PGA) of aftershocks magnitude 5.5 or above