Long-term forecasts

While no one can yet scientifically predict earthquakes, we can provide forecasts of future aftershocks using computer models that are updated as the aftershock sequence continues. The forecasts for aftershocks from the September 2010 Darfield (Canterbury) Earthquake are in the table below. These forecasts are for the entire aftershock zone, not just for Christchurch city, as indicated by the box on the map below. Earthquakes can and do happen outside this box, but the box represents the most likely area for aftershocks in this sequence.

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.

Canterbury aftershock sequence long-term probabilities

Issued on 1 September 2020. This table shows a forecast for future aftershocks for1 year from 1 September 2020, for the area from 171.6-173.2 degrees east and 43.3-43.9 degrees south (see map).

This table says that:

• Within the next year, there is a 38% probability (unlikely) of one or more earthquakes of magnitude 5.0 to 5.9 occurring in the area shown in the box in the map. It is expected that there will be between 0 and 2 events of this magnitude within the next year.
• It is very unlikely (4%) 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.

How do aftershock sequences change over time?

As time passes the chance of further earthquakes generally becomes lower. The table shows how the probability of larger magnitude aftershocks (e.g. over M7.0) is lower than smaller magnitude aftershocks (e.g. M5.0-5.9). In fact, a magnitude increase of one means a rate 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.

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.

What you can do about this:

As described in this article about our forecasts, the best thing we can do is be prepared for earthquakes. You can follow the National Emergency Management Agency on Twitter and Facebook for the latest earthquake and tsunami preparedness information. EQC also have a great guide to Quake Safe your home and you can also follow your local Civil Defence Emergency Management Group.

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

This table shows details for the more significant aftershocks that have occurred so far.