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.
|Average number||Range||Probability of one or more||Average number||Range||Probability of one or more||Average number||Range||Probability of one or more|
|Within 1 year||0.5||0 - 2||38%||0.04||0 - 1||4%||0.005||0 - 1||<1%|
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:
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.
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.
This table shows details for the more significant aftershocks that have occurred so far.
|Earthquake Date/Time||Magnitude (Mw)||Location||Depth
|Distance from station recording
highest PGA to epicentre (km)
|Sep 4 2010, 4:35:42 am||7.1||25 km south of Oxford||11||1.26||Greendale||9|
|Sep 4 2010, 4:37:03 am||5.8||25 km west of Christchurch||10||unable to be determined||-||-|
|Sep 4 2010, 4:37:36 am||5.5||25 km west of Christchurch||12||unable to be determined||-||-|
|Sep 4 2010, 4:52:56 am||5.5||20 km west of Christchurch||7||0.23||Rolleston School||2|
|Sep 4 2010, 4:59:20 am||5.5||35 km west of Christchurch||8||0.07||Lincoln Crop and Food Research||21|
|Feb 22 2011, 12:51:42 pm||6.3||5 km southeast of Christchurch||5||2.20||Heathcote Valley Primary School||2|
|Feb 22 2011, 1:04:19 pm||5.8||5 km southeast of Christchurch||6||0.93||Christchurch Cathedral College||6|
|Feb 22 2011, 2:50:30 pm||5.9||5 km south of Christchurch||7||0.76||Heathcote Valley Primary School||6|
|Jun 13 2011, 2:20:49 pm||6.4||10 km east of Christchurch||7||2.00||Godley Drive||3|
|Dec 23 2011, 1:58:38 pm||5.8||15 km east of Christchurch||10||0.98||New Brighton Library||6|
|Dec 23 2011, 3:18:04 pm||6.0||10 km east of Christchurch||7||0.66||Heathcote Valley Primary School||7|
|Jan 2 2012, 5:45:17 am||5.5||15 km east of Christchurch||12||0.21||Pages Road Pumping Station||18|
|May 25 2012, 2:44:49 pm||5.5||20 km east of Christchurch||12||0.17||Lyttelton Port Oil Wharf||17|
|Feb 14 2016, 1:13:43 pm||5.7||10 km east of Christchurch||8||0.36||New Brighton Library||8|