Earthquake forecasts



The Darfield (Canterbury) earthquake is generating thousands of aftershocks, although most of them are not felt. What are the numbers so far? What is the expectation for the coming months and years?

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
M5.0-5.9 M6.0-6.9 M≥7.0
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 0.6 0 - 2 45% 0.05 0 - 1 5% 0.005 0 - 1 <1%

Issued on 1 September 2017 for the coming year.

Aftershock probabilities read from the table for the coming year:

This table shows an updated forecast based on a model from international expert elicitation. The forecast starts from 1 September 2017 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).

Map of the central Canterbury area showing the rectangular zone covered by the model.

Map of the central Canterbury area showing the rectangular zone covered by the model.

How many have there been?


Numbers of Canterbury region earthquakes from 4 September 2010 to 3 September 2015
Magnitude range Number
7.0 and above 1
6.0 - 6.9 2
5.0 - 5.9 34
4.0 - 4.9 383
3.0 - 3.9 3921
TOTAL 4341

A graph showing the significant effect of the February, June and December 2011 earthquakes re-energising the Darfield aftershock sequence. No other earthquakes have affected the decay rate in so obvious a fashion. Thanks to Chris Mance for this graph.

A graph showing the significant effect of the February, June and December 2011 earthquakes re-energising the Darfield aftershock sequence. No other earthquakes have affected the decay rate in so obvious a fashion. Thanks to Chris Mance for this graph.

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


Earthquake Date/Time Magnitude (Mw) Location Depth
(km)
largest recorded
PGA (g)

Station recording
largest PGA

Distance from station recording
highest PGA to epicentre
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