The expected probabilities (below) of further earthquakes cover the entire Canterbury region aftershock zone. The figures 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
|Within 1 year||0.80||0 - 3||55%||0.07||0 - 1||7%||0.006||0 - 1||<1%|
Issued on 21 July 2016 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 21 July 2016 and is for the region from 171.6-173.2 degrees east and 43.3-43.9 degrees south (see map). This region is smaller than the one covered by our previous forecasts, and corresponds more closely to the aftershock region of the Darfield earthquake. See below for more information about this change.
The table shows that 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).
Updated model for Canterbury
Aftershock probabilities for Canterbury went down a bit more than usual after November 2014. This is because the area for which the probabilities were shown was reduced. The smaller area better represents the earthquakes that could have similar impacts as the 7.1 Darfield and subsequent aftershock sequence on central Canterbury (Christchurch city, and parts of Waimakariri and Selwyn Districts). Earthquakes at long distances from Christchurch are now excluded from the table. The probabilities of earthquakes close to Christchurch have not been affected by use of the smaller area. The probabilities for Christchurch, and for the entire Canterbury region, continue to decrease as expected.
This follows an updated model that was implemented for Canterbury forecasts in April 2014. There was an increase in the forecast rates and probabilities of aftershocks between March and April 2014 due to switching to the use of a more detailed earthquake catalogue in our modelling. Through additional analysis, many more small and moderate earthquakes were identified that had occurred during the early part of the aftershock sequence.These earthquakes had initially been difficult to locate in the data and have been found using a detailed analysis of the earthquake waveform data. These additional earthquakes have been added into the model, causing the numbers to be recalculated based on improved data. There was no increase in the Canterbury earthquake rates during March and April 2014.
After the Darfield earthquake and its major aftershocks we published more finely grained forecasts up until 12 October 2011.
How many have there been?
Numbers of Canterbury region earthquakes from 4 September 2010 to 3 September 2015*
|7.0 and above||1|
|6.0 - 6.9||2|
|5.0 - 5.9||34|
|4.0 - 4.9||383|
|3.0 - 3.9|
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.
Peak ground accelerations (PGA) of aftershocks magnitude 5.5 or above
|Earthquake Date/Time||Magnitude (Mw)||Location||Depth|
|largest recorded |
|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|