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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 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

 

M5.0-5.9M6.0-6.9M ≥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 year0.800 - 355%0.070 - 17%0.0060 - 1<1%

Issued on 21 July 2016 for the coming year.

Aftershock probabilities read from the table for the coming year:
  • Within the next year, there is a 55% 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 3 events of this magnitude during the coming year.
  • It is very unlikely (7%) 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 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*
Magnitude rangeNumber
7.0 and above1
6.0 - 6.92
5.0 - 5.934
4.0 - 4.9383
3.0 - 3.9

3921

TOTAL4341

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

Earthquake Date/TimeMagnitude (Mw)LocationDepth
(km) 
largest recorded
PGA (g)

Station recording
largest PGA

Distance from station recording
highest PGA to epicentre
Sep 4 2010, 4:35:42 am7.125 km south of Oxford111.26Greendale9
Sep 4 2010, 4:37:03 am5.825 km west of Christchurch10unable to be determined--
Sep 4 2010, 4:37:36 am5.525 km west of Christchurch12unable to be determined--
Sep 4 2010, 4:52:56 am5.520 km west of Christchurch70.23Rolleston School2
Sep 4 2010, 4:59:20 am5.535 km west of Christchurch80.07Lincoln Crop and Food Research21
Feb 22 2011, 12:51:42 pm6.35 km southeast of Christchurch52.20Heathcote Valley Primary School2
Feb 22 2011, 1:04:19 pm5.85 km southeast of Christchurch60.93Christchurch Cathedral College6
Feb 22 2011, 2:50:30 pm5.95 km south of Christchurch70.76Heathcote Valley Primary School6
Jun 13 2011, 2:20:49 pm6.410 km east of Christchurch72.00Godley Drive3
Dec 23 2011, 1:58:38 pm5.815 km east of Christchurch100.98New Brighton Library6
Dec 23 2011, 3:18:04 pm6.010 km east of Christchurch70.66Heathcote Valley Primary School7
Jan 2 2012, 5:45:17 am5.515 km east of Christchurch120.21Pages Road Pumping Station18
May 25 2012, 2:44:49 pm5.520 km east of Christchurch120.17Lyttelton Port Oil Wharf17
Feb 14 2016, 1:13:43 pm5.710 km east of Christchurch80.36New Brighton Library8
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