The Darfield (Canterbury) earthquake is generating thousands of aftershocks, although most of them are not felt. What are the numbers over the first two years? What is the expectation for the coming month and year?
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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.
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How many have there been?
Numbers of Canterbury region earthquakes from 4 September 2010 to 3 September 2012 | |
| Magnitude range | Number |
|---|---|
| 7.0 and above | 1 |
| 6.0 - 6.9 | 3 |
| 5.0 - 5.9 | 55 |
| 4.0 - 4.9 | 494 |
| 3.0 - 3.9 | 3870 |
| This table was last updated on 16 January 2013 | |
Long-term forecasts
The table below summarises the expected probabilities of further earthquakes anywhere in 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.
It shows that as time passes these probabilities 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 are 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. The zone extends from Hororata in the west to large parts of Banks Peninsula, and from Kaiapoi in the north to Lincoln in the south.
Canterbury region long-term probabilities | ||||||
| One month: 1 June 2013 - 30 June 2013 | One year: 1 June 2013 - 31 May 2014 | |||||
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| Magnitude range | Expected range | Expected average | Probability | Expected range | Expected average | Probability |
| 5.0 - 5.4 | 0 - 1 | 0.099 | 9% | 0 - 3 | 0.97 | 62% |
| 5.5 - 5.9 | 0 - 1 | 0.028 | 3% | 0 - 2 | 0.29 | 25% |
| 6.0 - 6.4 | 0 - 1 | 0.0079 | 1% | 0 - 1 | 0.080 | 8% |
| 6.5 - 6.9 | 0 - 1 | 0.0021 | <1% | 0 - 1 | 0.022 | 2% |
| 7.0 - 7.9 | 0 - 1 | 0.0008 | <1% | 0 - 1 | 0.008 | 1% |
This table was last updated on 12 June 2013 | ||||||
 | Further informationAfter the Darfield earthquake and its major aftershocks we published more finely grained forecasts up until 12 October 2011. |