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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?

How many have there been?

Numbers of Canterbury region earthquakes from 4 September 2010 to 18 February 2014*
Magnitude rangeNumber
7.0 and above1
6.0 - 6.93
5.0 - 5.951
4.0 - 4.9496
3.0 - 3.9

3648

This table was last updated on 18 February 2014
*Aftershock numbers/magnitudes may change as our analysts continue to fine-tune the aftershock sequence data.

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 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 Arthur's Pass in the northwest to Banks Peninsula, and from Ashburton in the south to Waipara in the north (see the map).

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 month

0.10 - 1

9%

0.0080 - 1

1%

0.00060 - 1<1%

Within 1 year

1.20 - 4

69%

0.090 - 1

9%

0.0070 - 11%

Issued on 1 July 2014 for the coming month.

This table shows an updated Canterbury forecast based on a model from international expert elicitation. All forecasts start 1st June 2014 and are for the region from 171-174 degrees east and 43-44 degrees south (see map above). For example, within the next 1 year, there is a 69% probability of one or more earthquakes of magnitude 5.0 to 5.9 occurring in the Canterbury region. It is expected that there will be between 0 and 4 events of this magnitude during the coming year. This forecast uses a new, more complete, version of the earthquake catalogue for the first time, resulting in higher estimates than those calculated in March 2014. The rate of aftershocks is expected to continue to decrease over time. 

Updated model for Canterbury

An updated model 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. The rate of aftershock occurrence is continuing to diminish as expected.

 

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After the Darfield earthquake and its major aftershocks we published more finely grained forecasts up until 12 October 2011.


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