Aftershocks
The Darfield (Canterbury) earthquake is generating thousands of aftershocks, although most of them are not felt. What are the numbers like so far? What is the expectation for the coming month and year?
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.
Our thanks to Chris Mance for compiling this and sending it in to us.
- View our map of the latest aftershocks in the area
- Read the list of the most recent earthquakes in New Zealand
How many so far?
| Magnitude range | Number |
|---|---|
| 7.0 and above | 1 |
| 6.0 - 6.9 | 3 |
| 5.0 - 5.9 | 52 |
| 4.0 - 4.9 | 399 |
| 3.0 - 3.9* | 2981 |
| * not all earthquakes in this range have been analysed yet. | |
| This table was last updated on May 10 2012 | |
24 hour forecast map
View the 24 hour forecast map for the Canterbury region.
Aftershock forecast numbers vs. actual numbers
The statistical modelling of the Darfield sequence is now focused on estimating the long-term probabilities of earthquakes greater than magnitude 5 in the Canterbury region (see next section). The table of forecast numbers vs. actual numbers (up until October 12 2011) has now been closed off.
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.
Please read GNS Science's latest media release regarding the computation of probabilities of future events.
| One month: April 15 2012 - May 14 2012 | One year: April 15 2012 - April 14 2013 | |||||
|---|---|---|---|---|---|---|
| Magnitude range | Expected range | Expected average | Probability | Expected range | Expected average | Probability |
| 5.0 - 5.4 | 0 - 1 | 0.17 | 15% | 0 - 4 | 1.4 | 76% |
| 5.5 - 5.9 | 0 - 1 | 0.05 | 5% | 0 - 2 | 0.4 | 34% |
| 6.0 - 6.4 | 0 - 1 | 0.01 | 1% | 0 - 1 | 0.1 | 11% |
| 6.5 - 6.9 | 0 - 1 | 0.004 | <1% | 0 - 1 | 0.03 | 3% |
| 7.0 - 7.9 | 0 - 1 | 0.001 | <1% | 0 - 1 | 0.01 | 1% |
| This table was last updated on April 30 2012 | ||||||
Aftershock felt reports
We invite you to fill in felt earthquake reports for the aftershocks by clicking on the "felt it?" button next to each quake that you felt. We'd really appreciate it if you could try and select the earthquake nearest to the time you felt it. More information about this is found in our Earthquake section.
View Canterbury aftershocks in Google Maps
- View the last 14 days (updates every 5 minutes)
- View the first 14 days after the M7.1 Darfield earthquake (updated at 6 am daily)
- View the first 14 days after the February M6.3 Christchurch earthquake (updated at 6 am daily)
- View the first 14 days after the June M6.3 Christchurch earthquake (updated at 6 am daily)
Aftershock animation
You can view your own animation of the Darfield sequence, or any set of New Zealand quakes, in Google Earth. You can control the events for selection, the speed of animation, and both area and time of interest. Just go to QuakeSearch, set your search criteria, then select KML output and press submit. The default on the time selection is the last week. N.B. Time are in Universal Time, either 12 hours behind New Zealand Standard Time or 13 hours behind New Zealand Daylight Time.
If you have Google Earth installed you will be prompted to open the results. You may need to fine tune the animation speed and window size in Google Earth - mouseover the time bar and click on the spanner icon.
