Canterbury Aftershocks

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.

How many have there been?

Long-term forecasts

Map of the central Canterbury area showing the rectangular zone covered by the model.

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 (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.