There were over 20,000 aftershocks in the year following the M7.8 Kaikōura earthquake. Since that first year, there has been 2,552 aftershocks in six months between 14 November 2017 and 14 May 2018. 167 of those aftershocks were M3 and above. You can use or Quake Search tool on the website if you want to know about other earthquakes.
While no one can yet scientifically predict earthquakes, we can provide forecasts of future aftershocks (based on probabilities), as well as some scenarios from what is most likely to happen to what is very unlikely, but still possible. Most earthquake aftershock sequences decay (i.e. the number of earthquakes generally decreases) over time, with spikes of activity that can include larger earthquakes.
|Average number of M5.0-5.9||Range* of M5.0-5.9||Probability of 1 or more M5.0-5.9||Average number of M6.0-6.9||Range* of M6.0-6.9||Probability of 1 or more M6.0-6.9||Average number of M≥7||Range* of M≥7||Probability of 1 or more M≥7|
|within three months||1.9||0-7||74%||0.2||0-1||16%||0.01||0-1||1.4%|
|within one year||7.0||1-16||>99%||0.6||0-3||47%||0.05||0-1||5%|
Forecast for rectangular box (see map below) with the coordinates -40.7, 171.7, -43.5, 171.7, -43.5, 175.5, -40.7, 175.5 at 12 noon, 14th May 2018; 95% confidence bounds.
The aftershocks of the magnitude 7.8 Kaikoura earthquake are mostly occurring throughout a broad area from North Canterbury through to Cook Strait that surrounds the faults that ruptured in that earthquake, although a few have occurred in the lower North Island. We forecast aftershock probabilities for the area in the red box on the map below. The area near the centre of the box (around Kaikoura) is more likely to experience felt aftershocks than areas towards the edge of the box. See the MMI map below for more information on the forecast shaking. Earthquakes can and do happen outside this box but the box represents the most likely area for aftershocks in this sequence.
For example, there is a 16% chance of one or more M6.0-6.9 earthquakes occurring within the next three months; and a 47% chance of one or more M6.0-M6.9 within a year. We estimate there will be between 0 and 3 earthquakes in this magnitude range within the next year.
The current rate of M6 and above earthquakes for the next three months is about 2 times larger than what we would normally expect for long term seismicity represented in our National Seismic Hazard model. As the aftershock rates decrease, this difference will decrease as well.
We have also calculated the probability of strong and severe earthquake shaking from aftershocks over the next year (starting 14 May 2018). On the Modified Mercalli Intensity (MMI) scale Strong shaking is classed as MM6, and severe as MM7. The MMI scale is different to earthquake magnitude – it describes the intensity and impacts of the shaking, which depend on the magnitude of the earthquake, how far away the earthquake was and the type of ground you are on. At MM6 intensity shaking levels, walking steadily is difficult, furniture and appliances may move on smooth surfaces, and objects fall from walls and shelves; glassware and crockery break; slight non-structural damage to buildings may occur. At MM7 intensity shaking levels, it is difficult to stand; furniture and appliances move; contents are damaged; there is minor building damage and liquefaction can occur in susceptible sediments.
The maps show the probability of MM6 and MM7 shaking within the aftershock region, which includes Wellington. Over the next year the probability of MM7 shaking is largest around Cape Campbell with around 20%. In comparison, the probability of MM7 shaking in the Wellington area is around 4% (dark blue) in the next year. While this probability is considerably lower in Wellington than in the areas around Kaikoura (15%), it is possible for shaking similar to what occurred during the mainshock to happen again in Wellington.
Can't get enough technical information? Here's the fine print on how we model aftershock probabilities.