Published: Mon May 22 2017 10:00 AM
Updated: Fri Oct 20 2017 11:00 AM
The expected numbers of earthquakes continue to drop. There is now a 14% chance of one or more M6.0-6.9 earthquakes occurring within the next two months.
We’ve had over 18,500 earthquakes since the M7.8 Kaikōura earthquake stopped shaking our islands (we ran the numbers on the 18th of August).
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 two months||1.6||0-6||69%||0.1||0-1||14%||0.01||0-1||1%|
|within one year||8.9||2-18||>99%||0.8||0-3||54%||0.07||0-1||6%|
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, Thursday, 19 October; 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 to the right. 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 for the Wellington area. 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 14% chance of one or more M6.0-6.9 earthquakes occurring within the next two months. We estimate there will be between 0 and 1 earthquakes in this magnitude range within the next two months.
The current rate of magnitude 6 and above earthquakes for the next two 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 earthquake shaking from aftershocks over the next year (starting 19 October 2017). 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 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 around the wider Kaikoura/northern area is around 20%. In comparison, the probability of MM7 shaking in the Wellington area is around 3% (dark blue) in the next year. While this probability is considerably lower in Wellington than in the areas around Kaikoura, it is possible for shaking similar to what occurred during the mainshock to happen again in Wellington.