A new year, a new M7.8 Kaikoura aftershock forecast

20/01/2017 1.30 p.m.

We’ve been busy crunching the numbers based on all the seismic activity in the aftershock area of November’s Kaikoura Earthquake. So what’s the big news? The expected numbers of earthquakes have dropped a lot since the last forecast.  There is now a 25 percent chance of one or more M6.0-6.9 earthquakes occurring within the next month; this has decreased from 54 percent from our last forecast (19 December 2016).  We like this downward movement in our forecast; it is good step in the right direction. 

But does this mean we are all in the clear and don’t need to worry about more big earthquakes? No, absolutely not. Another big earthquake is still well within the probabilities in our models. A 25% chance in a month is still a concerning probability. We need to continue to be prepared for earthquakes as these will go on for years to come. The ongoing Canterbury Earthquake Sequence is an example of aftershocks that can last for years after the initial mainshock (which was the M7.1 Darfield quake in 2010). 

Remember: Drop, cover and hold in an earthquake. If the earthquake is long or strong and you are near the coast, evacuate as soon as the shaking stops. Our friends at the Ministry of Civil Defence and Emergency Management advise to not wait for an official warning or sirens.

What’s happened so far

We’ve had a total of 11956 earthquakes since the M7.8 Kaikoura earthquake stopped shaking our islands (we ran the numbers at 10 a.m. on the 19 January, so these will change). 452 of those earthquakes were M4-4.9, 53 were M5-5.9 and 4 have been M6.0 or greater. Yes, that is a lot of earthquakes but line up with what we’ve been forecasting.

Aftershock Forecast starting on 19 January 2017 

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. Our previous forecast, from 19 December, is here

 

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 30 days

3.2

0-9

89%

0.3

0-2

25%

0.03

0-1

3%

 

within one year

16.6

7-29

>99%

1.5

0-4

77%

0.1

0-1

12%

 

Forecast for rectangular box with the coordinates -40.7, 171.7, -43.5, 171.7, -43.5, 175.5, -40.7, 175.5 at 12 noon, Thursday, 19 January. * 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 25% chance of one or more M6.0-6.9 earthquakes occurring within the next month. 

The current rate of magnitude 6 and above earthquakes for the next month is about 10 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.

Why has the forecast dropped so much?

There are two reasons to explain the drop in expected numbers: A month has passed since our last update and we have included another model into our forecast mix. The longer a sequence goes on, the more information we have (based on location, depth and size of earthquakes) to improve our forecasts based on what we are seeing from the Kaikoura earthquakes. We know more now and, as time goes on, we’ll continue to refine the models further to give you the best information we can. The more we observe, test, develop, and refine, the better we can forecast what happens next.

For example, for the 19th December forecast we estimated 8.4 earthquakes of M5.0-M5.9 within the next 30 days. With the updated model, that number for December would have been 5.2 earthquakes in the M5.0-M5.9 range; this was closer to the actual observed earthquakes of that size, which was three.  

Scenarios

The scenarios specifically address the probabilities of what we might see happen within the next year and were estimated in mid-December 2016 (this differs from the forecast table above which is more recently updated). The scenarios cover a wider geographic area than the aftershock probability forecast area. The probability numbers in the table above are slightly different to the scenarios.  This is because we have used new information we have gathered from the slow-slip events, and their potential impact on the plate interface and other faults, to help define our probabilities in scenario three. 

There are very different probabilities for each scenario; some of these may be more unsettling to you than others. We recognise that while these scenarios may increase anxiety the best thing is to be prepared. Remember: To drop, cover and hold in an earthquake. If you feel a long or strong earthquake and you are on the coast, evacuate immediately.

Scenario One: Likely (approximately 70% within the next year)

The most likely scenario is that aftershocks will continue to decrease in frequency (and in line with forecasts) over the next year and no aftershocks of magnitude 7 or larger will occur. Felt aftershocks (e.g. over magnitude 5) can occur in the area from North Canterbury to Cape Palliser/Wellington.

Scenario Two:  Unlikely (approximately 25% within the next year)

An earthquake smaller than the mainshock and between magnitude 7.0 to magnitude 7.8 will occur. There are numerous mapped faults in the North Canterbury, Marlborough, Cook Strait and Southern North Island areas capable of such an earthquake. It may also occur on an unmapped fault. This earthquake may be onshore or offshore but close enough to cause severe shaking on land. This scenario includes the possibility of an earthquake in the Hikurangi Subduction Zone. Earthquakes originating from here or in the Cook Strait have the potential to generate localised tsunami. The Hawke’s Bay earthquake sequence in 1931 provides an analogy to scenario two, as a magnitude 7.3 aftershock occurred approximately 2 weeks after the initial magnitude 7.8 earthquake.

Scenario Three:  Very unlikely (5% within the next year)

A much less likely scenario than the previous two scenarios is that recent earthquake activity will trigger an earthquake larger than the magnitude 7.8 mainshock. This includes the possibility for an earthquake of greater than magnitude 8.0, which could be on the plate interface (where the Pacific Plate meets the Australian Plate). Although it is still very unlikely, the chances of this occurring have increased since before the magnitude 7.8 earthquake, and have also been also been slightly increased by the slow-slip events.

Initially our scenarios covered what might happen over the next 30 days, but we are now shifting to covering what might happen over the next year.  This is because the aftershocks are generally becoming smaller and less frequent (decaying) over time, and this lower aftershock rate increases the uncertainty of what might happen over shorter time periods. The change in forecast does not hugely affect the scenarios at the moment; we will review these again later in the year. While we will continue to update the aftershock probabilities regularly, we will not update the scenarios as often.

Can't get enough technical information? Here's the fine print on how we model aftershock probabilities.

Aftershock shaking forecasts

We have also calculated the probability of damaging earthquake shaking from aftershocks over the next year (starting 19 January 2017).  Damaging earthquake shaking is defined as MM7 on the Modified Mercalli Intensity (MMI) scale.  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 MM7 shaking within the aftershock region, which includes Wellington. Over the next year the probability of MM7 shaking around the wider Kaikoura/northern area. 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. 

Christchurch's aftershock probabilities are not greatly affected by the magnitude 7.8 Kaikoura earthquake.  The most recent update of the Christchurch aftershock probabilities are here. We update the Christchurch aftershock probabilities annually, as now they do not change much from month to month. 

Tsunami and landslide hazards

A tsunami was created by the magnitude 7.8 Kaikoura earthquake, and our scientists are still analysing the tsunami data and collecting information on its impacts. Remember, if an earthquake is too strong to stand up in, or lasts longer than a minute, move inland or to higher ground immediately. Do not wait for a siren or an official warning.

The earthquake also caused tens of thousands of landslides in North Canterbury, Kaikoura and Marlborough. These landslides remain dangerous and can move at any time. Please be careful around landslides and cracks in slopes. Heavy rain can pick up and carry landslide material and cause debris flows and debris floods (flash floods). Landslides have also dammed several rivers. These dams could breach, particularly in heavy rain. Please be careful and avoid riverbeds downstream of dams.

Take care of yourselves and others – physically and mentally

Earthquakes can be scary.  It is normal and okay to be a bit scared about things that are scary.  But the best thing you can do is take action and be prepared. 

You can follow our friends at the Ministry of Civil Defence & Emergency Management on Twitter and Facebook for the latest earthquake and tsunami preparedness information. You can also follow your regional Civil Defence Emergency Management GroupsIf you are anxious about the earthquakes and this is affecting your ability to go about your daily life the All Right? Hotline (0800-777-846) is a great resource where you can talk about any anxieties or concerns that you have regarding the earthquakes. Remember to also seek support with friends and family, and to take time out to do things you enjoy. 

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