20/04/2017 12.00 p.m.
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.
Want to know why we produce aftershock forecasts? As well as helping people to understand what they might expect next, many people use the information in their work. This includes engineers, infrastructure planners, Civil Defence planners and insurance risk assessors who use our forecasts in their planning and decision making. See more in our Why do we do earthquake forecasting story.
What’s happened so far
We’ve had a total of 15918 earthquakes since the M7.8 Kaikoura earthquake stopped shaking our islands (we ran the numbers at 11:30 a.m. on the 20 April). 486 of those earthquakes were M4-4.9, 56 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 April 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. For a comparison, here's our older forecast from 19 December.
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
within one year
Forecast for rectangular box (see map on the right) with the coordinates -40.7, 171.7, -43.5, 171.7, -43.5, 175.5, -40.7, 175.5 at 12 noon, 19 April; 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 13% chance of one or more M6.0-6.9 earthquakes occurring within the next month. We estimate there will be between 0 and 1 earthquakes in this magnitude range within the next month.
The current rate of magnitude 6 and above earthquakes for the next month is about 4 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.
Aftershock shaking forecasts
We have also calculated the probability of damaging earthquake shaking from aftershocks over the next year (starting 19 April 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 and a zoom into the Wellington area. Over the next year the probability of MM7 shaking around the wider Kaikoura area is around 25%. 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. EQC also have a great guide to Quake Safe your home. You can also follow your regional Civil Defence Emergency Management Groups. If 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.