Published: Tue Dec 19 2017 11:45 AM
Some people feel better when we share our earthquake probabilities, others don’t and that’s okay – the important things are to be aware, understand what they mean and know what to do.
Many of you may be familiar with our earthquake forecast (at GeoNet our earthquake forecast is a probability of something happening over a certain period of time) – and after large earthquakes we use earthquake forecasts to describe what the aftershock behaviour might look like.
Our scientists update the forecasts as we learn more about each individual earthquake sequence. Recently, our scientists, with a group of highly experienced international colleagues, estimated a heightened likelihood of further large earthquakes in central New Zealand.
You can see the estimates below in the forecast table – and as you are reading through the article, remember, we share this information so that you are aware about the earthquake forecast, understand what they mean and know what to do (MCDEM have really useful information about what you can do and there’s also more below).
We estimate that there is a 2% to 14% chance – in verbal likelihood terms this is a very unlikely chance – of a magnitude 7 or above earthquake occurring within the next year in central New Zealand (the map below shows what we mean by central New Zealand).
Our best estimate is a 6% (very unlikely) chance, which is about a 1 in 16 chance. This has decreased over the last year (in December 2016 it was greater than 20% within the next year), but it is still a higher chance than before the 2016 Kaikōura earthquake.
The table below shows the estimated chance of large earthquakes within the next year, and within the next 10 years. For example, within the next 10 years, there is a 10% to 60% chance (best estimate is 30%, unlikely) of a magnitude 7 or higher earthquake occurring in the area shown on the map.
|Magnitude Range||Chance of occurence:
Range (the figures in the brackets
are the best estimate)
|Within the next year||M7.8 or greater||0.3% to 3% (1%)|
|M7.0 or greater||2% to 14% (6%)|
|Within the next decade||M7.8 or greater||2% to 20% (7%)|
|M7.0 or greater||10% to 60% (30%)|
Updated probabilities table for central New Zealand.
The magnitude ranges are for a magnitude 7.8
or greater and magnitude 7 or greater within the next year and within the next 10 years
The chances of earthquakes can be calculated from a computer model (you can learn about the earthquake models that we use here).
The nice thing about using forecasts models is that after a large earthquake they can provide us with information about the average behaviour of aftershock sequences (but of course, we learn more about each particular earthquake sequence as time goes on).
On the other hand, a model is not going to think through what it is processing, it just does it. Therefore, we need scientific expertise to think through the model outputs (the numbers) and what these numbers actually mean for the current situation. So, we have called together a group of experts from around the world to consider and think through models of different possibilities such as, how slow slip events (also called slow slip earthquakes) change things, long-term trends, and the likely behaviour of specific faults. Normal aftershock forecast models on their own can’t yet include those factors.
When it comes to scenarios and forecasting earthquakes, we are reliant on the advice of experts.
Another way to think about the forecast models is to think about the process of timing how long it will take you to get from A to B. You enter where you are now and where you want to get to into Google maps and you will get an estimate of the time it will take.
There are things you can change in this calculation, like whether you are going to walk, cycle, drive or take public transport. Regardless, we know that this time is only an indication (although Google maps is fairly accurate on driving estimates, it is not so good at walking estimates).
When it comes to actually going from A to B, though, there are many things that can change how long it takes – you might take a wrong turn (twice), traffic might change while you are en route, you might drive/walk slower (or faster) compared to what Google maps calculated, stop to get fuel on the way … there are lots of different things that Google maps is not able to consider in its estimate and some are beyond our control.
Sometimes, to help us make a more accurate estimate of how long the trip would take, we could ask our friends who are familiar with the route how long they think it will take to make it from A to B. This helps us work out when we need to leave A to get to B by taking into account extra bits of information that can’t go into the computer model.
So, the Google maps model is like our earthquake forecast models, and talking to our friends about the route is like the scientists meeting and discussing the extra information such as slow slip events.
Scientists from Japan, Taiwan, and USA met with our scientists to estimate the chance of a large earthquake occurring in central New Zealand. Together they assessed all the earthquake models, plus newly developed models of how slow slip events impact the probability of future earthquakes.
The results of these models were then combined with other information, including observations of how the numbers of earthquakes change during slow slip events, and evidence of earthquake clustering over the past few thousand years to estimate revised probabilities for large events in central New Zealand.
The range of probabilities show the experts’ uncertainty around what we can expect. For example, small ranges mean they are more certain of the probability, large ranges mean they are less certain. The experts also give their ‘best estimate’ of the probability.
The best estimate over the next year for a magnitude 7.0 or higher earthquake is 6%. This is an increase of 20% over the long-term estimates from the National Seismic Hazard Model (i.e., it is 1.2 times higher).
The best estimate for a magnitude 7.8 and higher earthquake is 1% within the next year. This is double the long-term estimates (i.e. it is twice as likely to happen now as it was before November 2016). The upper bounds for both magnitude range estimates are much higher than the long-term estimates.
That being said, the chance of a very big earthquake has been going down over the past year, since we first estimated the numbers following the Kaikoura earthquake.
Back in December 2016, there was a 5% chance of a M7.8+ earthquake within the coming year (December 2016 to November 2017), now the best estimate is 1% within the next year (December 2017 to November 2018).
This exercise has been focused on earthquake forecasts for larger magnitude earthquakes over central New Zealand rather than the Kaikōura aftershock sequence- (those will still be regularly updated here).
While no one knows how to predict the exact date, time, magnitude and location of the next ‘Big One’, scientists can estimate the chance of having large earthquakes to help people make decisions about what to plan for, or even things as simple as whether to put the nice china back out on the shelf.
Earthquake forecasts and chances of occurrence help infrastructure providers (such as our railways, water pipe networks, and electricity lines companies) compare different risks and decide on what measures to take to make their networks more resilient.
Government agencies also use the forecasts to inform public education and preparedness campaigns, for their risk management plans, and for exercises.
MCDEMs earthquake and tsunami safety campaign will hit TV screens from Friday 23 December. Find out more at www.civildefence.govt.nz
Whatever the numbers say, our beautiful country sits on top of a tectonic plate boundary. The forces involved in the collision of the two plates are huge and because of this we get a lot of earthquakes, some of them big. Therefore, as an island nation some of these earthquakes will create tsunamis.
Living around this risk can sometimes be scary, but there are things we can do to make life a bit easier when a big earthquake does happen.
The Ministry of Civil Defence & Emergency Management has some great information on what you can do before, during and after an earthquake. It also helps to talk to friends and family about what they are doing – you can even share what you have learnt from this article with them.
Science contact: Matt Gerstenberger firstname.lastname@example.org
Science input recieved from Matt Gerstenberger and Sally Potter (GNS Science) as well as valuable contributions from our colleagues at MCDEM and USGS This research was funded by the Natural Hazards Research Platform Kaikoura Earthquake short term research projects