Earthquake forecasts – So why do we do earthquake forecasting?

Published: Tue Feb 21 2017 1:00 PM

We get asked two questions a lot: what is an earthquake forecast and why do I need to know about it?

Answering that second part quickly: some people need to know this for their work, some people are interested as to what we think could happen next, while others just want to move on and not hear about earthquakes anymore. But, for those of you who want more detail, we thought we’d take a moment and answer some questions about the forecasts.

What’s the difference between a forecast and prediction?

GNS Science has been producing earthquake forecasts since the late 1990s, but it wasn’t until the 2010/11 Canterbury earthquakes that people got really interested in them.

The earthquake forecasts we produce are not earthquake predictions. A forecast is a probability of something happening over a certain period of time. A prediction gives a specific timing and location for something to happen.

Some people say they can predict earthquakes. However, at GeoNet we stick to our knitting: we are a science organisation and base our work on things we can observe and measure. At present there is no scientific way to accurately and reliably predict when and where a big earthquake is going to happen next.

Another way to think about the forecasts: Introducing our Grandma’s China-ometer!

What many people want to know is “what should I do with this forecast”? Perhaps you could think of the forecasts as a sort of Grandma’s China-ometer – should I put Grandma’s heirloom tea set back on the mantelpiece or bookshelf yet? I'm not an engineer or a seismologist, so I use the forecasts like this: I have a special china tea set that my grandmother left me when she passed away. In the Kaikoura Earthquake, some of the tea set moved (but didn’t fall…thank you Blu Tack!). My grandmother’s tea set means a lot to me, so, as an extra precaution, I wrapped the tea set in tissue paper and put it in a cardboard box. I look at the numbers and think to myself “maybe just a few more months until I’ll risk putting my tea set back up".

grandma china-ometre

grandma china-ometre

Level Action
4 The china's probably best securely packed away in a box somewhere safe just at the moment
3 Keep the china in the china cupboard
2 The china is ok to put out again, but we'd suggest using Blu Tack
1 The china's probably fine to put out without Blu Tack (but nowhere in NZ is 100% safe from earthquakes, so for peace of mind perhaps use a non-slip mat!)

At the moment (three months since the M7.8 Kaikoura Earthquake), I think we are still at Level 4 on the Grandma’s China-ometer in the North Canterbury/Kaikoura area, but this will gradually lower back to a Level 2, which is the background, normal level for this area. In Wellington, we are probably at Level 3; but I’ve still got my tea set in a box packed away. I’m naturally a bit of a pessimist, so I act like Wellington is at Level 4.

How do you produce the forecasts?

The past gives us clues for the future. Much like detectives putting together evidence to solve a crime, scientists use evidence from observations and models to understand the processes happening in the earth.

The models that GNS Science use to generate the earthquake forecasts are based on observations of how earthquake sequences work, from all around the world over more than 100 years. In general, most aftershock sequences decay, which means the number of earthquakes decreases over time. This is called Omori’s law. Although, a large aftershock can cause a spike of activity any time.

These models tell us about the average behaviour of aftershock sequences, but we learn more as a particular sequence unfolds. Think of it like family behaviour: we might expect that your family might behave in a certain way at a family get together. But if we randomly grab one member, we might get your weird Uncle Kevin or more stable Aunt Caroline. At this point, we think the Kaikoura sequence is more like stable Aunt Caroline, but crazy Uncle Kevin can still show up and ruin the family gathering.

The initial aftershock model we use was developed by GNS Science. It was based on one used by the USGS that one of our scientists developed (we loaned him out for a few years and then brought him back). The model has been improved over the last decade to suit New Zealand’s unique conditions. If you want the technical details of the models, they are explained on GNS Science’s Earthquake Hazard Modelling page.

We don’t know exactly what is coming when, however, knowing what is most likely can help us make decisions as individuals and communities. Want to know more technical details about earthquake forecasting for the Kaikoura earthquake sequence? Go here.

These numbers don’t really help me, who uses them?

The earthquake forecast probabilities are really useful for engineers, infrastructure managers, private companies, Civil Defence, government planning, and insurance organisations, including EQC. Infrastructure managers and Civil Defence can use the probabilities to plan for the next few months – they only have so much time and resources, so knowing what is likely (or not) helps them decide where to focus their efforts and what to plan for. The probabilities are fed into new building standards (as they were after the Canterbury earthquakes), so that our buildings will be more resilient to earthquakes in the future. And when probabilities are quantified like this they can be used by risk assessors at insurance companies to compare risks from different hazards (e.g. flooding, snowstorm and earthquakes). Some members of the public also want the numbers to know what to expect about how many earthquakes they might feel and how many might be large enough to cause more damage.

Other people admit that they don’t really understand the numbers, but they say that the numbers provide reassurance; they are comforted by the thought that some people understand what is going on and what is happening is generally within the range of the forecast. Others would rather get a poke in the eye than see another forecast.

What is important is having a general indication of what we can expect and figuring out how to live around the possibility of another large earthquake – either as part of the current earthquake sequence, or a separate one (we live smack bang on the top of a tectonic plate boundary, so getting big earthquakes every now and then is not surprising). The best thing we can do is take a few steps to help ourselves. As the probability of a moderate-sized earthquake in the aftershock area is still significant three months on from the Kaikoura earthquake, you might want to be extra careful and prepared.

What do I do about these earthquakes? These earthquakes are really getting to me.

As fascinating as they are, earthquakes can be really scary for some people. Even if you are not that disturbed by the earthquakes themselves, just constantly getting a fright every time one arrives can be enough to rattle your nerves. Or you may just be plain scared of them, and it is normal to be scared of something that is scary. 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.

If you want more advice on how to prepare your household, 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.

Story written by: Helen Jack, Sara McBride, Annamarie Christophersen, Matt Gerstenberger

China stunt coordinator: Helen Jack