We get asked two questions a lot: what is an earthquake forecast and why do I need to know about them?
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.
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 all around the world have behaved over the last 100 years. In general, most aftershock sequences decay, which means the number of earthquakes decreases over time. This is called Omori's law. But a large aftershock can cause a spike of activity in an aftershock sequence at any time.
These models tell us about the average behaviour of aftershock sequences, but each sequence is unique and we learn more about a particular aftershock sequence as it unfolds. The basic aftershock model we use was developed by GNS Science. It was based on one used by the US Geological Survey 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.
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 living in areas affected by aftershocks are also interested in the forecast probabilities so they know what to expect around how many earthquakes they might feel and how many might be large enough to cause more damage. Some people say they don't really understand the numbers, but say that they provide reassurance that what is happening is generally within the range of the forecast. Other people just want to move on and would rather not hear about future earthquake probabilities.
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 and to be prepared.
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 an aftershock 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 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. Remember to also seek support with friends and family, to take time out to do things you enjoy, and to do things to help others - these are all known to help you feel better during stressful times.
If you want more advice on how to prepare your household, you can follow 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, and you can also follow your local Civil Defence Emergency Management Group. EQC also have a great guide to Quake Safe your home. You can also follow your regional Civil Defence Emergency Management Groups.