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.
The past gives us clues for the future, 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. The model has been improved over the last decade to suit New Zealand's unique conditions.
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 Toka Tū Ake 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.
Remember Long or Strong, Get Gone: If you are near the coast and feel a strong earthquake that makes it hard to stand up OR a weak rolling earthquake that lasts a minute or more move immediately to the nearest high ground or as far inland as you can, out of tsunami evacuation zones.
The National Emergency Management Agency (NEMA) has a great website with information on what to do before, during and after an earthquake.
There’s a lot we can do to make our homes safer and stronger for earthquakes. Toka Tū Ake EQC’s website has key steps to get you started.