Geonet has revealed in a blog post how New Zealand forecasts earthquakes.
The geological monitoring agency says forecasting an earthquake doesn't work the same way as a weather forecast - it can't predict the day it will happen or the magnitude, but rather the probability one will strike.
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The forecasts at Geonet are driven by hundreds of years of international observations, as well as models that have been reviewed and tested by international scientists, Matt Gerstenberger writes in the blog.
"Seismologists at GNS Science are part of an international collaboration between New Zealand, Japan, the USA, and Italy where we continuously test each other's forecast models to see how well they compare to what actually happens, ie whether earthquakes happened as forecast in the model," he said.
Short-term forecasting (days to years)
Short-term models (called STEP and ETAS) are used in the immediate aftermath of an earthquake to establish the probability of aftershocks.
They're based on the idea that, most of the time, the number and magnitudes of aftershocks lessens over time. This is called the Omori Law.
In the beginning scientists apply a version of the Omori law modified to include how they know New Zealand earthquakes behave, and then once they know how the sequence is behaving more refined forecasts are made.
Medium-term forecasting (months to decades)
A medium-term forecast uses the same models as a short term forecast, but with the addition of another model (EEPAS), which is based on the observation larger earthquakes can sometimes follow minor earthquakes over a long time period.
Long-term forecasting (decades)
Looking further ahead requires the use of three different models, the first of which is called 'background seismicity'.
It uses the National Seismic Hazard Model to look at the number and size of earthquakes that could be expected in an area under normal circumstances.
Measured shear strain looks at measurements from continuous GPS stations across the country to record how much of the land is being twisted and stretched in different places due to tectonic plate movements.
The final method is looking at the pattern of recorded historic earthquakes in New Zealand from 1840 -1950.
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