In the Canterbury Quakes section of our website (here) there is a tab called aftershocks.
|Aftershock Forecast Map|
Probabilities are calculated using a background of geological and earthquake information and all of the earthquakes that GeoNet records are looked at. For each event the probability that it will be followed by an earthquake large enough to cause strong shaking is calculated from the known behavior of aftershocks. The shaking that would be produced by such an earthquake is then predicted from the known relations between earthquake size and shaking patterns. The likelihood of that shaking is then added to the background probability on the map.
More detailed info here
Below the map are a few tables:
The top show the number of afershocks that have occured (vs how many were expected) and then there is a
table that shows a one month forecast of expected aftershocks of magnitude 4.0-4.9, and 5.0 +.
Dr Matt Gerstenberger produces these aftershock probabilities for one month ahead and they cover the entire aftershock zone, which extends from Sheffield in the west to Rangiora in the north to Lincoln in the south, and right across Banks Peninsula.
NB: We are currently looking into extending these forecasts (covering a few months, rather than just one) on our website, shortly.
These longer range forecasts are where the figures in todays news came from. And as far as today's news, the figures for Christchurch metropolitan area are roughly one quarter of the figures given to CERA. So 23% probability for Canterbury becomes 6% probability for Christchurch.
In rough terms, it means the quake probability for Christchurch has become similar to many other parts of NZ such as Wellington, Hawke's Bay, Wanganui, and Poverty Bay where quakes are more frequent. The initial driver of this increased activity was the September M7.1 quake. It started an aftershock sequence that will continue for many months, these aftershocks will gradually get less frequent, but there is still a small chance of a larger aftershock for a while yet.
And.... Its not saying there will be another large event, its saying what the probabilities of one occurring are - there is a difference, and they may not happen. All you can do is be prepared, it is the earth we are talking about here!