So are we getting the number of aftershocks expected in the forecasts? The short answer is: we are on the low side of what we’ve forecast, but the numbers are mostly still within the forecast range. There’s a bit more to the story, so let’s back up for a second and take a look at the big picture.

How many aftershocks have there been?

By noon on Monday 12 December we had detected 8,735 aftershocks from the M7.8 Kaikoura earthquake (with the area of detection being the forecast area represented by the box). Most of these aftershocks have been small (8,669 earthquakes <M4.9) and would have only been felt close to the epicentre. As of Monday 12th December, there were also 49 aftershocks in the M5.0-5.9 range, and 3 aftershocks in the magnitude M6.0-6.9 range.

When a large earthquake like M7.8 Kaikoura occurs, aftershocks happen thick and fast. When that happens, our seismic detection network can miss smaller aftershocks as their energy is overshadowed by the larger aftershocks - so not all aftershocks are detected. Our seismic network is very sensitive and typically picks up even the smallest of shakes. But now, due to the big earthquakes coming through, it is more difficult to detect all of the quakes. Imagine a clear lake. Most of the time, when you skip small pebbles on the clear lake, you can spot the associated ripples easily. But then a giant rock is thrown into the clear lake. The splash and ripples it creates can cause the smaller ripples from the pebbles to go unnoticed. Once the giant rock’s waves subside, the smaller pebbles and their ripples become noticeable again. That is similar to what is happening here.

At this stage, we haven’t been able to detect all of the smaller aftershocks in amongst the waves of the larger earthquakes. Therefore, the total number of aftershocks in the earthquake catalogue below magnitude 5 is currently lower than what has actually happened. The total of 8,735 aftershocks is the bare minimum of what we’ve detected. When we have more time for data processing we will likely find further small aftershocks in the seismic waves of the mainshock. Our earthquake catalogue will be changed to reflect this in future.

For example, at 12 noon on 14 November, we forecast that there would be between 5 and 17 aftershocks in the M5.0-5.9 range, for the following 24-hour period. Once this time period had finished, 12 aftershocks in this magnitude range were actually detected, which is around the average of what we forecast

Forecast and detected earthquakes for a rectangular box with the coordinates -40.7, 171.7, -43.5, 171.7, -43.5, 175.5, -40.7, 175.5. Note: Our aftershock forecast models are based on previous New Zealand aftershock sequences.

* The first forecast is calculated is about a quarter of an hour after the mainshock to avoid the period of most undetected aftershocks.

How do the detected aftershocks compare to our forecasts?

At the moment, the aftershock sequence is falling within or just below the lower end of our forecast range. The table above shows the range in the number of aftershocks that we have forecast for 24-hour time intervals, compared to the number of earthquakes that we have actually detected so far, for three magnitude ranges.

The graph shows the number of aftershocks over M5.0 that we can expect per day, on average (with the uncertainty range in grey). The stars show the actual number of detected aftershocks that have occurred on each day. The graph shows that the forecast number of aftershocks will continue to decrease on average. There may be the occasional spike of activity as larger aftershocks occur with their own aftershock sequences. This follows the normal pattern of what we can expect following an earthquake.

We are working to understand why the Kaikoura sequence has been less productive than the average New Zealand earthquake sequence. However, the variability in aftershock productivity between sequences can be up to a factor of around ten. So far the aftershocks have decreased with the initial productivity of the sequence and as such the sequence is behaving normally.

In general, more small aftershocks will continue to occur than big ones. As a rule of thumb, there is a tenfold increase in the number of earthquakes for every one-magnitude decrease. For example, for one M6.0 earthquake, we expect around 10 earthquakes of M5.0-5.9 and around 100 earthquakes of M4.0-4.9 on average. This applies to all seismicity experienced, as well as that occurring as part of the aftershock sequence.

What does this mean?

In summary, the aftershocks are at the lower end of the forecast range. Just because we are in lower end of the forecast, it doesn’t mean that this will stay that way.

What you can do about forecasts: be prepared!

We know that these events can make people anxious or worried. That is perfectly normal; earthquakes can be scary! If you are feeling overwhelmed by the earthquakes or the forecasts, there are people who are there to listen and support you on 0800-777-846..

Our best advice is to be prepared for future aftershocks. You can find out more about getting prepared by visiting our friends at the Ministry of Civil Defence and Emergency Management.

Can't get enough technical information? Here's the fine print.

(This story will be next updated on the Monday 19th December, when the current weekly forecast period is over)