I spoke with this weeks (lucky) duty officer Anna Kaiser who said :
At this stage we might call it an earthquake "doublet" with two quite closely spaced similar sized events in very similar locations. And there have been several aftershocks over M4 following this mornings earthquake (where there was only one following Fridays).
Historically there have been several quakes over M5 in the general Cook Straight region in the last decade, and there was also an earthquake swarm in 2005. Back in 1977 there was an M6 in a similar location and depth.
What is an aftershock?
Aftershocks are earthquakes that follow the largest of an earthquake sequence. They are smaller than the mainshock, can continue over a period of weeks, months, or years, and In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.
|A section of the aftershocks following 5.8(yellow) on July 21|
Aftershock Rules?A common aftershock 'rule-of-thumb' applies to the magnitude, quite often ‘the largest aftershock will be one unit below the magnitude of the main shock'. So if you have a magnitude 8 earthquake, you would expect a magnitude 7 aftershock. So in this case with this mornings magnitude 5.8 earthquake, we would anticipate a magnitude 4.8 aftershock. and we got a 4.9!
There are other rules involving a bit of math and the amount of aftershocks and their rate of decay, with the second day having half as many aftershocks as the first, and so on. But in general the magnitudes will get smaller and the amount of quakes less frequent over time, though this can vary., such as this weekend when you get another large quake.
Of course as this is the earth we are speaking of, it does not always play by the rules!
Why so many earthquakes?In New Zealand, the Australian and Pacific plates push against each other along a curving boundary. At the southern end of the South Island, the Australian Plate dives down (subducts) below the Pacific Plate whilst in the North Island the opposite situation occurs with the Pacific Plate being pushed under by the Australian Plate. In between, through most of the South Island, the two plates grind past each other along the Alpine Fault. The Hikurangi Trough marks the collision boundary to the east of the North Island, and is where oceanic lithosphere (the Pacific Plate) descends into the Earth’s interior as a huge inclined slab.
Is this normal?Yes! Because of where we live (see above) we get lots of earthquakes, some of them are large and scary. It does occasionally seem like we are getting more than normal, but its just how the earth works.
For example here is a map showing quakes over magnitude 5 from 1992 -> 2012, and you can see they tend to follow the line of the plate boundary.
There is also the fact that we have the internet, smartphones etc. and are more aware of what is happening around us, where as before you might not know unless you felt an earthquake.
What does this mean?Unfortunately we don't have earthquake crystal balls just yet, so we can't say if this means 'the big one' is coming, or if this sequence will stop 'the big one' etc. All we can do is be prepared - so check out the Get Ready Get Thru website for what to do before, during and after an earthquake.
You can also read more on a post i did here Lots of earthquakes - what does this mean?