In the North Island the Pacific plate is going down underneath the Australian plate (this is called subduction) and this is why we get those deep earthquakes in the middle of the North Island.
This mornings M5.1 quake was 167km deep and we received over 3000 felt reports from people who felt it. Due to its depth the quake was not damaging, and most of the reports were of light/weak intensity. (you can read more on intensity here).
|Felt reports from the M5.1 event.|
This is shown well on the M5.1 quake today, the image to the left shows the felt reports we received. As you can see most of the reports are away from the quake (highlighted by a rad star).
|Deep quakes (100km+) in NZ over the past year|
What about the rest of NZ?
In the bottom of the South Island the opposite is happening with the Australian plate subducting under the Pacific plate, so we also get deep events here.
And in the middle, for most of the South Island, the two plates are colliding and grinding past each other (why we have the cool Alps)
You can see exactly how the plates are moving under NZ, on a cool diagram here.
Here is a handy video by GNS Science, which explains how the tectonic plate boundary creates faults and earthquakes in NZ.