Thursday, September 24, 2015

Whats up with those deep quakes?

As many of you know New Zealand is right in the middle of the boundary of the Australian and Pacific tectonic plates and this is the cause of all of our earthquakes and volcanoes!

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.
The energy released by deep NZ earthquakes travel more efficiently up the denser subducting Pacific plate, rather than the overlying Australian plate and as a result these earthquakes are often felt very strongly to the south-east of where they occurred. So you will often see more felt reports away from the quake than directly above it.

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.