Friday, September 13, 2013

Plates and Quakes

We often get people commenting on the earthquakes moving up or down New Zealand and the pattern you can sometimes see, and why we get so many when our neighbors in Australia get so few!  Well the answer is plate tectonics, the earth's outer shell is made up of plates, they are in constant motion and where they interact is where you get earthquakes, mountains and volcanoes. So here in wee New Zealand we are right in the middle of the boundary of two of these plates (this is why we get all of the fun geological hazards) Australia on the other hand are right in the middle of the Australian plate, so only a few quakes for them!

Here is a map showing the last 1500 quakes in NZ of weak intensity and above (you can see these maps and select all of NZ or just a region under the 'Map & Stats' tab on our website).   I've added in (an awesomely drawn) very rough red line that shows the tectonic plate boundary that we live on, and you can see the quakes follow this.   Its very interesting as the two plates interact differently along this boundary.

In the North Island the Pacific plate is going down under the Australian plate (subduction) so that's why the quakes are more spread out, and why you can get deep events in the middle of the North Island such as this 2012 Taupo event  The seismic waves travel very well along the boundary between these two plates and so the earthquakes are often felt very strongly to the east of where they occurred, i.e. Hawke's Bay, Wairarapa and Wellington.  This is also why we have all of our volcanoes in the North Island.

In the bottom of the South Island we have the opposite with the Australian plate subducting under the Pacific plate, so we also get deep events here.   In the middle, for most of the South Island the two plates are colliding and grinding past each other, and this is how we get our Southern Alps (its also home to our famous Alpine Fault.)

You can see exactly how the plates are moving on a cool diagram here.

For those interesting at looking at quake patterns we are still working on our 'Quake Search' function on our website, which at the moment stops at September 2012 (when our new automated system came in).  This feature allows you to search for quakes using specific dates / location / depth and size and have then the quakes as dots on map (which you can click on) or the data in a spreadsheet.   We hope to have this up and running very soon!, below is an example of what it will look like (its still being jazzed up).

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