Friday, July 17, 2009

Rock and roll

You may have heard that NZ’s been getting a good shake up over the last few days. We’ve had around 30 decent shakes plus hundreds of small ones following the big one on Wednesday which measured 7.8 on the Richter scale.  I felt the ground rumble briefly this morning and it prompted this post on the subject.

A 7.8 quake is pretty massive, in fact it was this magnitude quake that killed 240,000 people in 1976 in Tangshan, in north-eastern China. The Japanese Kobe quake was a 7.2 and nearly 7000 lost their lives. Luckily the quake this week had an epicentre in NZ’s most sparsely populated region at the very bottom of the South Island. It was only 12 kilometres down too which is extra dangerous. By all accounts it lasted nearly 30 seconds which is a very long time by earthquake standards. It was a ‘rolling’ quake rather than a shake which means the ground appears to move in a wave formation – I saw one years ago and I’ll never forget it.  TectonicsNew Zealand straddles the Indo-Australian tectonic plate and the Pacific plate. These are 2 of the 15 plates on the Earths surface. The plates are enormous, rigid plates of rock up to 100km thick. These plates are always moving and at their edges they are constantly grinding into each other. This movement causes stress to build up in the brittle, upper layers of the plates. When this brittle rock finally breaks, an earthquake results.

The Pacific plate is forcing itself under the Australian plate at a rate of about 48-50mm a year. The Pacific plate can move this fast because it carries only a minimal amount of land mass. The edges of the Pacific plate form what is known as ‘the ring of fire', a zone of frequent earthquakes and volcanic eruptions stretching from New Zealand’s Southern Alps mountain range across to Washington state’s Mt St Helens. The image below shows the location of volcanic areas along the rim of the Pacific plate. ringfireThe city I live in, Auckland, has around 50 volcanoes. In theory they are classified as dormant though in practice they are unlikely to erupt again. More likely is the formation of new volcanoes due to Auckland's position on a magma ‘hot spot’.

Never a dull moment.


  1. That's good tectonics. Hope you're all OK. The British media, as usual, ignored the story.

  2. I heard this story this mornign and i wondered if you were all doing okay.
    Thanks for the update and remember: doorways.

  3. We don't get much of that in the Vancouver area but I have felt a "rolling" quake here a few years back.Our house shakes from the trains some distance away but this was no shake. You felt the house move up and down.
    When Mt. St. Helens blew we heard the explosion here and felt it too. It was a good 7 hour drive from here.
    Lets hope you don't end up with your own version of "Lou VS the Volcano". (no movie quote for that one)

  4. Lou as you know I was concerned for all my NZ friends and family and so pleased that all was well. Yes you dodged the bullet but we also like to think that these are good ones - they ease up all the movement and ease the pressure (not to be crude but like a good fart -- blush).
    Like Lori E said when My St Helen's blew I remember watching the curtains blow into our house - like a huge sonic boom

  5. Oh no: it gets worse -

  6. Yeah I know, I read that :-) we're doomed to end up sitting in Aussi laps.


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