Saturday, 4 September 2010

Christchurch earthquake ~ things that went bump in the night

UPDATES CAN BE FOUND AT THE FOOT OF THIS ARTICLE ~
4th September 2010:
Even here south of Dunedin the earthquake that hit Christchurch gave us a thorough shaking.   We're quite used to earthquakes, and this one wasn't nearly as long or as dramatic as the one we felt from the Te Anau area a year or so ago, but it was a good shaking for all that.  I happened to be awake with the light on, and took note of the time while warily watching the light fitting gyrate above me.  Everyone I spoke to was woken.  Over the hill on the coast the jolt seems to have been much sharper.  I had no idea that Christchurch had been severely hit until I had an early morning toll call.  Truly a wild night for them and fresh shakes and aftershocks keeping it all vivid for its 350,000 residents throughout the day.

For those interested in news coverage here are the links to:
For those interested in the scientific side of it here is the GeoNet link.  Just look how many quakes there have been during the day!  Wild weather is forecast for tomorrow - this time in the form of wind and rain.  Canterbury is getting quite a pounding.

8th September:
There has been another sizeable quake this morning, so structural engineers will be going back over previously inspected buildings yet again, a very busy and disturbing time for everyone.

9th September:
19th September ~ update:
For those with an interest in issues relating to restoration and rebuilding  projects following the earthquake a thought-provoking perspective can be found at the  "Rebuilding Christchurch" site of James Dann.  

For those with scientific interest here is more data about earthquake measurement and liquefaction:
In discussion of the earthquake there has been considerable comparison between the Haiti earthquake and the Christchurch / Canterbury one, and many people have concluded that the difference in terms of the cost of human life and building failure is due to New Zealand's strict building code.  While there is some truth in this, the two earthquakes were quite different in a number of ways, both with regard to population density, time of day, and  earthquake type and intensity.  It would seem that the Richter scale, which is the common point of reference in any public mention of earthquakes, is no longer current in scientific circles, and in any case relates to only one part of any earthquake equation.

Grace Dalley of Rata Design and Rata Weekly has generously supplied the following information:
(1) To summarise about the Richter scale: apparently the Richter scale went out with the ark, and the figure of 7.1 (relating to the Christchurch earthquake) is on the Moment Magnitude scale which is much the same only more accurate. Basically it's the amplitude of displacement, and hence the energy released during the quake. 
     Each step of moment magnitude (eg. from 4-to-5 or 6-to-7) is 10 times the movement and 32 times the energy released of the previous step.
     You can read more here:  
http://en.wikipedia.org/wiki/Richter_magnitude_scale

Shaking intensity, on the other hand, is measured on the Modified Mercalli scale (as in the USGS chart, see point 2). 
The shaking intensity varies with:
     a) depth of quake, 
     b) distance from epicentre, 
     c) type of ground.

2) The All-geo site has a chart comparing shaking intensity (on the Modified Mercalli scale) and also population, in the Christchurch And Haiti quakes: http://all-geo.org/highlyallochthonous/2010/09/tectonics-of-the-m7-earthquake-near-christchurch-new-zealand/ Look for the coloured chart right at the bottom above the comments.
[They do say they think the USGS (who made that chart) has underestimated the intensity in Christchurch]
The explanation of liquefaction with diagrams is on the left near the top.

Thank you Grace, for bringing all this to my attention!

10th November 2010
AFTERSHOCKS ~ a visit to Christchurch
The Canterbury earthquake is far from over.  As the ground settles after the initial big earthquake hundreds of aftershocks are occurring.  The Christchurch Quake Map shows over 2,700 at this point, and a number can be expected each day and night: for the 8th November twenty are listed; the 9th was quieter with a mere twelve.  Some of these come in sharp jolts while others are barely felt.  Added together they  certainly give a feeling of instability.  The Quake Map's 'FAQ' and 'About This Site' pages give further useful information.  Aftershocks may continue for weeks or months to come.

I experienced these myself when in Christchurch last week.  The first occurred when I was sitting at dinner with a friend: our conversation ceased abruptly as the house shook and things rattled.  She remarked "That would have been about a 3", and we resumed our meal.  Others were mild, many more like a slight trembling which was not all that noticeable.  One night I even dreamt there was an earthquake: in my dream I crawled from one room to another.  I fancy I came close to waking as I got an impression of things in the house rattling.  Sure enough, there had been a shake in the night, two of them in fact.

'The surfaces of the blocks of land that have moved along a fault are usually irregular, and even after the main movement has occurred, small areas continue to shift and readjust, producing smaller quakes known as aftershocks.'
The explanation is expanded on the page linked to above.  Scroll down towards the bottom of their page to find it.

In a Press article published on 25th September University of Canterbury doctoral student John Holdaway describes the earthquake as a massive release of energy, with the sum of the following aftershocks being very minor by comparison.  In an article published on 15th October Debbie Roome reports Mr Holdaway's points more fully, and describes the stressful effect all the on-going shaking is having on residents.

I must say it got to me a bit, and I was only there for five days.  I'm reasonably relaxed about this sort of thing but by the end of that week I found I was looking forward to being back on firmer ground Otago.  
 
But how stable is Otago and its city, Dunedin?  The answer is 'not particularly!' Scientific predictions indicate we should expect and prepare for an earthquake at any time at least as large as the one in Christchurch and Canterbury. This is outlined in the ODT article of 18th September 2010: "Quaking in our boots: how prepared is Dunedin?"

In an article "Bigger earthquake predicted to come" published on the 'Stuff" website on 5th September 2010, scientists make this same point, saying that it was not the big one they had been predicting as it was not related to the well known alpine fault line which is overdue to release a known build-up of stress.
26th December 2010 / Boxing day aftershock ~ update:  The big aftershock of magnitude 4.9 shook central Christchurch more violently than the original September quake.  Peak ground acceleration levels recorded reached 48% of gravity compared with 15 to 20% of the September 4th quake, but caused less damage as shaking did not continue for as long.  ODT article published on 29th December 2010

My further article 'New Zealand ~ land of earthquakes and volcanoes' can be found by clicking the link provided.  In it I discuss the relative geological instability of New Zealand's major cities and touch on how we know the history of the planet we live on.  

All my articles about the Christchurch earthquakes and aftermath can be found via the page linked to below, or at the upper right of this screen:

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