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Saturday, 24 December 2011 21:01

A theory on the Christchurch earthquakes

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The spate of earthquakes that have inflicted major damage upon Christchurch in New Zealand display a very worrying trend.  Is the nearby volcano about to erupt?

There are a couple of websites that provide a detailed display of the entire set of earthquakes since September 4th 2010 (the magnitude 7.1 event that essentially started this sequence).

The first site - Geonet has an image of all earthquakes of magnitude 3 or greater, split into three sequences separated by three main events.  The first is Sept. 4th 2010 to 22nd February 2011, the second sequence through to 13th June 2011 and the final set until December 4th (the point at which the image was compiled).

This image very clearly shows that each of the three major earthquakes (on the dates separating the three time periods) marks an obvious change in the primary location of the earthquake swarms.  Each swarm is located further east than the previous one.  And in fact the earthquakes of the past 36 hours have generally been centred to the east of the city itself, marking a new centre of activity further east from the previous centre.

The other website of interest - Christchurch Quake Map allows the visitor to replay the sequence of all earthquakes between September 4th and the present day.  Of interest is that this presentation uses a colour coding to indicate the depth of each event.  We suggest users limit the display to magnitude 3 or greater events in order to get a clear, uncluttered view.

These two charts show a number of interesting trends.


Both charts (one of them optionally) shows the newly discovered Greendale Fault, although the significance of this structure is (at least to this author's eye) is dubious at best.  For readers unfamiliar with this region of New Zealand, the entire Canterbury Plains are covered in alluvial material and the underlying geology is very difficult to ascertain.

However, of greater interest is that the entire set of earthquake swarms are in a number of very distinctly defined alignments. 

At the western end of the overall structure is a north-east / south-west aligned element which is loosely connected to the primary east-west structure.  To the north, passing between Darfield and Kirwee is a north / south aligned structure. 

Of greater interest is the narrow zone almost completely earthquake-free running between Templeton and Springston.  Both of the planes of activity either side of this zone show shallower events to the west and deeper to the east.  Finally there is a very clear rupturing structure running south-east under the Banks Peninsula volcanic structure with a mix of both shallow and deep events all occurring under the same location.

Beyond the earthquakes (9,356 at the time of writing), there are also unconfirmed suggestions of water temperatures rising and escaping sulphur dioxide gas (rotten egg gas) in Lyttelton harbour.


Contrary to the many suggestions this author has read on a variety of web sites, the Canterbury region of New Zealand has been geologically very stable for a considerable period of time; it is completely unconnected to the trans-alpine fault and plays no part in the fault displacement earthquakes seen elsewhere. 

In addition, it is on the wrong side of any nearby subduction zone to be related to eruptions such as have been seen in the middle of the North Island.

Instead, the Banks Peninsula, just like Otago Peninsula to the south of it was formed from a sub-crustal 'hot spot,' just the same as the Hawaiian islands were.

And, in the opinion of this writer, it is this sub-crustal hot-spot that is about to erupt again.  The earthquake pattern suggests that there are two magma sources (one in the general direction of Methven, the other towards Burnt Hill) which are combining to the north of Dunsandel and moving towards Christchurch city before turning southeast to erupt between the two current calderas of Lyttelton and Akaroa harbours.

Current events suggest that the eruption is not imminent; it is still probably many months away (and we may well be entirely wrong!) but it would be wise to factor the possibility of a major eruption into future planning for the region.

 


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David Heath

David Heath has had a long and varied career in the IT industry having worked as a Pre-sales Network Engineer (remember Novell NetWare?), General Manager of IT&T for the TV Shopping Network, as a Technical manager in the Biometrics industry, and as a Technical Trainer and Instructional Designer in the industrial control sector. In all aspects, security has been a driving focus. Throughout his career, David has sought to inform and educate people and has done that through his writings and in more formal educational environments.

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