Looking at the Earthquake maps on Google - it's very noticeable how quiet everything got along the middle of the South Island all last year (and for the two months of this year), at the same time as Christchurch was beginning to experience a very marked cluster of quakes; the stresses that were normally getting dissipated over the wider region were focussing on Christchurch - in a 'wunner' so to speak, so Christchurch was copping the punch of the whole lot of normal regional activity - plus a possible extra (somebody could calculate if ..)
We're two months into the year (2011) and there is still not an earthquake along that western side of the South Island. Christchurch will not return to normal until that pattern of dissipated release along the western part of the Island is resumed.
There might well be more to come. By the look of it the Earthquakes in the immediate vicinity of Christchurch are continuing at the same rate as last year.
Fig.1 Christchurch quakes Jan / Feb 2011. Looking along the line of the Chatham Rise and the suspected fault transecting South Island, which is also marked by the linear topographic incisions along strike of the line on the westsern side of the Island. Click image for a bigger figure.
Christchurch is in a special position - on the crest of the Chatham Rise, and the line of the rise and the line of the Earthquakes is the same, ..cutting right across the middle of the South Island (see Google). A similar line divides the North Island from the South Island (where there was a little cluster of quakes opposite Wellington in 2005).
The quakes appear to be related to the dislocation of the two islands as the South Island is getting hung up on the rise whilst the North Island continues to override the ocean floor to the southeast, so this could be not just something restricted to the South Island, but could include transfer of momentum from the North Island to the South Island. If the fault zone marked by the line has not previously been recognised then it could be the start of a new seismic pattern. It's right at the southern end of the steeper subduction zone, where this would flatten off over the rise.
What I find disturbing is the regional extent of the quiet zone this year and last year, compared to previous years. Last year the quiet zone was only around the Christchurch area. This year the quiet zone is the whole of the South Island and half of the North Island as well (though mind you we're only two months into the year).
Doesn't look good. That there has been a bad one or two already doesn't mean it's over. As mentioned above, Christchurch will not be clear till the regular pattern of quakes in the South Island is resumed. Two months into the year already and no sign of this starting up again. (See what happens in another month.)
(See also:- ftp://hazards.cr.usgs.gov/maps/sigeqs/20110221/NZspecial.pdf )
Addendum 31st March, 2011
Fig.2. Darfield and Christchurch earthquakes.
2010/09/04 = Darfield, 7.1M, depth 12km, green star
2011/02/22 = Christchurch, 6.3M, depth 5km, red star
Green = aftershocks before Christchurch quake
Red = aftershocks after Christchurch quake
Fig.3.
(In progress - call back later for this)
[ See also - Debunking Plate Tectonics - at :-
http://www.platetectonicsbiglie.blogspot.com/
I have been corresponding with GNS as a totakl lay person. Just something I noticed. The line of aftershocks lines up from mount somers to the greendale fault to Akaroa and now is heading east towards the other volcanoes on Banks penisula. Given they all are of teh Cretacious period (including Chatham Island) does this indicate that there is an underlying Volcanic aspect to all of this. the guy at GNS does not think so.
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Martin O'Sullivan
Well, .. yes to the extent that volcanism is the result of movement on the subduction / overriding zone. But more generally the magmatism is seen as secondary to the tectonics, where movement on the fault (generally described as the subduction zone) depressurises the mantle and generates the magma which bores its way up fractures to make volcanoes. I guess that's why the guy at GNS is saying 'No', but he could be a bit more equivocal; they're related. (interesting about the liquifaction, though I guess that's water in the floor plain.) The old volcanoes on the Peninsula will be the same origin (probably) as those on North Island, which would also be related to the subduction zone. The different dips of the subduction zone between the North Island and South Island I would see as an effect of a high-level 'push' by the Chatham rise, which has overturned it (giving the big kink in the subduction zone as it crosses on to South Island from the north);(if it was a 'deep-push' it would make the general westerly dip of the subduction zone to the north even flatter here.) The fault you're referring to is a small one along the axis of the rise - a rift along the axis, .. maybe a little normal fault. There should be some first motions on them somewhere on the GNS site, but slips tend to be many and varied.
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