Christchurch's December Earthquakes, 2011

Making New Zealand's third island?
( Blog for Website at http://users.indigo.net.au/don/ )

Fig.1.  New Zealand drifts eastwards over the deflection in the subduction zone (red line) around the Chatham Rise.  Axis of the rise is shown by the yellow line and the white dots constructed in Google Earth.  White line around New Zealand = 'continental' shelf.  USGS arrow showing rate of subduction (88mm) is a furphy, .. movement is the other way ("overriding").  (Right click / new window all figures for bigger images.)


It was noted in an earlier post how, seismically,  the South Island went very quiet during 2010 before the Christchurch Earthquakes started up in February this year (following September the previous year).  The New Zealand earthquake seismic warning centre said those earthquakes were related to a new fault that hadn't been noticed before, .. cutting across the island.  Since the fault lies right on the crest of the Chatham Rise (which seems to be important in swinging the subduction zone from the leading side of crustal drift on the North Island, to the trailing side on the South Island), I suggested we might be seeing a change to include this fault as a new and important part of New Zealand structure.  Makes me wonder if this fault might be implicated in the break-up of the South Island - making New Zealand in three, rather than just two, and eventually have an expression like that separating the present two islands.   Is this maybe the way the separation of North Island from South Island started millions of years ago (possibly as it too once crossed the Chatham rise)?

Of course, such an event would probably take longer to be noticeable than the lifetime of the human species, much less yours and mine, .. and just because the fault has been seismically inactive till now doesn't mean to say it hasn't been sleeping for a while.  Well, if it was, then it seems to be wakening up again.  That volcanic complex forming the peninsula just to the south of Christchurch could be related to the same thing

Fig.2.  Current crop of earthquakes, past 7 days or so

It's like Japan in a way, situated on the crest of the Marianas Ridge.   Both appear to be 'sticking' on ocean-floor 'highs', which are stopping the natural eastwards crustal drift, .. then letting go with quakes when the stress is released.

The liquifaction of the soils is interesting.  Being right on the coastal plain I guess it's a fairly common expression of earthquakes in suchlike situations, but I wonder if, with time, we might expect an increase in the salt content of the groundwater, a change in the vegetation, ground subsidence and ponds of saltier water to appear that will get bigger, with the margins eventually separating to become lakes then shorelines then coastlines.  Of course, no-one will be around to see it, .. probably not even the human species unless evolution finds a way for it to outlive the erosion of the mountainous hinterland, so as a prediction it's pretty useless, but the rumbling on that fault for the last year - and the silence over the South Island for the preceding year (2010) suggests a 'salty separation' might well happen in the future.

The old pattern does seem to be resuming, but it is still not back to normal by a long way.  Maybe we are witnessing the start of severance of the South Island to make New Zealand into three, instead of two?

Fig.3.  Earthquakes for the last six years, showing the silence of 2010 prior to the onset of the Christchurch earthquakes.



[ See also - Debunking Plate Tectonics - at :-
http://www.platetectonicsbiglie.blogspot.com/ ]