On earthquakes, and other weighty matters...

As I was driving in to work (through the sewer) this morning just before the 0800 Morning Report news, Kim Hill (temporarily running the studio end until the Christchurch earthquake is passed by some greater event) was doing the “next hour” promos. These included an interview with a gent who puts the cause of the earthquake down to “rogue seismic waves” apparently. There is another very learned gent who puts the cause down to the gravitational effect of the moon. He was interviewed by John Campbell (no, I was watching netball at that time I think) who promptly got himself into trouble by "getting his personal opinion between the interview and the audience".

Now I am not about to critique the reasoning behind either of these theories; I know little to nothing of the rationale behind them. But the thought came to me that perhaps I should put onto (virtual) paper the probligo theory of the cause of the Christchurch earthquakes. I can present the theory with as much justification as either of the other two. Who knows? The ol' probligo could end up being interviewed on national tv!!! What a HOOT!!

Now, my caveat. I am a long-worn retiring accountant. My knowledge of seismic events would perhaps cover a reasonable sized postage stamp (do they still exist?). I know the difference between an up-thrust and a slip. I also confess that the last time I did three dimensional mechanics was about 45 years ago, and then for a period or two in a week.


The probligo theory of Christchurch earthquakes.

Recall that the first earthquakes were under Darfield, about 30km southwest of Christchurch.

Roughly, the geology of that area is continental rock underlying a broad expanse of alluvial plain, eroded from the Southern Alps. That underlying rock level is general fairly level though it is subject to increasing deformation toward the west. It forms a basin in which there is a considerable artesian water resource. The floor is not in one piece, but broken by more faults than I suspect the experts know about.

Principle 1 – Equilibrium

The theory is based upon the idea of equilibrium. The various pieces of the rock floor under the Plains are held in place horizontally by each other, and vertically by pressure from beneath countered by the mass of what is above. As the upper layers – the alluvial deposits – are comparatively fluid (compared with solid rock they are) it is a bit like a saucer of milk except that it is floating in a bigger bowl of golden syrup. To add to the picture, the saucer is not in one piece. It is fractured into a lot of smaller pieces. Those are being held in place by a flexible wall around the outside of the saucer. That is sufficient to prevent the saucer (which is somewhat denser than the underlying syrup) from leaking from beneath and sinking. A very good description of the geology can be found here.

That whole system is in an equilibrium. The saucer is replenished as quickly as the milk can leak out of the saucer. The total mass of the overlying deposits (the milk) is relatively constant.

Principle 2 – Cause of loss of Equilibrium

Land use in Canterbury has changed considerably over the past five years or so. Large areas have converted from low-intensity arable, beef and sheep farming to high-intensity dairying.

One of the consequences of that change is the very high increase in demand for water. Dairying is a high water use industry; the water content of the milk has to come from somewhere in addition to the normal by-products.

I have not found comparative statistics of the volumes of water drawn off each year over a longer period of time. Nor have I found the far more critical statitics in the change in artesian “head” in the various parts of the Canterbury area. The article I referenced earlier quoted costs in pounds rather than dollars, and mentioned a head of 9 feet in an artesian source 200 feet below surface. I interpret that to mean that water out of that bore was reaching 9 feet above ground level without pumping. The kind of detail I need is what height that bore runs to now. That number might give a clue as to the amount of water drawn from the aquifer and not replaced.

Why is this important? For the simple reason that a very good part of the mass above that saucer is (at least was) water. Draw that water off, reduce the mass holding the system in equilibrium, and the system then will try find a new equilibrium.

Principle 3 – “Local” and “area” effects of changes in the mass equilibrium.

Let us assume for the purposes of this exercise that 200 feet artesian layer covered the area between northeast Christchurch and Darfield. If I can assume that the 9 feet head has reduced to zero (3m loss) over the area of a 30km diameter circle, that could indicate the loss of something like 2 billion m3 (Pi * 15000*15000*3) of water, which unless the Blenheimers has really taken hold still represents 2 billion tonnes less of water holding that saucer together.

The question is whether that kind of regional (over a 30km circle) effect could have a local effect like the movement of two pieces of the saucer near Darfield. I believe that is a distinct possibility.

Further, once that initial movement has taken place the neighbouring pieces then have to readjust to form (hopefully) a new equilibrium.

Conclusion.

So there you have it. The probligo theory of what caused the Canterbury earthquakes. Too many cows drinking too much water and eating too much grass irrigated by too much water… Well, you gotta admit, it is just about as reasonable as George Gair (bless his cotton socks) suggesting a cow fart tax to alleviate global warming.