Wednesday, March 02, 2011

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.

7 comments:

Monkfish said...

I think your theory is seriously flawed. Assuming that most of the water is consumed by humans and cows (in the form of grass, which is mostly water anyway), it will be excreted over the same, more or less, region.

Thus equilibrium is maintained by cow shit.

QED

The probligo said...

:D :D Not bullshit?

The cow excretia I wondered about. Having a sister in the business might provide some clues there. On the whole, I had thought the biggest hole was in the irrigation use. But, once on the surface, evaporation and expiration takes over as well...

The human contribution though is generally piped "somewhere else"...

As a generalised argument, I am happy to let it stand at present. ;)

Monkfish said...

Aahhh...in support of your theory (nobody can say I'm biased here) evaporation of irrigation and urine from the domestic ox (so's not to differentiate between cows and bulls) is of course exacerbated by the drying north westerlies that Canterbury is famed for. Thus, there will be some considerable mass lost through evaporation (this is known as the "Piss and Wind" hypothesis).

But then again, population increase in the area, plus enlargement of dairy herds through payouts to farmers will most likely result in an increase in biomass, in turn increasing the quantity of unwanted mammalian impedimenta. You have also neglected the overlying mass increase due to the popularity of large 4WD SUVs. In fact, it's arguable that the extra mass of cheap 2 tonne SUVs has in fact been responsible for increased instability of the Canterbury area.

So I think my crap hypothesis is more tenable than yours, which essentially devolves around taking the piss out of the area.

Thanks for the intellectual stimulation, prob.

In a less lighthearted vein, the Christchurch event has been truly horrific, a timely wake-up call for all New Zealanders. My heart goes out to those poor people who have lost family, friends, house and work, not to mention the things we normally take for granted: hot showers, telephones that work, lavatories...

The probligo said...

Absolutely. Well said.

And one of the start points has to be the systems put in place by CD for assessment of damage and community requirements.

I heard Robinson's interview with Leanne Dalziel the other morning (it prompted a probligo missive to Granny Herald - unpublished). One thing she got right is that CD depend upon incoming "complaints" for the early information they use for planning. It is apparent that CD has to take a far more active role in the gathering of data on damage, immediate community requirements, and available resources.

And, quite by the way, where the hell was the Minister of Civil Defence? Who was leading the political front? The jonkey and the wobblybrownlee. The only time I heard from John Carter was the initial imposition and extension of the civil emergency.

Monkfish said...

I agree. The CD has to learn from this disaster, because sure as eggs, there will be another to cope with before too long.

Tracey Barnett's column in this morning's Herald is a genuinely uplifting comment on the response of 'ordinary' Kiwis who took matters into their own hands. A pity though, that it takes such a catastrophic event to unite a country. The next will possibly occur later in the year if the ABs lose the Cup...

And while we're with Granny, Rudman has hit the nail on the head (again) when he says that the gummint will use the earthquake as a very convenient excuse to punish Len Brown for being such an upstart mayor from the wrong ideological camp.

The probligo said...

I am waiting for TB's op-ed to appear on her webpage. I have a couple US-mates I want to send it to. They just doan unnerstan kiwis and iwunna make the point to them fullas.

Re Rudman - I commented on very similar lines to Cactus Kate. She moderates so that one never saw the light of day. I also left the same comment with Karl du Fresne (c&p is wonderful!). That is still there but without response.

Re LB, it is more insidious than that I believe. There is far greater political ground to be made in other pastures. And a lot more of the bullshit to go with it. Targets at this point are Goffy and the rhinohide. Do not be surprised if the Nat's campaign in Epsom concentrates on the mess created by the more extreme elements of rhino's ideas; like the stop-gap solution for Maori representation.

Oh, and du Fresne is rabbiting again so I took the opportunity to draw his attention to TB's article.

Last thing - "jellybrownee" occurred to me on the way to work this am instead of "wobblybrownlee". The sewer brings out the best... :D :D

Prognos said...

I believe the above theory that irrigation extraction in Canterbury may have triggered the quakes is well worth investigation. The trouble is the economic value of dairy conversions and its political significance to the government is huge. In the same wek that GNS(govt science dept) 'persuaded' the geology department of Canterbury university not to release the seismic data gathered since the quakes, the government announced its $420million funding for new irrigation for central Canterbury....coincidence?
The volume of water extracted from Central Canterbury aquifers in recent years is approxiamately equal to 10 Benmore dams...when the Benmore dam was filled, small quakes were recorded.
It is an established phenomenon that changing water tables can cause new faults. The International Guide to seismic Engineering lists numerous examples of mining damming and drilling, causing faults where they previously didn't exist.
Scientists (geologists) I have spoken to have voiced concern regarding the irrigation theory, but acknowledge that to voice such views publicly, would be to risk their continued employment.
Ecan was sacked in 2010 because of the governments urgency to increase irrigation, using the already seriously depleted aquifers of Canterbury.
The Greendale fault is smack in the centre of the 'Red' zone of over allocated irrigatrion permits in Canterbury.