Nature at Work: the Canterbury Earthquake

It is now about 42 hours since Christchurch, surrounding towns, and rural communities were hit by a massive earthquake at about 4.35am on Saturday 4 September. It measured 7.1 on the Richter scale, which is very large, but not exactly huge on a global scale. However, for those of us who live in Christchurch, it felt really big. First, the epicentre was only about 30 km away, and second, it was only about 10km below the surface. That shallowness makes a big difference. Also, the size of it came as a major surprise, as the experts have always said that the big one would be on the Alpine Fault, which is more than 100 km away.

Like many people, I assumed that this was indeed likely to be on the Alpine Fault. My only doubt was that I can see from our house, which is on the lower Port Hills, right across to the Alps. The moon was shining on the snow, and there were no signs of massive snow and rock falls stripping the mountains bare. I have seen what really big alpine earthquakes can do in Peru, and the lack of any signs left me hopeful but perplexed. Given the violence we had just experienced, I knew that if it was the Alpine Fault then in the mountains it had indeed been huge. We now know it was not the Alpine Fault, which has been building tension for 300 years, and is still to go. In fact the mountains were unscathed.

At the time, I had no idea how long the quake  lasted. When it started I was initially somewhat relaxed, as a modern house such as ours is supposed to be able to withstand whatever nature can throw at it. But as the shaking increased, I did indeed begin to fear the house was about to fall down around us. My wife and I simply lay there in bed, as trying to stand up did not seem an option.

In fact we were lucky. Apart from broken crockery, a TV that fell over, a sewing machine that bounced from a table to the floor, and books strewn around as they were flung from bookcases, we have suffered no apparent damage. And after about 34 hours, the power and then the water returned. Of course others were much less lucky, and many have lost their homes.

In trying to make sense of which buildings were extensively damaged and which were not, some things seem to stand out. Houses with timber structures, with or without non-timber external cladding, have withstood the force better than those that rely on brick for structure. Old houses with chimneys have typically lost them regardless of their structure. Houses that lay over any one of a myriad of emergent mini faults had no chance regardless of structure. But there is a lot more to it than that. In some places well-built modern houses also suffered greatly. Over the coming months, the structural engineers are going to have to look again at some of the soils on which houses have been built.

The experts seem to be telling us that soil liquefaction has been a problem. In fact they had warned of this in advance. Liquefaction occurs when shaking causes wet sandy soils to turn to jelly, and house foundations sink. No doubt this has been an issue, but I think there has been more to it than that, at least in the region of town where I live.

The Canterbury Plains have extensive aquifers below them. We know quite a lot about these aquifers, but not as much as we would like. They are tapped extensively for irrigation, in some cases from close to the surface, and in other cases from as deep as 200 metres. We also know that these aquifers well-up naturally as springs to feed the Heathcote and Avon Rivers which flow through Christchurch, and then out to the sea to the East via the Heathcote Estuary.

On the urban fringe over my side of town, at Halswell, there is a third river, called the Halswell River. It has its source within sight of my home, and flows in a southwest direction through Tai Tapu and then to Lake Ellesmere.

All of these rivers begin in what was originally low swampy country. Yes, much of Christchurch was built on a swamp! The paddocks below our house are only about 20 metres above sea level, but the Halswell River still has about 20 km to flow to its outlet. In the other direction, the Heathcote has a similar distance to flow and much of this is tidal.

Within 20 minutes of the big earthquake, I could see by the moonlight that the paddocks below my house were flooded. Where had this come from? Once daylight came we went searching for bottled water, which we found at a local shop, which remarkably, was still open. What we also saw was lots of surface flooding in the streets, with silt-laden water still welling-up in many places. I was subsequently told, but did not see, that immediately after the earthquake the water was spouting to 3 metres. At the local service station the underground fuel tanks had been uplifted about 10cm, and in some places there was evidence of slip and slide movements of about 30 cm that were both lateral and vertical. This included an extensive fault across a finger of volcanic rock that juts out into the flatlands.

Some three hours after the earthquake I noticed the Heathcote was carrying large quantities of silty water. I am told the river rose within less than an hour of the quake.

Today I climbed to the Summit Road on the Port Hills and wandered around the Crater Rim Track to the southwest. It was strikingly clear that the flatlands were inundated for several km out from the foot of the Port Hills. It has been a wet winter, but I am often up on the Port Hills, and this was something different.

So what does all of this mean? My theory is that I was observing something different than liquefaction caused by shaking, although that is not to question the experts who say that liquefaction is a big part of the overall Christchurch story. I think I was observing a wave transmitted through the aquifer, somewhat like an undergound tsunami. When the wave hit the Port Hills it had nowhere to go except to well-up as spouts of water. The force was sufficient to break through bitumen, which it has done in many places. In other words. the quake greatly augmented the natural flow of artesian water and it found many new breakout points. Also, I think I was seeing slip and slide movements that were both vertical and lateral, despite the end of the newly found fault being supposedly 21 km from Christchurch. This is also consistent with what I have seen with a major earthquake in PNG, where it was transmitted as a visible wave through the earth, in some places leaving no mark behind, and in other places, where there was differential resistance, creating tears or mini faults. I think the wave theory, and differential resistance, can explain some things which liquefaction by itself does not.

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On Cashmere Road about 1 km from our house.  The mini fault extends away from the camera and down to the flats below.

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A broken water pipe blew out this hole about 10 minutes after the earthquake.

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Larsen’s Rd Halswell at about 8.30am, 4 hours after the earthquake.  The water is silt-laden artesian uplift, with some sewage contamination from broken mains

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Halswell Road uplift

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Silt left behind after welling up of artesian water, which has broken through the concrete drive, leaving silt mounds at each breakthough point. This photo was taken about 30 hours after the quake.

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Petrol station forecourt with uplifted underground fuel tank. It might look minor but it probably is major.

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Paddocks below our house. We forgot to take a photo of this until 36 hours after the quake, by which time most of the flood water that welled up from below the ground had drained away to the Halswell River.

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A wider angle showing water still flowing on Monday afternoon, about 58 hours after the quake. (Update: In fact what is left here is actually ‘silt volcanoes’ and not water.  See subsequent posts on silt volcanoes)

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About Keith Woodford

Keith Woodford is an independent consultant, based in New Zealand, who works internationally on agri-food systems and rural development projects. He holds honorary positions as Professor of Agri-Food Systems at Lincoln University, New Zealand, and as Senior Research Fellow at the Contemporary China Research Centre at Victoria University, Wellington.
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