Tuesday, 1 March 2011

Why did the Christchurch earthqauke occur ?

Although I am working overseas for the Red Cross I have a house in Christchurch in which my daughter lives, and I call Christchurch  my 'home town'. My heart and prayers go out to the people in Christchurch who are having a really tough time after two major earthquakes and so many aftershocks..

I am not a scientist but I trained as a seismological technician and wintered in over in Antarctica  in 1970 at Vanda Station recording earthquakes on a 24 hour basis for 12 months.  So I picked up quite a knowledge on earthquakes and I had the honour to work with famous seismologists such as George Eiby, Robin Adams,  Frank Evison, Rex Orr and geologists such as Trevor Hatherston before my departure for Antarctica. And also to Ray Maunder, a seismological technician who taught me so much.

I am currently reading articles on why this earthquake happened and I find these words sent to me the other day by Gary Lewis who wintered over with me in Antarctica as a senior science technician, very interesting. This is what Gary Lewis said

"My own view is that because some parts of Christchurch are on a thin pan, we should not rebuild in all cases. It might be better to compensate the owners and dedicate those parts to market gardens, which could be an acceptable blend of rural and urban spaces.


I remember the words of an old deceased DSIR mate, Harvey Cummack, a physicist, who always maintained that Chch was like a bowl of instant pudding with a coin floating on the surface.
When the bowl shakes, the coin sinks to the bottom.


Because there is no such thing as bedrock down there, the building piles are only set into soft wet stuff – scary.


Sadly, I don’t think it is over yet, and could turn very ugly. The thin pan is now cracked all over the place with the remaining coins on the pudding looking rather fragile."



John Clague, Canada Research Chair in Natural Hazards Research, Simon Fraser University answered these qusetions:

Why was the damage so extensive?
The main reason is that the epicentre was only about 5 km from the city centre. Even a magnitude-6.3 earthquake, which seismologists consider "moderate", will cause major damage if its epicentre is that close to a major urban area. In addition, the earthquake was shallow - about 5 km deep; again the seismic energy swept through Christchurch with little attenuation.

Many of the buildings in Christchurch are old and made of brick. These buildings are vulnerable to the strong shaking of an earthquake of this magnitude. The damage to buildings in the city was highly variable - some collapsed during the shaking; others were so severely damaged that they will have to be demolished as soon as possible; other buildings can be repaired; and others suffered little damage. Some buildings may have been weakened by the September 2010 earthquake and failed during the earthquake yesterday.

I find it interesting that some newer buildings in Christchurch were severely damaged. New Zealand's building code has earthquake provisions equal to those of any country. I’m not sure what went wrong; damage to modern buildings could be because of liquefaction. Liquefaction is a huge problem in Christchurch because the city is built on an alluvial plain, on sediments that is vulnerable to liquefaction. When shaken, these sediments transform into a liquid, causing irregular settlement of the ground, which is extremely damaging to buildings and buried structures, like water lines.

Was this related to the previous quake?

The New Zealand quake appears to be related to a system of faults that was implicated in the earlier earthquake. It's a minor fault, not main active faults that run the length of the South Island farther west. The faults that produced the September earthquake wasn't even known until the earthquake occurred. Yesterday's earthquake can be considered a large aftershock of the September event; we’re going to see a lot of aftershocks, hopefully smaller, in the days and weeks to come.

Something seismologists wrestle with is whether a large earthquake like this can set the stage for future ones? The September quake released strain on one fault, but perhaps it loaded stress on the ends of that fault or another nearby fault. Whether one calls the second quake an aftershock and a mainshock is a bit fuzzy.

What’s next?

Officials will have to do an inventory of damage to the building stock. They will declare some buildings a 'write-off', to be torn down as soon as possible, probably in about a week or so, because they will be vulnerable to collapse during aftershocks. Structural and civil engineers will examine the buildings for signs of damage that might not be obvious to the untrained eye, like cracking of key structural elements. Other buildings will have to be repaired before they can be occupied, and still others will be declared safe for immediate occupation.

Mike posted these comments on my blog which I add here:

The quake was a "strike-slip event with oblique motion" - mostly horizontal movement with some vertical movement - with reverse thrust (ie. vertical movement upwards). The vertical acceleration was far greater than the horizontal acceleration. The intensity felt in Christchurch was MM VIII. The peak ground acceleration (PGA) in the Christchurch area exceeded 1.8g (i.e. 1.8 times the acceleration of gravity), with the highest recording 2.2g, at Heathcote Valley Primary School, a shaking intensity equivalent to MM X+. This is the highest PGA ever recorded in New Zealand; the highest reading during the September 2010 event was 1.26g, recorded near Darfield. The PGA is also one of the greatest ever recorded in the world, and was unusually high for a 6.3 quake.


In contrast, the 7.0 Mw 2010 Haiti earthquake had an estimated PGA of 0.5g. The acceleration occurred mainly in a vertical direction, with eyewitness accounts of people being tossed into the air. The force of the quake was "statistically unlikely" to occur more than once in 1000 years, according to one seismic engineer, with a PGA greater than many modern buildings were designed to withstand. By comparison, the 2010 quake - in which damage was predominately to pre-1970s buildings - exerted 65% of the design loading on buildings. The acceleration experienced in February 2011 would "totally flatten" most world cities, causing massive loss of life; in Christchurch, New Zealand's stringent building codes limited the disaster.

The loss of life and property and ongoing extreme stress for so many people is tragic, but I do think it's a testament to New Zealand's building codes that so many structures remained standing long enough for people to get out alive. It could have been far worse.

Professor John Wilson is Chair of the Australian Earthquake Loading Standard and Deputy Dean of Engineering at Swinburne University of Technology had this to say:

“This quake was pretty much a bullseye - it was quite a large Mn 6.3 event and so close to Christchurch that we weren’t surprised to see significant damage; at that close range, the level of shaking is quite severe. We expected the older buildings with unreinforced masonry to suffer - their masonry is heavy, brittle and vulnerable to earthquake shaking. In general the contemporary buildings performed well, although a few contemporary buildings have collapsed which did surprise us. New Zealand has very good loading standards and a strict regulatory environment and since the mid-70s onwards the buildings have been designed for earthquake resistance very well. What’s more, the standard of design has still been improving over the last 20 years or so - which is why most buildings performed well, with the exception of a few buildings that were severely damaged or partially collapsed. The immediate challenge is to allow the Urban Search and Rescue (USAR) teams to respond and rescue in what is a very hazardous environment with continuing aftershocks.

It is definitely possible to make earthquake-proof buildings nowadays. The most elegant approach is to use ‘base isolation’ - basically you found the building on springs and de-couple it from the ground. It adds about five per cent to the building cost but makes it totally secure. The design has been around for about 30 years, though really became more common about 15 years ago. It’s widely used in California and Japan, which see so many earthquakes, but is also used in Wellington for buildings like their large museum.”

Anonympus posted this rathet interesting piece:

I think that there is not enough people in Christchurch to make a proper assessment of building collapse who are competent.


The CTV building was built to a BI model but the structure was not able to withstand rotational torque stress and suffered a punch through failure.

If the structural corners had be braced it would have withstood this.

If you tested the CTV building construction profile on a shake-table test environment, you would have discovered these weaknesses.

I have studied the constructive methods in Christchurch as I live here quite a lot and they are sadly lacking in vision and modern construction practices.Most architects and engineers are using outdated methodologies and practices.

Building a suburban home without reinforcing the slab and footings and less that 400mm in thickness is just insane and I have visited hundreds of homes and seen the poor construction methods employed.Yet these people still get paid $400 an hour to design these buildings.

And added later:

AS 1170.4 is the building code that Christchurch needs to utilise.


Australia is far more realistic in its building codes and disallows "shortcuts" I have seen done in NZ.

In rebuilding damaged heritage structures all internal walls and externals need to have their double cavity structures braced with steel fingers between the courses that tie them to eachother to give flexibility and stress relieve from the surface waves that enter a structure.The absorption and dispersal of wave energy is paramount to its survivability.

I have built heritage walls and houses and reused site materials(deconstructed the structures with a deconstruction crew) to keep the historical significance of the structures and identity intact.

Christchurch can do this, but people ( who hold a monopoly of constructive services in Christchurch )neededlessly charge exhorbitant rates when they need not to and it annoys me that greed will cause many such efforts and recontruction projects to be disregarded in Christchurch.

The Lyttleton Time Ball Tower is a classic example. It could be completely rebuilt for less than $3 million NZD if you remove the greed factor.

21 comments:

Marja said...

Very interesting Bob. I read in the star that about 800 buildings in the central district are going to be demolished and about 900 have to be fixed. One of our friends told us that most buildings are build for 1 G-force and that this quake had a strength of 2 g-force, whatever that means.
We also discussed that Christchurch might have to choose different locations for some buildings or even relocating the centre, although I don't think that wil happen. Thanks for al your great posts

MikeM said...

Hi Bob. I'm not a geophysicist, though I know a few. To extend John Clague's comments the quake being so shallow and so near the city, various Wikipedia editors have collated some additional notes about Peak Ground Acceleration and what made this earthquake stand out so much. I haven't verified them, but the source text includes references for anyone wanting to follow up. Emphasis below is mine:

The quake was a "strike-slip event with oblique motion" - mostly horizontal movement with some vertical movement - with reverse thrust (ie. vertical movement upwards). The vertical acceleration was far greater than the horizontal acceleration. The intensity felt in Christchurch was MM VIII. The peak ground acceleration (PGA) in the Christchurch area exceeded 1.8g (i.e. 1.8 times the acceleration of gravity), with the highest recording 2.2g, at Heathcote Valley Primary School, a shaking intensity equivalent to MM X+. This is the highest PGA ever recorded in New Zealand; the highest reading during the September 2010 event was 1.26g, recorded near Darfield. The PGA is also one of the greatest ever recorded in the world, and was unusually high for a 6.3 quake.

In contrast, the 7.0 Mw 2010 Haiti earthquake had an estimated PGA of 0.5g. The acceleration occurred mainly in a vertical direction, with eyewitness accounts of people being tossed into the air. The force of the quake was "statistically unlikely" to occur more than once in 1000 years, according to one seismic engineer, with a PGA greater than many modern buildings were designed to withstand. By comparison, the 2010 quake - in which damage was predominately to pre-1970s buildings - exerted 65% of the design loading on buildings. The acceleration experienced in February 2011 would "totally flatten" most world cities, causing massive loss of life; in Christchurch, New Zealand's stringent building codes limited the disaster.

The loss of life and property and ongoing extreme stress for so many people is tragic, but I do think it's a testament to New Zealand's building codes that so many structures remained standing long enough for people to get out alive. It could have been far worse.

Bob McKerrow said...

What a tragedy and so, so much damage. I hope you and al the people in christchurch are slowly but surely working through your grief and pain. Thinking of you all a lot.

Bob McKerrow said...

Mike, many thanks for your comments, references and observations. I have posted some of them on my blog. Many thanks. Bob

MikeM said...

Thanks Bob. I should just clarify that I can't take credit for those notes -- apart from the bits at the start and end of my comment, it was lifted the Christchurch earthquake article on Wikipedia which was linked to in my comment. (I inserted a paragraph break, but that's about it.)

Cheers.
Mike.

Anonymous said...

Your picture/map is wrong

Gary Lewis said...

Thanks Bob.
I have a bad feeling about Christchurch.
My friend, Bruce Comfort of Oamaru, a retired engineer and industrial historian said to me back in September, that things in Chch gould "get ugly"
I have since modified that to "very ugly" and I have an analogy by way of a true story that involves you;-

In the early winter of 1969 with the midnight sun gone, but still some noon daylight, Bob McKerrow and I were driving a 1955 Ferguson 35 tractor across the ice of Lake Vanda in the Antarctic dry valleys.
For some reason I don't remember, I got off and walked behind the tractor while Bob roared off across the lake ice, when about 300 metres in front of me, I saw a huge "diaphram" of ice about 100 metres diameter, bouncing up and down with the tractor at it's centre - an unbelievable sight.
Realising he had driven onto thin ice, I started running, trying to attract his attention to no avail.
A sudden geiser of water erupted into the air and the tractor shot vertically downwards like a giant piston.
After the water settled, I could see only Bob's head and shoulders. He was waving his arms and shouting something. I feared the worst - that his legs were stuck and I realised as I ran towards him that we had no ice screws or ropes to pull him out with, and he would not survive the time it would take to get them.
We were in deep shit, or so it seemed at the time.

50 metres off, I was able to understand his shouting and waving - "Take a bloody photo, I'm getting cold"
I took the photo and he climbed out of the tractor, stepped onto the ice, which I could see was only about 1 inch thick, and said "thank you" as if nothing much was wrong.
By sheer luck, the lake was only about 5 feet deep at that spot, and we were only a couple of hundred metres from Vanda Station and warmth. The rest is another story.
Suffice to say it was Bob who later jumped back into the water to attach a wire rope to the towing hook which was away down under the radiator.

The point of the story is the analogy of water trapped under a pan, that when fractured, lets water escape, as heavy objects sink.
If Christchurch was once a swamp with flax or coastal scrub on it, with a load of maybe 100kg per square metre, and it now has tall buildings at maybe more than a thousand tonnes per square metre, Aren't we in trouble if the diaphragm is a thin clay pan or similar when the whole thing is shaking?
Is there any chance that a tall building can fall through like the tractor on the ice diaphragm?

I fear it is time to leave, but how do you tell people that when their life investment, life memories and dreams are all locked up in there?

The warning is in the liquifaction. If I see silted water squirting up like that, I wonder what else is lowering to balance the hydraulic equation.
Is it a mean lowering of the entire city, or localised settling?
The surveyors will already know the answer to that.
Market gardening seems to be the attractive option now.
This event is hugely damaging to our whole country, not just Christchurch.
How much worse in Japan?

Best regards to you Bob - many fond memories. You wouldn't like Scott Base these days - I hear they have ice cream dispensers and floor polishing machines, and they even have indoor toilets, bah humbug.

Gary Lewis.

Bob McKerrow said...

Dear Gary

What can I say about that event which was in 1970, not 1969.

A lot of memories flood back. Do you remember Harold lowe flapping about the shore in a mad panic, shouting "don't panic." You and I were as calm as the ice.

I like the way you use that as a lead in to what is happening to Christchurch.

I like this comment of your:

The loss of life and property and ongoing extreme stress for so many people is tragic, but I do think it's a testament to New Zealand's building codes that so many structures remained standing long enough for people to get out alive. It could have been far worse. Thanks, let's keep this alive.

Take care. Bob

Gary Lewis said...

That wasn't my quote Bob. It is credited to another of your correspondents, and well said.
You are right, it was indeed 1970 and my hair was thick and jet black. Now I have bugger all hair and I don't want to discuss the colour any more.
People who flap around in an emergency like penguins avoiding an Orca, perform an important function by reminding us that we need to shut our ears and get on with what has to be done.
I remember it took more than 9 hours to break out an ice channel to tow the tractor out, with ice as hard as concrete, with just picks and crowbars. We had drills, but no explosives to load the holes with, so it was hard yakka and there were only 4 of us. What was the air temperature? Maybe around minus 20? Quite warm compared to the winter when it dropped to minus 56.5 or thereabouts.

Keep the powder dry mate.
Gary

Bob McKerrow said...

An interesting debate Gary. I am coming back to NZ in July and on the 14th of July in auckland, I am giving a keynote address to insurance brokers. I will aslo be talking to a Red Cross audience on Friday 15 July in Wellington. Any chance of connecting?

Gary Lewis said...

That would be great.
I will bookmark the date.
My mobile is 021 242 0007
or you can usually get me through the operator at PNHQ during work hours, Wgtn 4749499.
I was at a funeral yesterday and saw Arnold Heine. He is looking well.
Gary

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