Although I am working overseas for the Red Cross I have a house in Christchurch in which my daughter lives in, and I call Christchurch my 'home town'.
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.
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.
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. 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.
No comments:
Post a Comment