The earthquake hazard In Christchurch: a detailed evaluation (1991 Report)

Hello everyone,

as some of you may know, I live in Christchurch, New Zealand and have been actively posting on most of the Christchurch Earthquake threads.

I contemplated adding this to those threads, but after some thought, decided it was worthy of its own thread based on the size of the information at hand here -

I came across many studies/documents on the Earthquake Comissions Website dating back to the 1990's/2000's

I have not had time to look at half of these interesting finds, but the one thats is most striking is this one from 1991 -

The earthquake hazard In Christchurch: a detailed evaluation

I just want to be clear, that i'm not suggesting any conspiracy here, just purely pointing out that it was an accepted and known thing that Christchurch was heavily at risk from Earthquakes and especially, huge liquifaction problems.

therefore, I would just like to put the question out there - If they knew of such risks, then why wasn't more done in the building consent of housing, the enforcement of strictest building codes for Earthquake proofing, etc?

From the 1991 document -

In considering the earthquake hazard in Christchurch it is useful to apply the law of precedence: the past is the best indicator of the future. In the first 80 years of the city's history, four large earthquakes significantly damaged the growing settlement, one seriously.
Any one of these four events today would cost the city millions of dollars in direct damage and could result in major disruption to the local economy. The largest of these events was virtually under the city, with an epicentre close to New Brighton

Analysis indicates that potential exists for relatively rare but very large earthquakes (approximately magnitude 8) along the Alpine fault, which essentially marks the western edge of the Southern Alps. More frequent moderate to large earthquakes (around magnitude 6-7.5) can be expected in the Canterbury Plains foothills and North Canterbury area, and less frequent moderate earthquakes under the Canterbury Plains and Christchurch itself.

And this next paragraph describes indentically what happened in the big one last week -

An important component of this study has been to consider the additional effect at Christchurch of the deep, relatively soft sediment underlying the city (the site response model). This creates major changes in the nature of the earthquake shaking by modifying the ground acceleration, velocity and displacement at any frequency.

In some areas of the city the earthquake vibrations are amplified. As a result the overall average hazard for the city increases when compared to areas on bedrock, (for example most of Banks Peninsular) by approximately 0 to 2 intensity units, or by 0 to 1 unit when compared to areas on 'average ground' (comprising shallow sediment). Within the city distinct local variation results in particular from gradational changes in the top 30m of sediment

sorry posted thread too soon, bear with me while i finish....

Continued -

Again, this is identical to what we have just experienced. the scientists here were bang on -

The greatest concern for Christchurch, located near a saturated, sand and silt rich, prograding coastline, is the potential for liquefaction. This phenomenon occurs when the tendency for loose granular materials to compact during earthquake shaking results in a pore water pressure increase, and reduction or total loss in strength.

This may cause subsidence, foundation failure and damage to services. Analysis shows that large areas of the city are underlain by sands or silts which, if sufficiently loose, would be highly susceptible to liquefaction. Although insufficient soil testing has been carried out to characterise densities in all areas, extensive investigation has been done in the central city. Some silts and sands in this area are loose and extremely vulnerable to liquefaction.

again, here we have aquote that exactly describes what happened by Sumner/Lyttleton -

Consideration of the likely effect of a large earthquake in the hill areas suggests damage by landsliding is likely

Houses below steep hillslopes in rural catchments are generally most at risk

Quite scary reading this now. It kinda makes me a biut angry that more was not done in Earthquake preparedness around the city -

Possibly the most significant physical impact on the city may be damage to water, sewer and power supply services. With the depths of relatively soft alluvium under the city, the strains experienced by pipelines are expected to be high, with corresponding high pipe stresses and pipe joint displacements. If liquefaction occurs the sewerage reticulation system and treatment station could be severely damaged

We have not attempted an in-depth lifelines study for Christchurch, or included economic or sociological analysis in this report. In addition to the need for this type of work, we recommend further action from the engineering profession including a review of the current seismic loadings code, local seismic design practices and building stock.

We suggest site specific studies for the Lyttelton tank farm, Bromley sewerage ponds, pumping stations, substations, hospitals, civil defence facilities, airport and key bridges. Major areas of further research include studies of sand density variations and susceptibility to liquefaction across the city; continued paleoseismic evaluation of adjacent active faults, particularly the Alpine Fault, and further investigation of the deep sediments below the city

Surely, if this was the line of thinking back in 1991 more could have been done to prevent this from being such a massive mess at the moment.

Buildings could have been strengthened, built elsewhere.....etc etc...

They are just pushing thorugh legislation now!!


But most of all, why wasn't this document made known to all of us so we could be aware? Were any building companies in the city aware of this? Is the mayor even aware of this?

Please have a read and see what you think....

cheers.

www.eqc.govt.nz...

Here's the link for lots of other interesting papers from the Earthquake Commision of New Zealand -

www.eqc.govt.nz...

Excellent find

A must read for everyone n New Zealand.

Here in Wellington new houses have been required to have 10 metre deep piles for the last 15 years.
The Wgtn/Porirua/Kapiti Council is pretty strict about retro earthquake strengthening old buildings, but admits its a hard task when the building owners don't have the finances to do it right away.

Whats the score in Christchurch regarding pile depth?

reply to post by muzzy


Hey Muzzy,

yeh there's a few good reports on Wellington on that website as well. Not sure about the pile depth here, but i'm sure all buildings from now on will be much more resistant to quakes.

bump

I wonder how the Post Office Buildng stood up to the shaking?
I worked on the foundations when it was being built, the main perimeter walls were 4- 6ft thick.
From memory it was 2 stories below ground, they had pumps going all the time until it was waterproofed on the outside ( thats what I was as doing).
Also worked on the Heathcote Grain Silos roofs, they were wobbly even when there were no earthquakes