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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.
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
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.
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
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