TC - Ross Wylie presentation
Verbal Submission to the Nelson City Council on the Trafalgar Centre
on Wednesday 14 May 2014
When faced with the term “earthquake prone” which features on the doors of the Trafalgar Centre one is stimulated to know what that term means. The most modern English dictionary that I could find does not mention this combination of words. I subsequently found reference to the meaning of these words in New Zealand legislation which is not the easiest to understand when it is tied up with the concept of a “moderate earthquake”.
So having overcome these hurdles what can specialist scientists say about the frequency, risk and magnitude of earthquakes in the Nelson Tasman area. After enquiry, to some extent this question is covered in a scientific paper “Assessment of the location and paleoearthquake history of the Waimea Flaxmore Fault system in the Nelson Richmond area with recommendations to mitigate the hazard arising from fault rupture of the ground surface” by M.R.Johnston and A.Nicol August 2013. As councillors you should be familiar with the content of this report as GNS Science has provided both councils with map overlays of the fault lines giving the set back distances for buildings etc. (reference the executive summary of their report.)
Their paper is reasonably lucid to the layman. As far as my quest for information is concerned I find the following very illuminating.
- 1)Active faults have been identified passing along the edge of the Nelson CBD which are adjacent to or within built up areas.
- 2)The Waimea Flaxmore Fault system contains several active fault traces arising from fault rupture.
- 3)Three fault line displacements are identified spaced approximately 6000 years apart.
- 4)“Our analysis indicates that rupture of the Waimea and Flaxmore faults at the ground surface may be achieved during earthquakes of magnitude 6.5 – 7.4”.
- 5)“The average recurrence interval of events on the Waimea fault is about 6 thousand years and approximately equal to the elapsed time since the last surface – rupturing earthquake”.
So therefore one can assume with some degree of confidence that earthquakes of magnitude 6.5 – 7.4 are on the cards for Nelson City and environs.
The engineering reports commissioned by the NCC are mixed bag in terms of clarity and manner in which they explored the opportunity to assess the Main Hall of Trafalgar centre. If there are deficiencies in design and execution in the construction of the building it should always be done in an open and frank evidence based manner without the help of assumptions.
For me it was hard to find evidence that the reviewers produce a dispassionate and methodical analysis of the plans and specifications for the building from the bulbs on the bottom of the reinforced concrete piles to the metal roofing over the glue laminated beams with steel perlins interlaced with metal rod strengthening. My assessment is that the engineering design of the building has been done with the purpose of constructing a building that will be relatively maintenance free and built to last. The question that needs to be put is, if the Main Hall was built today what would be different about its specifications compared with those that were used and modified in the 1970”s?
Perhaps I am sticking my neck out by asking, if there were a 6.5 -7.4 earthquake, which would be the safer place to sit, in this room or on a seat inside the Main Hall of the Trafalgar Centre? Much would depend on the peak ground acceleration during the earthquake. If Cowles Stadium in Christchurch is similar in construction to the Main Hall of the Trafalgar Centre, which survived and has been mended without huge expense then I would choose the Main Hall to sit in. Whereas this building, I have been told, has used high tensile reinforcing steel rods and they may, when subject to a Christchurch type earthquake scenario go “ping” and the building crumble.