Editorial: Quake work must be considered

Colombo Street in Christchurch after the earthquake. Photo / Simon Baker

Back in April, when a royal commission was set up to investigate the collapse of buildings in Canterbury's earthquakes, it was asked for an interim report within six months to help rebuilding get under way. Right on deadline, the commission has delivered some tentative conclusions for the construction or strengthening of buildings not only in Christchurch but in all parts of the country.

It suggests local authorities should keep an up-to-date register of unreinforced masonry buildings, which caused 42 deaths in the February 22 quake. It wants these old, often historic, structures urgently made safer by bracing their parapets, installing roof ties and securing anything that might fall in a public place. In cities where the earthquake hazard is high, such as Wellington, Napier, Hastings and Gisborne, it suggests additional measures.

It says this work should be done as a matter of urgency. Auckland's council has given owners until 2045 to strengthen the 393 unreinforced masonry buildings on the isthmus that are listed on a preliminary register of structures likely to collapse in a moderate earthquake. The city's building policy manager sees nothing in the commission's findings to bring forward that date.

It is hard to inject much urgency into precautions for places that scarcely ever feel an earthquake thanks to their distance from the tectonic plate boundary that New Zealand straddles. Auckland and Dunedin have a hazard rating of just 0.13 as compared with Wellington's 0.4. But then, Christchurch was previously rated 0.22. Now that uncharted faults have made their presence known the rating is 0.3.

The commission notes that the national seismic hazard model assumes an earthquake up to magnitude 7.2 (Canterbury's 2010 quake was 7.1) "could occur on an unknown fault virtually anywhere in New Zealand". However, it adds that "in areas of low seismicity the likelihood of such an occurrence is very low".

With mixed messages such as this Auckland and Hamilton (0.16) probably should relax and weigh up the cost of earthquake precautions against the fact that seismic faults release their stress at intervals of many thousands of years. It is worth keeping a register of buildings prone to them, and require the commission's recommended work to be carried out. Parapets and pieces that might fall on public areas should be strengthened. More costly structural work is unwarranted.

New buildings invite more stringent design standards and the commission finds a number of deficiencies in the national building code have been exposed in Christchurch. The city was the focus of greater ground forces on February 22 than the code envisaged. The commission recommends improvements that it suggests should be made mandatory for Christchurch by an order-in- council before they are incorporated in the national code in due course.

Its interim report does not deal with the collapse of particular buildings. The CTV, PGC, Forsyth Barr and Hotel Grand Chancellor failures are still under investigation by the Department of Building and Housing. But from submissions and consultants' reports so far, the commission has given some pointers for Christchurch's reconstruction. It finds the liquefaction-prone alluvial gravels under the city to be too varied for any single foundation to be suitable for every site. Each new building will need careful consideration of the soils beneath.

Its interim report could be the basis for a start at last to some reconstruction in the shattered city and more robust building designs for the whole country.