Wellingtonians told to be prepared

The Kaikoura earthquake won't be Wellington's last, so be prepared for the next one.

So said a panel of experts at the NZILA annual conference. At June, insured losses were $NZ1.84 billion, of which $1.4 billion was commercial losses and 80% of losses were in Wellington.

Geologist Kelvin Berryman, general manager strategic relationships at GNS Science's hazards division, said Kaikoura was in New Zealand's highest quake hazard zone so was not unexpected. NZ sits on top of the Australian and Pacific plates, which move continuously. There were no set repeat times for quakes, but scientists could identify clusters. For example, from 1929 to 1942, NZ earthquake activity was more frequent than the long-term average. Since 2003 there had been another cluster.

The magnitude 7.8 Kaikoura quake, which hit on 14 November 2016, was felt in Wellington 45 seconds after it started. There was sideways movement up to 12m on the Kaikoura fault line. Coastal uplift was 6m-8m in some areas north of the epicentre. Mr Berryman said ground motions were "very similar to Christchurch". Forces were "phenomenal and some exceeded the force of gravity".

The quake imposed "a lot of stress" into faults in the Wellington region and, within a week, "all slow slip patches were active at the same time". Slips were evident under the Marlborough region that had never been seen before.

Mr Berryman advised Wellington residents to be prepared and make plans because there was "heightened probability" of another quake after Kaikoura.  It was 70% likely aftershocks would continue to decrease and there would be nothing larger than M7, but there was a one in 20 chance of a M8+ in the coming year, "and that's a big scary one".

He said it was "hard to work out the prior history of something 320km below us, but we can see evidence of 500 and 800 years ago of large M8 events with tsunamis". Wellington was "in the window" because every 300 to 500 years there was a big earthquake. "Planning is better than panic."

There was a 30% chance of a M8 quake on the Alpine fault (which runs for 600km up the spine of the South Island) in the next 50 years and a 50% chance of a M7.2 on the Hope fault (in the northeast of the South Island). "NZ will continue to experience very large hazard events," he said.

Mr Berryman's advice was to:

• Continue quake research
• Judge levels of acceptable risk
• Use risk management options of accept, reduce, transfer or avoid
• Plan ahead at personal, community, city and country levels
• Develop contingencies.

Asked about scientific methods to release strain on earthquake faults, Mr Berryman said the earth's system was complex and "we know so little, it's very risky", akin to introducing rabbits to NZ or cane toads to Queensland.

Drilling had occurred to 1km depth on the Alpine fault, which provided "a piece of the puzzle", but didn't get to the fault itself and cost $30m.

Engineer Hamish McKenzie, principal at Holmes Group, said the 1931 Napier earthquake saw the birth of seismic engineering and design codes in NZ. The mid-1970s, after soft-storey and column failures occurred in overseas quakes, saw a philosophical change to developing building codes. Saving lives became the focus, so buildings could get damaged, providing it was controlled, enabling people to exit safely. The NZ Building Act focused on safety, not business continuity or damage avoidance, although they were desirable.

Mr McKenzie said Wellington suffered liquefaction around the port and other reclaimed areas with softer soils after the Kaikoura quake. "Amplification impacts on poorer soils," he said. The type of shaking that occurred determined what type of buildings sustained the worse damage. In the Kaikoura quake, Wellington's mid-rise, eight-to-10 storey concrete-frame buildings fared worst because they resonated with the earthquake's force. "Not every earthquake will be like that; a sharper, shallower quake will affect more shorter, squatter, stiffer buildings."

A lot of mitigation work was now being done, particularly with pre-cast concrete floors, where damage was hard to identify immediately after the quake because it was not readily visible.

Structural engineers made rapid visual inspections to determine if buildings could be used, access was restricted, or entry was prohibited.

Engineers established a clearing house after the quake to meet regularly to share intelligence and built a website to disseminate information. An engineering industry leadership group established a targeted damage evaluation (TDE) procedure.

Engineers were continuing investigations, conducting crack mapping, and testing structural and reinforcing steel to see how it had been stretched.

Research on the amplification of ground shaking contributed to low-damage design, including more base isolated buildings and viscous damping systems for vibration control, much of which went beyond the Building Act's minimum requirements.

Mr McKenzie said seismic engineering was "still a young profession and we have a lot to learn".

Wellington City Council (WCC) chief resilience officer Mike Mendonca said 20 Wellington buildings were "munted" in the quake. "Some were already on death row and the earthquake hurried them up." Demolishing them gave the city "opportunities to build back better". There were 150 buildings with damaged pre-cast concrete flooring and 72 the TDE procedure identified had significant damage.

There were hundreds of buildings across the city built from unreinforced concrete and the council was six months into a 12-month plan to "deal with them". There was "a lot of pressure on owners".
WCC's resilience strategy, sponsored by the Rockefeller Foundation, was about to be released when the Kaikoura quake occurred. Its vision is "survive and thrive". Mr Mendonca said the strategy had three goals:

• putting people first, including encouraging people to be better neighbours
• gaining information on at-risk buildings and infrastructure, including understanding the risks of building on steep slopes or different soil types
• focusing on making homes warm, safe and dry. “Our homes should be the cornerstone of our resilience.”

The strategy's planning scenario was for 50,000 people being displaced. As the population increased, more people were living in high-rise buildings, which were more vulnerable

He said WCC was supporting an insurance literacy campaign because many in the city were underinsured.