16 Feb, 2023 By Rob Hakimian
New Zealand’s North Island was struck by a 6.1 magnitude offshore earthquake yesterday, coming shortly after the nation’s most destructive cyclone in decades, but the infrastructure and resilience systems are in place to cope with it more effectively than a few years ago, according to a former resident and chartered geologist.
Progress since the country’s 2011 Christchurch earthquake, with a magnitude of 6.3, and the 2016 Kaikōura earthquake, which had a magnitude of 7.8, shows where New Zealand can – and has – improved in its response to massive weather events.
Aecom associate director in ground engineering Jamie Codd moved to New Zealand in 2011, after the main Christchurch earthquake. He said that Wellington – the nearest big city to this week’s earthquake – was previously in a particularly vulnerable position. “Wellington is a very different city to Christchurch, it’s got a higher population and, geographically, it’s a little bit more isolated,” he said. “There were only a few ways in, so there was a risk of it becoming isolated in a big earthquake.”
To combat this, Waka Kotahi NZ Transport Agency has invested money into improving resilience of the roads and access for people, resources and materials in and out of the city. This has seen them construct the Transmission Gully Motorway, a new 27km road that stretches past Wellington with the express purpose of improving safety and offering better access. Construction of the NZ$1.25bn (£650M) road started in 2014 and was completed in 2022.
The strategy for utilising this access is also part of building resilience. “I know the local authorities on the Kapiti Coast, where this quake happened, will have resilience plans in place that will kick into action,” Codd said. “They know which roads would be affected, which lifeline infrastructure would be affected and then respond to them. Routes will have been devised for the machinery and plant needed for repair operations to get to where they are needed.”
Meanwhile, one critical piece of infrastructure that was damaged in the 2016 Kaikōura quake was State Highway 1, the country’s longest and most used road. The transport corridor north and south of Kaikōura suffered extensive damage and was inundated by numerous rockfalls and more than 750,000m3 of debris from 85 landslides. One particular stretch of the road, below a sheer 90m high cliff face, has continued to experience dangerous rock falls since, which can damage the road – or worse.
To combat this, the option of a self-cleaning rockfall canopy was decided to be the best mitigation for the scenario. The one designed and installed for this location, following detailed rock fall modelling, is 104m long and is the first of its kind in the southern hemisphere. It is “self-cleaning” because the structure’s elasticity is enough to use the falling rocks’ kinetic energy to bounce them into a safe zone – off the road. The canopy was completed in June 2021.
When it comes to the buildings, Codd recalled that “after the Christchurch earthquake, there was a lot of talk about building regulations and how important they are”.
He continued: “In Christchurch, Wellington and probably Auckland too, there was a period where teams were checking which buildings complied and which didn’t. Some buildings were marked as not complying and needing improvement. That was in 2012 to 2013, so they’ve had several years to retrofit those buildings and apply new technologies.”
One technology to reduce earthquake impacts on buildings, pioneered by New Zealand scientist William Robinson in the 1970s, is what’s known as base isolators or a base isolation system. This is a similar idea to car suspension and sees the superstructure of a building decoupled from its substructure, isolating it. The base isolation takes the weight of the building, dissipates the seismic forces and allows the foundations to move horizontally – separate from the superstructure. There are various techniques to create base isolation, generally using rubber bearings, friction bearings, ball bearings and spring systems.
Many important buildings in New Zealand have been built with base isolation. Examples include Te Papa museum in Wellington, Parliament House, the William Clayton building in Wellington and Christchurch Women’s Hospital.
Te Papa’s base isolation system takes the form of 152 flexible bearings, described as a “rubber-and-steel sandwich with a core of lead”, that sit between the main building and the concrete slab of its foundation. In an earthquake event it will move independently of the building, taking much of the impact, while the main Te Papa “rolls with the punches”, experiencing a longer but gentler motion. There is a 400mm seismic gap to allow for this movement – during the 2016 earthquake it “slid” around 15-20mm.
New Zealand is well prepared for earthquakes, but is always improving and never complacent about it. This latest quake will show the country how prepared it is for the next big one, and where it can improve.
This is particularly important due to the location of New Zealand’s South Island, which sits directly on top of the Alpine Fault, an 800km fault line between the Australian and Pacific tectonic plates. For the last 8,000 years there has been a major earthquake from the Alpine Fault roughly every 300 years. With the last one having occurred in 1717, the next one, known as Alpine Fault magnitude 8 (AF8), is anticipated in the next half a century.
Codd notes, however, that “people have short memories” but says that New Zealand’s government – its institute of geological and nuclear sciences GNS Science and the Toka Tū Ake Earthquake Commission – ensure earthquake resilience remains in mind for residents. “They do a good job of keeping it on people’s agenda, making sure people know how to respond when they need to. I think that’s perhaps one of the lessons they’ve learned from Christchurch,” he said.
New Zealand based subsurface software firm Seequent, who provide earth modelling, geo data management and team collaboration solutions, has also studied the changes in New Zealand’s resilience to earthquakes after the Christchurch quake. Seequent geotechnical analysis content development manager Dennis Waterman noted that, prior to 2011, residents were not widely aware of the natural phenomena of liquefaction – where sands or silts that are saturated with water turn almost into liquid when shaken – but are now regularly confronted with it.
“Now we know that especially the Canterbury plains, due to the nature of their formation, consists of lots of liquefiable sands which should be taken into account for future building projects,” Waterman said. “One could imagine, for instance, to reserve the most liquefiable areas for sports fields or parks rather than residential areas, and build new residential areas on less sensitive areas. This is something that was never considered before.”
He continued: “The Christchurch earthquakes raised a lot of awareness and one of the concerns for Wellington is that the reclaimed land between Lambton Quay and the waterfront is actually also loose material and therefore liquefiable. In fact, the 2016 Kaikoura earthquake already caused significant liquefaction in the Wellington Port area and an equivalent earthquake closer to Wellington will probably do that once more, but on a larger scale.
“Having said that, it is not easy to prevent liquefaction from occurring, nor to build buildings that won’t really be affected by it. The lesson from Christchurch to try to prevent building on liquefiable soils cannot be applied to the existing areas of Wellington anymore and what is left is to apply measurements to minimise the risk and damage in Wellington in case of an earthquake. Not that that is an easy solution: such measurements are expensive and difficult to apply inside a city area. Which proves that the gap between learning a lesson and being able to apply the lesson can be large. We know Wellington has a potential problem, but now we have to find a way to solve it.”
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