A new University of Auckland study has put hard numbers on something Wellington has been warned about for decades. If a magnitude 7.5 earthquake ruptures the Wellington Fault, the great majority of people who want to drive out of the region in the days afterwards simply will not be able to, and roughly half of those trips would still be impossible three months later.
The research was published in the International Journal of Disaster Risk Reduction and reported by RNZ on Sunday. It takes an existing engineering assessment of the damage a Wellington Fault rupture would do to roads and bridges, then runs a traffic simulation over the top of it, modelling things like driver reaction time and the point at which a queue of cars stops moving altogether.
The results are stark. In the first three days after the quake, the model found 82 percent of trips out of the Wellington region and 68 percent of trips into it would be infeasible. Three months on, with some repairs done, that improves to 49 percent of trips out and 25 percent of trips in. Lower Hutt would be the worst affected part of the region, followed by Wellington City. The Hutt Valley and the Wairarapa would still be effectively isolated by road a full quarter of a year after the shaking stopped.
The damage scenario the researchers used closes multiple stretches of State Highway 1 along with Hutt Road, Grant Road, Curtis Street and Chaytor Street. Those are not obscure back roads. They are the arteries that carry Wellington in and out of itself, and the simulation shows what happens when traffic that normally uses them is pushed onto suburban streets instead. Trips that could still be completed took far longer, because detour routes filled up and stayed full.
One detail is worth sitting with. As the Science Media Centre noted in its summary of the paper, those figures assume Transmission Gully and the Peka Peka to Ōtaki motorway both stay open. Both are recent builds to modern standards, and the modelling leans on them heavily. If either went down, the picture would be considerably worse than the numbers above suggest.
The authors, Anish Khadka, Prakash Ranjitkar and Seosamh Costello, are careful about what the model can and cannot tell us. Their simulation assumes people keep trying to make the same trips they made before the earthquake, which is almost certainly not what would happen in real life. Faced with a four hour crawl to Porirua, most drivers would stay home.
“While this assumption enabled stress-testing of network performance under disruption, it may overestimate congestion and trip cancellations by not accounting for behavioural adaptation during recovery,” the paper said.
That caveat cuts both ways. It means the congestion numbers are probably a worst case, but it also means the underlying finding is unchanged. The roads themselves would not be there. No amount of sensible decision-making by individual drivers puts a bridge back up.
The conclusion the researchers draw is about money and where it goes. “Overall, the findings highlight that proactive planning, demand management, and targeted investment in critical corridors are essential to maintain network functionality and enhance resilience under extreme but plausible post-disaster scenarios,” they wrote. The work was supported by QuakeCoRE, a research centre funded through the Tertiary Education Commission, and by the University of Auckland. The full paper is available through its publisher.
The word plausible is doing real work in that sentence. The researchers point to a 2011 assessment putting the chance of a Wellington Fault rupture in the next 100 years at about 10 percent. GNS Science research on the Wellington to Hutt Valley segment of the fault has produced a similar conditional probability of around 11 percent over a century, with a sensitivity range running from roughly 4 to 15 percent. The fault last broke between 300 and 500 years ago and produces a large earthquake every 500 to 1000 years or so. This is not a tail risk dreamed up to make a paper interesting.
The study also lands two days after a separate piece of research raising much the same alarm about freight. Writing for RNZ on Friday, University of Waikato researcher Cécile L’Hermitte set out a scenario in which a magnitude 8.5 rupture of the Hikurangi Subduction Zone puts the Wellington and Picton ferry terminals out of action for three months. The Cook Strait link carries more than 500 trucks a day and 5.5 million tonnes of freight a year across just four ferries.
Her team identified 43 separate constraints on the freight system in that scenario, tied together by 99 cause-and-effect relationships, which traced back to only 18 root causes. Air freight is expensive and short of staff. Coastal shipping has little spare capacity, made worse by the closure of Pacifica’s domestic service. Ports that would have to absorb redirected cargo are themselves constrained, with Tauranga having spent years seeking consent to expand its berths.
“Our findings suggest many of the biggest transport problems would stem not from the disaster itself, but from policy decisions made years earlier,” L’Hermitte wrote.
That is the thread joining both studies. Neither is really a paper about earthquakes. They are papers about the choices made in the decades before one, about which corridors get strengthened, which ports get consented, and how much slack a small country is willing to pay to carry. The shaking only reveals the answer.
For households, the advice has not changed. The National Emergency Management Agency told RNZ people should make a personal household plan for emergencies such as earthquakes.
“In the event of an earthquake, remember to drop, cover and hold. If you experience a long or strong earthquake near the coast, seek higher ground or go as far inland as possible. You can find more earthquake safety tips, and make a plan, at getready.govt.nz,” the agency said.
What the new modelling adds is a sense of how long the plan needs to last. Three days of supplies is the standard advice, and in the first three days after a Wellington Fault rupture most of the region would have no way of driving anywhere useful. Three months later, half the trips out of Wellington would still be off the table. A household plan built around waiting a weekend for things to return to normal is a plan built for a different earthquake.
What do you think? Should road resilience spending in the Wellington region be moved up the queue, or is a 10 percent chance over a century too remote to build around? Leave a comment below.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂
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