This week marks the second anniversary of the magnitude 7.8 Kaikōura earthquake that ruptured a world record 25 faults in the upper South Island. GNS Science reflects on some of the key features and implications of this complex event – and talks to principal scientist Dr Kelvin Berryman about what we’ve learned.
It was New Zealand’s best ever recorded earthquake. The magnitude 7.8 event involved widespread surface rupturing, landslides and other ground failures, and a tsunami.
The rupture began near Waiau in North Canterbury and ripped through the upper South Island landscape at about 2km-a-second and ended up off Cape Campbell in Marlborough. It travelled 170km in about 74 seconds.
It was a very different event to the devastating Canterbury earthquakes of 2010-2011. A contrasting feature was the compressed aftershock sequence in which all the main aftershocks occurred within the first 24 hours of the mainshock.
In that respect, nature smiled on the amazing efforts of the North Canterbury Transport Infrastructure Recovery alliance. The lack of a drawn-out aftershock sequence was a major benefit in the heroic effort to re-open the road and rail links between Picton and Christchurch.
Despite these differences, the Kaikōura quake raised many of the same questions. This included risk management, preparedness for infrequent but damaging events, the resilience of key infrastructure in New Zealand, and the ability to provide timely and accurate advice on locally generated tsunami.
“It was certainly unusual that 25 faults ruptured simultaneously in the Kaikōura earthquake, and while a number of these faults are named separately we now understand there is greater connectivity between faults at depth,” Dr Berryman says.
“For scientists, it was not a huge surprise to see the faults in Marlborough – the Jordan-Kekerengu-Needles - all rupturing together. However, the big surprise was that faults in North Canterbury joined in with such enthusiasm.
“Also, it’s fair to say that all earthquakes of magnitude 7.5 and above are invariably very complex. In New Zealand earthquakes of this magnitude are rare so we have limited experience in this range. At the same time, advances in science means we have many more tools than we had 10 years ago for extracting useful information from data recorded during earthquakes.”
Another unusual feature was the strong ground motions experienced on soft soil sites in Wellington. This was because of the direction and style of rupture which pushed a ‘beam of strong seismic energy’ towards Wellington.
“When you reflect on the age of the harbour reclamations in Wellington, it’s not a surprise that so many buildings around the margins of the harbour had a pretty rough time during the earthquake. They are sitting on soft, unconsolidated sediments that accentuate earthquake shaking.”
Berryman said it was surprising how quickly memories of large damaging earthquakes such as the February 2011 Christchurch quake have faded from the public consciousness. Kaikōura acted as a reminder for all of New Zealand that we cannot be complacent.
In one sense, New Zealand was lucky the Kaikōura earthquake was well away from major urban areas. The biggest impact was on smaller settlements, tourism – and particularly transport infrastructure. Because New Zealand is a narrow, mountainous, and geologically active country many important road and rail routes are vulnerable to natural hazards.
“Anyone who has travelled the Kaikōura coastal route will have noted the way the road and rail links are cut into the bottom of steep cliffs in a number of places. It shouldn’t come as a surprise that this is a vulnerable section of transport infrastructure,” Dr Berryman says.
“It’s not feasible to engineer every piece of infrastructure to a gold-plated level of resilience. We have to be prepared to live with a level of risk, but we should be asking how much risk is acceptable?”
New Zealand has a similar land area to Japan, Italy, California, and the UK, but it has far fewer taxpayers. “So it’s a challenge for us to provide expected levels of services when confronted by such high levels of geological hazard.
“As a nation, we manage this pretty well, but there is room for improvement by using impact modelling tools to identify the most effective resilience investment so that future events have lower social and economic impact.”
The best time to beef up preparedness and build resilience was in quiet periods in between the damaging earthquakes. “It’s a bit late to recall what we should have done when the earth is already shaking.”
There are roles for everyone in quiet times - from individuals to governments - to make the future less susceptible to relentless geological processes.
“One of the things scientists can do better is to get more information into the hands of the public and to all levels of government and business to provide an evidence basis for better decision-making about natural hazard risk management to ensure long term social and economic prosperity.”