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How the Kaikōura earthquake changed our view of seismic hazards

How the Kaikōura earthquake changed our view of seismic hazards

Kaikōura was a complex earthquake with multiple faults rupturing in sequence to create a magnitude 7.8 earthquake. We were able to observe impressive land rise and deformation, and today it continues to help us learn about the highly seismic country we live in. Chief Resilience and Research Officer, Dr Jo Horrocks.

When the magnitude 7.8 Kaikōura earthquake shook central New Zealand on 14 November 2016, we had no idea of how complex this event would be. It gave us a huge amount of information about earthquakes and what they are capable of, while at the same time, it created big questions for us to answer.

From a seismology point of view, it is considered the most complex earthquake ever recorded, with more than 20 different faults involved in the single earthquake, which lasted almost two minutes and was felt around the country. What started in North Canterbury and finished in Cape Campbell, Marlborough, caused an impressive and dramatic land lift of up to 12 metres vertical displacement along the Kaikōura coast as well as significant surface deformation inland.

The earthquake also caused a somewhat ‘surprising’ tsunami, which occurred when a small fault perpendicular to the main fault ruptured, lifting the seafloor, and causing a surge of water to come onshore along the Kaikōura coast.

We also saw buildings designed to the latest standards, some many kilometres away such as Statistics House or Defence House in Wellington, that suffered damage so substantial they had to be written off and demolished. The public’s faith in the ability of new buildings to perform well in an earthquake was deeply shaken.

Although this was a traumatic experience for those who experienced the event, it gave us the ability to analyse the earthquake and what it did to land and buildings – not only in Kaikōura but further afield, such as in Wellington.

But questions remain about exactly how the earthquake started and why so many faults were involved.

Our beautiful land is one of the highest-risk countries in the world for a range of natural hazards, but there is a lot we can do to be prepared for them. The work we do at the Earthquake Commission (EQC) is not only to provide natural disaster insurance for when something happens, but increasingly we also aim to build the best possible understanding of our natural hazard risk through science, research, and data, and work to reduce the impact of future natural hazards on communities before they happen.

Since 2016, EQC has funded research that has helped engineers, planners and insurers understand the Kaikōura earthquake and to start answering some of the questions that arose because of it. This research aids our commitment, alongside GNS Science and Ministry of Business, Innovation and Employment (MBIE), to improve the National Seismic Hazard Model, and to increase our overall resilience and preparedness as a nation.

Wherever we look there’s something we can learn from. The changes to the land along the coast and inland along the fault line, as well as the more than 30,000 aftershocks recorded after the earthquake, continue to be studied five years later. Scientists are looking at whether earthquakes like the one in Kaikōura could be a ‘new normal’.

The series of faults that slipped at the time of this earthquake have been documented and mapped, and we now have an excellent understanding of what the faults look like at the surface thanks to an incredible effort by New Zealand geologists, as well as by using advanced data sources such as dense seismic networks and satellite imagery. Now it’s about trying to find out what happened below the surface, as well as whether this could happen in other locations, is a more complex and ongoing endeavour.

A key area of inquiry has been trying to understand why so many faults were involved in this earthquake, and how and why earthquakes ‘jump’ across faults. Multi-fault ruptures were known to exist before the Kaikōura earthquake, but this earthquake was unprecedented in the number of faults involved, and the distance involved in the ‘gaps’ between them – recorded at up to 15 kilometres.

We’re now also getting results on research that focused on understanding why buildings on Wellington’s waterfront land were so badly affected.

The extent of the damage had to do with how Wellington is situated in a basin of soft sediment. Think of it like a concave mirror or a bowl of jelly.

When seismic waves hit, it causes lots of shaking in different directions. The shaking can be amplified compared to areas of Wellington on hard rock. This sort of seismic activity could happen in any one of the 14 basins around New Zealand and cause similar damage.

This knowledge is helping us work on improving our resilience and preparedness as a nation. Because resilience of our homes and buildings – and towns and cities – is fundamentally created (or not) by where we build and how we build. These are conversations everyone can be involved in.

One focus for EQC is around refining and updating the National Seismic Hazard Model – a model that catalogues everything we know about fault lines and earthquakes in New Zealand and tells us about the likelihood and strength of earthquake ground shaking that might occur at any given site, over specified time periods.

The model is an important part in our step towards resilience as it contains our best estimation of the risk from earthquakes in any one location, and how we need to build, plan, and prepare as a result. The information is key for decision-makers; whether they are homeowners, government policy makers, council land use planners, developers, people in the construction industry or the many others that make decisions that determine how resilient our homes and buildings are.

We know the effects of what happened in Kaikōura will last for a long time yet and will continue to help us improve our ability to reduce risk for New Zealanders.

What we know and continue to find out we are committed to sharing with engineers, insurers, and others so that the right decisions can be made for our homes, towns, and cities – so that we have stronger homes on better land, with more resilient infrastructure, and ongoing access to insurance.

 

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