Scientists reveal new insights into New Zealand’s 7.8 . earthquake

In this 3D simulation, the fragments illustrate the network of faults—including the Hope Fault, the Leader Fault in the south, the Kikringo Fault, and the Needle Fault in the north—that erupted in the Kaikura earthquake. Seismic waves, ground motions, and rupture propagation along a complex network of faults are captured simultaneously. Source: Ulrich / Gabriel, LMU

Scientists have reconstructed the 7.8 Kaikoura earthquake in 2016 in unprecedented detail to reveal fascinating new insights into one of the most complex tremors observed anywhere in the world.

Starting just after midnight on November 14, the earthquake near Culverden shook the country, lasting two minutes, releasing the equivalent of 400 atomic bombs and triggering a seismic wave that made more than 20 faults.

Nowhere was this domino effect more dramatic than the Kekerengu Fault in Marlborough, which in places appeared to the surface as prominent folds of land stretching across farmland.

After several years, tens of thousands of aftershocks, and nearly two billion dollars in insurance claims later, scientists are left with many unresolved questions to answer.

Among them: exactly how the earthquake started, why there were so many faults and why the earthquake ended at Cape Campbell, 216 km from its epicenter in the heart of north Canterbury.

The most intriguing question was its possible association with our greatest geological hazard – the boundary of the huge tectonic plate which is the Hikurangi subduction zone – and its interface deep below the northern South Island.

Dr Callum Chamberlain, a seismologist at Victoria University of Wellington and lead author of the study, which has just been published in JGR: Solid Earth, said.

It was the sheer complexity of the fault that made it difficult to understand the earthquake, he explained, after nearly five years.

“Although we have an excellent understanding of what faults look like at the surface, thanks to the incredible efforts of New Zealand geologists, this mapping does not directly tell us what is happening at depth – where most slip seismic events occur.”

The piece of land that was raised when the commander’s fault ruptured became known as the “Wayau Wall”. Photography / Kate Bedley, University of Canterbury

After building a decade-long data set of earthquake locations and their exact strength, the study team relied on a technique that allows them to correlate known earthquakes with other earthquakes that were too small to be included in the GeoNet catalog.

They then calculated the exact locations of each of the 33,328 earthquakes in their database between 2009 and 2020, and modeled the faulting pattern for 1,750 of these events.

“This provided the most detailed and complete view of the rifts in depth beneath the ruptures in the Kaikoura earthquake to date.”

As previously suspected, they were able to confirm that the event started as a simple slip of the hump fault, before spreading north through the South Island, as well as offshore thrust faults, which are likely the main source of the associated tsunami fault. .

The Papatea fault, which was not mapped before the earthquake and runs along a similar path to the Lower Clarence River at Marlborough, caused a surface rupture of 19 km length and transformed a large area of ​​mountainous country 8 meters high in a matter of seconds.

“We suggest that fault links between the Naval Thrust Faults and the Pappatia Fault help explain the unusually high slip on the Pappatia Fault, by providing it with more room to accommodate this slip,” Chamberlain said.

The study team, including scientists from GNS Science, the University of Victoria, the Massachusetts Institute of Technology (MIT) in the US and the Swiss Seismological Service, added another bug to a tally of more than 20 already identified.

It was this previously unknown that linked the Papatea fault to the Clarence fault, which was another major component of the broader Marlborough fault system.

The Papatea fault, which had not been mapped before the earthquake, caused a 19 km surface rupture and transformed a large area of ​​mountainous country to a height of 8 meters in a matter of seconds. Photo/Mark Mitchell

“All of these structures play an important role in explaining the strongly variable deformation and slip around this fault junction,” Chamberlain said.

Our earthquake catalog also shows that the Kaikura aftershock series has reactivated faults that erupted in the 2013 Cook Strait and Grasmere Lake earthquakes.

“This suggests that the rupture could have persisted beyond those faults, but it is possible that they slowed it down as they had slipped as recently as a few years ago.”

But when they analyzed relevant activity along the Hikurangi core subduction interface, they found almost no satellites.

“We showed that the Hikurangi subduction zone here did not cause significant aftershocks, despite the slip after the earthquake,” he said.

“This suggests that the interface here may be less likely to be a direct source of earthquake tremors.

“However, this is now a topic of ongoing research to look at more data and try to determine if there are any other earthquakes at the interface.”

Interestingly, they also confirmed that there were a few aftershocks on Papatea and Kekerengu’s faults, despite their dramatic parts of the event.

State Highway 1 has been left out of action for several months due to damage caused by the 2016 Kaikoura earthquake. Photo/Nelson Marlborough rescue helicopter

“We conclude that these faults set off near all the stress accumulated on them, resulting in very little or no slip after the earthquake.”

Ultimately, Chamberlain said, the study highlighted the chaotic nature of the fault in the upper half of the South Island.

“Kaikoura is still a very complex earthquake, but by resolving fault structures at depth by mapping the aftershocks, we can understand how these complex earthquakes work,” he said.

“New Zealand has a lot of faults, and the possibility of linking these faults in rupture chains during individual earthquakes is not currently understood.

“By knowing how faults in the Kaikoura earthquake relate to each other, we will develop a better understanding of the range of potential earthquakes here.”

What did we learn from the Kaikura tsunami?

Meanwhile, another team of researchers who took a closer look at the tsunami accompanying the earthquake said that the event left many of its own questions to explore.

While the earthquake is best remembered for the devastation it wrought on highways—not to mention the thousands of landslides and kilometers of coastal elevation it caused—the tsunami it caused was also a major feature of the late-night event.

The pebbles and seaweed that appeared on the beach at Needles Point on the Marlborough Coast were likely washed away by the tsunami that accompanied the 2016 earthquake. Image / GeoNet

More than 25 minutes after the quake, the tide level at the Kaikoura tidal scale fell about 2.5 meters, before the water level rose by about 4 meters and a series of waves began to erupt within several hours.

It was later discovered that it had reached a height of about 7 meters in Goose Bay, about 17 kilometers south of Kaikoura, and in the nearby city of Awaru, leaving marine debris scattered across the river’s floodplains.

To the south on the Banks Peninsula, the only building in Little Pigeon Bay, an uninhabited vacation home, was exposed to its foundations by an estimated 5-meter-high tsunami.

In a study published in the New Zealand Journal of Geology and Geophysics, Niwa Hydrodynamic scientist Dr. Emily Lane and colleagues analyzed five years of research conducted in the aftermath of the tsunami.

While much has been learned, they find that many open questions remain.

It has included what is the best approach to rapid modeling of a tsunami – at night, simpler models are found to perform as well or better than more complete ones – and how the lessons of this local event can be used to better understand the characteristics of older ones.

The only building in Little Pigeon Bay in Banks Peninsula, an uninhabited vacation home, was demolished to its foundations by a tsunami estimated to be 5 meters high. Photo/NZ Herald

“Learning as much as we can from recent events is crucial to advances in these areas of tsunami research; such efforts may even lead to questioning previous scientific conclusions.”

The main challenge ahead, they said, is to find more effective ways to increase the community’s ability to respond to local tsunamis.

During the Kaikoura tsunami itself, they found that there was confusion in some areas, due to “a combination of the complex and evolving nature of the event and inconsistent messaging from various levels of responding agencies and the media”.

“This led to several evacuation stages where people were not sure if, when and where to evacuate.”

In particular, they suggested that more could be learned through mutually motivated relationships and co-designed research with the iwi, hapū and whanau communities.

“This is particularly relevant for remote, exposed, and vulnerable communities.”

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