Mysteries of quake unravelled
Why did some faults rupture and others didn’t?
Scientists have reconstructed 2016’s 7.8 Kaiko¯ura Earthquake to reveal fascinating insights about one of the most complex shakes observed anywhere in the world.
Kicking off near Culverden just after midnight on November 14, the two minute quake shook the country with the equivalent energy release of 400 atomic bombs and triggered a seismic ripple that set off more than 20 faults.
Nowhere was this domino effect more dramatic than along Marlborough’s Kekerengu Fault which, in some places, appeared at the surface as raised-up folds of earth.
Several years, tens of thousands of aftershocks, and nearly $2b of insurance claims later, scientists have been left with many lingering questions.
Among them: precisely how the quake started, why so many faults were involved, and why the quake ended at Cape Campbell, 216km away from its epicentre in the heart of north Canterbury.
A yet more intriguing question was its potential link to our largest geological hazard — the huge tectonic plate boundary that is the Hikurangi Subduction Zone.
Dr Calum Chamberlain, a seismologist at Victoria University of Wellington and lead author of the study, just published in the journal JGR: Solid Earth, explained the sheer complexity of faulting made the earthquake so
difficult to understand, nearly five years later.
“Although we have an excellent understanding of what the faults look like at the surface, thanks to an incredible effort by New Zealand geologists, this mapping doesn’t directly tell us what is happening at depth — which is where most of the slip happens in earthquakes.”
After building a decade-long dataset of precise earthquake locations and magnitudes, the study team drew on a technique allowing them link known quakes with others that were so small they hadn’t been in
cluded in GeoNet’s catalogue.
They then computed accurate locations of each of the 33,328 quakes in their database between 2009 and 2020, and modelled the faulting style of 1750 of them.
As had earlier been suspected, they were able to confirm the quake began as a simple slip of the Humps Fault, before propagating northward across the South Island, as well as out onto offshore thrust faults, which were likely the main source of an associated tsunami.
The Papatea Fault, unmapped before the quake and running along a similar path to the lower Clarence River in Marlborough, produced a 19km-long surface rupture and shunted a large area of mountainous country up by 8m in a matter of seconds.
“We suggest that fault-linkages between the offshore thrust faults and the Papatea fault help to explain the unusually high slip on the Papatea fault, by providing it with more area to accommodate this slip,” Chamberlain said.
The study team, including scientists from GNS Science, Victoria University, the Massachusetts Institute of
Technology (MIT) in the US and the Swiss Seismological Service, also added a further fault to the tally of more than 20 already identified.
That was a previously unknown one that linked the Papatea Fault to the Clarence Fault, which was another major component of the wider Marlborough Fault System.
“All of these structures play an important role in explaining the strongly variable deformation and slip around this fault junction,” Chamberlain said.
“Our earthquake catalogue also shows that the Kaiko¯ura aftershock sequence reactivated the faults that ruptured in the 2013 Cook Strait and Lake Grassmere earthquakes.
“This suggests that the rupture could have continued past those faults, but they likely slowed it down as they had recently slipped.”
But when they analysed related activity along the underlying Hikurangi subduction interface, they found almost no aftershocks, despite slipping after the earthquake, he said.
“This suggests that the interface here may be less likely to be a direct source of earthquake shaking.”
This is now the subject of ongoing research.
Interestingly, they found few aftershocks on the Papatea and Kekerengu faults.
“We infer that these faults released close to all of the stress accumulated on them, resulting in very little or no slip happening on them.”
Ultimately, Chamberlain said the study highlighted the scrambled nature of faulting in the top half of the South Island.
“By learning how faults in the Kaiko¯ura earthquake linked together, we will develop a better understanding of the range of possible earthquakes here.”