International scientists have claimed the 7.8 Kaikoura earthquake has raised the potential threat of another large event facing New Zealand in the future.
But a top GNS Science natural hazards expert says there’s no indication that the November 14 quake would prove a trigger for “the big one”.
In an article this month in Science Bulletin, scientists from the US, China and Singapore wrote the rupture created by the Kaikoura quake – which set off at least 10 faults after first striking at an epicentre near Culverden – was “far too complex to have even predicted, even if we knew all of the previously unrecognised faults in the system”.
They suggested that scientists may now have to reconsider the way earthquakes were modelled in complex systems.
Further, they said stress changes caused by the quake warned that faults in the southern part of the North Island, near Wellington, may be “closer to failure than the past”.
The fact that the Kaikoura earthquake occurred along the comparably smaller Marlborough Fault System, instead of the much larger Alpine Fault on the west of the South Island, could also signal “the coming of a great earthquake along this plate boundary fault”, they wrote.
On average, earthquakes of magnitude 7.5 or larger strike along the Alpine Fault every 300 years – a relatively frequent rate in geological terms – and scientists say it’s highly likely the next big quake will happen in the lifetime of today’s population.
It last ruptured 299 years ago – producing a massive earthquake of about magnitude 8.0 – and has an average 30 per cent probability of rupturing in the next 50 years.
But GNS Science principal scientist Kelvin Berryman said investigations had so far indicated the Kaikoura quake was unlikely to have loaded pressure on the Alpine Fault, and instead had applied pressure north-eastward toward the Cook Strait.
The article’s authors also suggested the increase in the number of slow-slip events along the Hikurangi plate boundary underneath the North Island, triggered by the Kaikoura earthquake, had “encouraged re-evaluation of the potential of a great megathrust earthquake in this region in the near future”.
Berryman said while there had been some loading on the subduction interface, there had been many major quakes the size of the Kaikoura event in the 600 years since the last big subduction quake – and none had proven enough to be a trigger.
“Sooner or later, there might be a trigger – but there’s no reason to think this is the one.”
GNS Science staff remain investigating the mechanics of the Kaikoura quake, and a team of geologists were still conducting field research in Marlborough this month.
The article comes as the quake’s impact on the South Island’s coast has been revealed in 3D images just released by government agency Land Information New Zealand (LINZ).
LINZ researchers drew on LiDAR, a 3D mapping technology that gathers data with airborne lasers, to create the latest images of areas of the Kaikoura coast transformed by the November 14 quake.
“We worked with the Australian Defence Force to gather LiDAR data showing a first look at the impact on the sea floor,” LINZ chief geodesist Graeme Blick said.
“We’ll use this to plan where more sea floor mapping might be needed so we can update the charts and other information we make for skippers.”
Mariners in the area had already been warned about the potential for uncharted hazards in the area as a result of the earthquake.
“We also worked with the NZTA to gather LiDAR about the extent of landslides and how they affected the roads there,” Blick said.
“By combining both sets of data in these images, we can show just how the fault cuts across land and sea.”
Taranaki’s mysterious quakes probed
Meanwhile, new research from Victoria University suggests a mysterious cluster of deep earthquakes beneath Taranaki may provide a vital clue to understanding how New Zealand’s landmass was created.
The study, just published in the United States journal Geology, investigated the “unusual” earthquakes that occur beneath the line that runs between the volcanoes of Mount Taranaki and Mount Ruapehu.
“The earthquakes are unusual for several reasons,” lead author and Victoria researcher Dr Jesse Dimech said.
“They are up to 52 kilometres deep, compared to most earthquakes in regular continental regions which are restricted to the top 20 kilometres – and they also lie in the Earth’s mantle, beneath the crust, defining an east-west boundary, or structure, which is in contrast to the strong northeast-southwest grain for most geological structures in New Zealand.”
Professor Tim Stern, co-author of the paper and Dimech’s PhD supervisor, said while the earthquakes were all less than magnitude 5 and did not represent a significant hazard, they helped to explain the rapid rise and fall of the mountain ranges of the western North Island in the last five million years.
“Geologists have long puzzled over the fact that much of central and western North Island rose above sea-level in the last few million years, whereas south of Whanganui a once high-standing mountain range now lies underwater,” Stern said.
“The earthquakes are indicators of a geophysical process beneath Taranaki that is fundamental to how continental regions like New Zealand evolve and change through time.
“We know that the tectonic plates have a deeper layer of mantle rock which is actually denser than the overlying crust.”
The authors believed the deep earthquakes beneath Taranaki were being triggered as the dense mantle layer peels off, and sank into the hotter and less dense regions below.
“This process creates both uplift and subsidence at the earth’s surface, as well as causing a specific type of volcanism that has been linked to zones of rich mineralisation – for example, gold and copper – elsewhere in the world.”
The authors argue that the “peeling off” process in the mantle may have been responsible for the uplift of the western and central North Island up to five million years ago, from a few kilometres below sea level to average elevations of about 500m above sea-level today.