GeoNet recorded close to 11,000 earthquakes around New Zealand over the first six months of 2023. Image / GeoNet
An early-morning tremor in Te Aroha, a March rumble in Kawerau, a 5.9 jolt in Pōrangahau and a 6.0 Kāpiti quake that struck amid a devastating cyclone.
Has New Zealand been shakier than usual this year?
Well, slightly.
A new GeoNet analysis has found the 10,957 recorded over 2023′s first six months is relatively close to average, given our seismometers normally register around 20,000 in a year.
Amid those numbers, however, seismologists have had plenty of interesting localised patterns to pick over – including the potential influence of a deep-seated, slow-motion earthquake that’s still rolling on right now.
Zoom out to the big picture, and we can see that this year’s 11,000 quakes appear grouped in a relatively straight line running down the east of the North Island and the west of the South Island - much like someone has splattered a dash of paint across a canvas.
That’s no coincidence: this line marks a continual scrum going on between the Australian and Pacific tectonic plates, each forming part of the wider Pacific Ring of Fire.
How these plates collide beneath New Zealand varies in different places.
At the southern end of the South Island, the Australian plate dives down, or subducts, below the Pacific plate, while in the North Island, the opposite occurs.
In between, through most of the South Island, the two plates grind past each other along the Alpine Fault that runs along the mountainous spine of the island.
As the two plates push together at a steady rate, the rocks along the boundary become more and more stressed until eventually, something has to give – forcing a quake along a fault somewhere in the plate boundary zone.
Scientists often compare this to a bending stick: as it becomes more deformed, it breaks and each of the pieces spring back in a relatively straight but new position with regard to each other.
This motion ultimately creates tens of thousands of quakes each year, of which around 100 to 150 are big enough to be felt by us at the surface.
Often, they can strike in places we don’t expect them.
Just a few days into 2023, at 5.39am, residents of Te Aroha were shaken awake by a 5.1 quake that sent shelves of crockery flying in local second-hand shops and opened up cracks in a nearby paddock.
That event - which was preceded by a smaller 3.9 quake and followed by a strong 4.8 quake a month later, amid more than 100 other aftershocks – proved the biggest since 1972 in what’s typically a seismically quiet town.
Yet, as is the case in much of New Zealand, the potential for earthquakes lay ever-present in the nearby Kerepehi Fault: a key feature of the Hauraki rift zone.
Later, on February 15, the country’s biggest quake of 2023 coincided with its biggest weather disaster, ex-tropical Cyclone Gabrielle.
That was a mid-evening 6.0 jolt recorded offshore of Paraparaumu, which likely occurred within the subducting Pacific plate, at a depth of around 55 kilometres.
“The energy from such events can travel quite strongly and widely to a broad region, and we did record a high number of felt reports,” GNS Science seismic duty officer Jen Andrews said.
A few weeks afterwards on March 18, a 4.9 quake near Kawerau kicked off a swarm of some 1,200 local events – 60 of them measuring above magnitude 3.
“These events occurred in a region known for seismic swarm activity, although this had a larger number of felt events compared to many other swarms in the area over previous years.”
Like the Te Aroha quake, scientists considered the episode to have been linked to tectonic processes, rather than local geothermal activity.
It was also unlikely to have been linked to what’s been another major source of recent quakes further south in the Taupō Volcanic Zone.
That was unrest within the enormous, lake-filled Taupō caldera volcano, where scientists recorded nearly 2000 quakes over a year-long episode.
Around 300 of those quakes were detected this year – the biggest being a 4.4 shake on March 5 – and likely stemmed back to magma jostling for space within the hidden system.
While GeoNet recently nudged Taupō's Volcanic Alert Level back to zero – essentially indicating typical background levels – the agency said quakes could and would still occur around the super-volcano.
“Activity seems to be continuing to taper at present,” Andrews said.
Elsewhere, another curious earthquake sequence began on April 26, with a 5.9 event that struck near the sleepy coastal town of Pōrangahau.
It was followed by around 700 further quakes – 42 of them above magnitude 3 - which occurred mainly in two clusters in late April and early June.
Pōrangahau hasn’t been a stranger to such swarms of moderate to large quakes. One in 2011 brought more than 30 within a few months, while another weeks-long episode played out off its coast in 2021.
Scientists were now investigating whether the recent spate was linked to an ongoing “slow slip earthquake” beneath Kāpiti and Manawatū.
Discovered only in the past two decades, these slow-burning quakes could produce up to tens of centimetres of displacements along faults.
But because they happened too slowly to be picked up by seismometers – or to be felt by humans – they had to be observed using sensitive GPS equipment measuring the slow movement of land.
In New Zealand, they tended to play out within areas where the subduction zone was transitioning from being “stuck” beneath the southern North Island, to an area where the subduction zone was “creeping” further north, around Gisborne and Hawke’s Bay.
In November 2016, a magnitude 5 earthquake that struck offshore from Pōrangahau was likely caused by a slow slip event that itself had been set off by the Kaikōura quake that month.
“It is very difficult to specifically tie earthquake sequences to slow-slip events because the system is complex and there is so much seismic activity,” Andrews said.
“However, scientists are exploring whether the ongoing slow slip event in Kāpiti and Manawatū could play a role in sequences like Pōrangahau.”
So, how did this year’s activity compare with some of our biggest for earthquakes?
“The mainshock-aftershock sequences of large events can change our local earthquake activity rates significantly,” Andrews said.
“For example, the last six months of 2016, including the Kaikōura sequence, had 22,275 earthquakes.
“The Canterbury sequence contributes to the 14,032 earthquakes recorded in the first six months of 2011.”
At the same time, however, recorded quake numbers had grown over time simply because of technology.
“As the monitoring network has expanded over time, we can detect more, and smaller, earthquakes, increasing the total numbers recorded,” she said.
“This sensitivity of our network also affects the activity recorded during big sequences, alongside factors like the maximum magnitude and tectonic setting.”
Putting all the recent activity into context and interpreting it was still a work in progress, she said, especially for complex regions such as the Taupō Volcanic Zone, or areas where earthquakes were infrequent, like Te Aroha.
“While no unknown faults have been or yet to be identified from data this year, this data will add to the rich seismic dataset that underpins lots of valuable ongoing research such as ground motion studies, hazard models [and] volcanic and geothermal systems.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
Armed police stood guard as the patched members were led out in handcuffs.