This map shows hundreds of earthquakes located by GeoNet in the Lake Taupo area from January 1 to September 25, 2022. Many more followed a 5.7 quake in November. Image / GeoNet
It rumbled on for a year, raising the ground beneath Lake Taupō and triggering thousands of local earthquakes.
Now that our famous super-volcano’s latest episode is over, scientists have some intriguing theories about what started it – yet they’re still left with more questions than answers.
GeoNet’s down-grading of Taupō's alert level to zero last week marked the end of one of its most significant unrest periods in the 150 years scientists have been observing it.
We can’t see it from the surface, but the giant, hidden caldera system at the centre of the North Island happens to be the world’s most frequently active “super-volcano”.
The lake above it essentially fills the hole left by one of its biggest blows: the Oruanui eruption around 25,400 years ago, which sent more than 1100 cubic kilometres of pumice and ash into the atmosphere, and as far as Antarctica.
Events of that scale are thought incredibly rare (Taupō's last sizeable eruption occurred in 232AD), but unrest episodes occur relatively often; 18 were recorded in the last century and a half.
Scientists say the latest was a stand-out for several reasons.
One was that it churned on for four months longer than the average bout.
Another was that a sophisticated array of instruments – from seismometers and GPS receivers, to regular chemistry samples and lake-levelling measurements – was in place to capture the unrest in unprecedented detail.
And then there were some dramatic moments that were felt at the surface: namely a magnitude 5.7 earthquake on November 30, followed by more than 800 aftershocks over the next few months.
“This earthquake was the largest to have occurred at Taupō for at least 50 years, and we have really strong evidence that it was directly related to the magmatic system,” Victoria University seismologist Dr Finnigan Illsley-Kemp said.
The quake was also powerful enough to trigger two tsunamis: one thought to have been directly caused by the lakebed being uplifted, and the other generated by a submarine landslide.
At Four Mile Bay at the southern end of Taupō township, the water had surged about 20m to 30m up the beach, destroying two boats, ripping wooden bollards from a park area nearby and eroding about 2m of soil away from the foreshore.
Illsley-Kemp said neither that ground deformation – nor an associated tsunami – had ever been recorded at Taupō before.
“Ongoing study of this earthquake will allow us to better understand the shallow processes that occur in the volcano,” he said.
“It also improves our understanding of the tsunami hazard in the lake, which is something that was probably underappreciated.”
As for the causes of the wider unrest, we could so far only point to some likely drivers.
The main catalyst was suspected to be a fresh batch of magma entering the system from below.
“This has to be quite a regular occurrence at Taupō, because the input of fresh magma and heat is the only way the volcano can stay ‘alive’,” Illsley-Kemp said.
“But there are also questions about how the volcano interacts with the wider tectonic setting of New Zealand.”
Before the episode even began, he and colleagues had begun exploring possible links with mysterious “slow-slip events” - silent, unfelt earthquakes that are known to displace faults over months or years.
As the episode progressed on through 2022, the activity was thought to have been fuelled by a combination of local faults slipping, magma jostling for space deep within the system, and hydrothermal fluids working their way through it.
“It’s always difficult to distinguish between these processes, particularly at Taupō, where we don’t have the ability to monitor for changes in gas or fluid chemistry,” he said.
“However, for the November 30 earthquake, we can say that it didn’t just involve simple slip along a fault like most earthquakes.
“There was an increase in volume, or inflation, that was a significant component of that earthquake, and this must have been a fluid of some sort - most likely magma.”
GNS Science volcanologist Brad Scott said studies of rock ejected in past Taupō eruptions had shown how molten material lay kilometres beneath the surface, in a mush-like form.
As such, it wasn’t capable of erupting without extra heat and gas driving it to shallower depths.
In unrest periods, Scott said the “mush zone” was active, and pushing on the material above.
Volcanic activity at Taupō Volcano is now back to background levels. The Volcanic Alert Level is lowered to Level 0. Read the full bulletin here: https://t.co/31p3ioNARP pic.twitter.com/jK0Upg3dZO
— GeoNet (@geonet) May 29, 2023
Geophysical studies had also shown there were faults and caldera boundary structures beneath the lake, which enabled hydrothermal fluids to move about.
“Pressure from below also induces some of these structures or faults to move, thus creating earthquakes,” Scott said.
“We also know there is a large geothermal system under the lake that discharges hot fluids into [it].”
The unrest – and the November 30 quake, particularly – had already enabled scientists to confirm some long-standing hypotheses about Taupō.
It had been assumed, for instance, that unrest could be directly linked to earthquakes, and that these quakes could potentially create tsunamis.
“We’re also able to see the ground deformation is driven from two sources: a deep one and a shallow one,” Scott said.
A wealth of earthquake data gathered from seismometers, deployed as part of the major Eclipse project, also appeared to outline a magma system beneath the lake’s Horomātangi Reefs.
That added more evidence to the existence of a hidden chamber near the reefs, which an earlier study estimated to be at least 20 per cent molten, and roughly 250 cubic km in volume.
In 2021, scientists also revealed how, in the past 42 years, areas north of Lake Taupō have sunk by about 14cm over that time, while the lakebed near Horomatangi Reefs has been uplifted by about 16cm.
Illsley-Kemp said there was still much more to be learned, and a large research effort between GNS, Victoria University and other institutes was now underway.
One focus was on comparing the episode with another that unfolded in 2019, when GeoNet logged more than 1100 local quakes.
“Why did this unrest event happen so soon after the last unrest event in 2019? Normally these are spaced a decade apart,” he said.
“Is this telling us something about the state of the volcano? We don’t know.”
The very biggest mysteries, meanwhile, waited to be solved.
“What would the unrest look like if it was moving towards an eruption? Would it look significantly different from this last year?” Illsley-Kemp said.
“We might expect an even higher rate of earthquakes and ground deformation, but we’re not sure.
“Is Taupō in an eruptible state at the moment? In order for Taupō to erupt, it needs to have a body of highly molten magma available.
“We don’t know if this exists in the present day, or if it is in an un-eruptible, mushy state.”
Based on what’s been observed over the past century and a half, he figured we could at least expect another unrest period like this within the next decade.
“However, this could happen at any time, and we don’t understand the volcano well enough yet to say when,” he said.
“This unrest event has reiterated that the volcano poses an earthquake hazard to people in the region.
“Communities should be prepared for this and remember to drop, cover, and hold during strong ground shaking, and if they are near the lake front, move to higher ground.”
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