This map shows some 700 earthquakes recorded this year around Taupo volcano, which now sits at alert level one for the first time. Image / GeoNet
For the first time, GeoNet has raised the alert level of New Zealand's ever-quiet "supervolcano", Taupō: but scientists stress that doesn't mean an eruption is on its way. Science reporter Jamie Morton explains.
What's just happened? And should we be worried?
After months of earthquake activity, target="_blank">GeoNet nudged the official Volcanic Alert Level at Taupō from zero to one, which signals "minor" unrest.
In itself, that's no small thing.
It's the first time our volcano-watching agency has ever shifted the alert dial on what's regarded as the world's most frequently active supervolcano system.
But the type of episode we're seeing now isn't rare for Taupo - and none of some 17 in of these in the last 150 years have ended in eruption.
Indeed, the area under Lake Taupō has been seismically active since the turn of the millennium, with just over 5000 earthquakes located by GNS Science instruments.
The Volcanic Alert Level for Taupō Volcano has been raised to Volcanic Alert Level 1 (minor volcanic unrest). This minor unrest is causing the ongoing earthquakes & ground deformation at the volcano. Read our bulletin with all of the information here: https://t.co/T9yTR3aF32 pic.twitter.com/FD31y9iYqD
— GeoNet (@geonet) September 20, 2022
"What we're talking about is the lowest level of minor activity we can have at a volcano," GNS Science volcanologist Dr Nico Fournier said.
Nearby Ruapehu, for instance, was locked at level two for much of the year, without major incident.
"And if there's no indication that this activity [at Taupō] is going to increase, or even continue, to be fair," Fournier said.
"There are a lot of steps we'd need to go through before we'd even be considering a likely eruption: and these would be the very, very early stages."
What actually is Taupō volcano?
We can't see it from the surface, but this giant, hidden caldera system at the centre of the North Island is known to have caused some of the most enormous eruptions in our planet's history.
The lake above it essentially fills the hole left by one of those monster 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.
Taupō's most recent major episode, some 1800 years ago, fired out more than 120 cubic km of pumice and ash and obliterated the surrounding landscape.
In any given year, the chance of something like that occurring was extremely slim.
One 2020 modelling study put the annual probability of a Taupo eruption at any size at a very low chance of one in 800 – or at between 0.5 and 1.3 per cent - within the next 500 years.
"We're unlikely to see an eruption in our lifetimes," the study's leader, Massey University geostatistician Mark Bebbington, told the Herald at the time.
What many scientists call "super-eruptions" - four of the 10 ever recorded in the past 2.8 million years have occurred in the Central North Island - were rarer still.
One of New Zealand's most prominent volcanologists, Victoria University's Professor Colin Wilson, earlier told the Herald our resident "supervolcanoes" like Taupō and Okataina might see periods of unrest every decade - but eruptions perhaps every 500 to 1000 years.
"We are very confident that these volcanoes are still alive," said Wilson, who's been leading a multi-million dollar research programme focused on them over recent years.
What's behind the latest unrest?
A network of local instruments has located nearly 700 quakes - mainly at a depth of 4km to 13km below the lake - over a sequence that began in May.
"The earthquake locations in this year's sequence are forming two clusters in the central part of the lake," GeoNet duty volcanologist Steven Sherburn explained.
One was centred around the central and eastern part of the lake, while a smaller one had been identified to the west, just offshore from Karangahape.
Alongside that, GNSS (GPS) instruments have revealed uplift – at a rate equivalent to 20mm to 60mm per year – near Horomatangi Reef.
In 2019, an analysis of more than 7000 recorded quakes helped them pinpoint a hidden magma chamber near the reef that was at least 20 per cent molten, and roughly 250 cubic km in volume.
"Modelling of the GNSS data indicates that the area that is rising beneath the lake is in the same place as the main region of earthquake activity," Sherburn said.
"This is also where we interpret the existing magmatic system to be."
All of this activity appeared to be caused by movement of magma and the hydrothermal fluids within the volcano.
"We have also sampled springs and gas vents around the lake for changes in chemistry that may be related to the earthquake and ground uplift."
In July, scientists revealed how, in the last 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 around 16cm.
Sherburn pointed out that unrest – typically caused by magma or steam forcing its way through the ground beneath a volcano – wasn't uncommon at other calderas around the world.
"Volcanic unrest at volcanoes like Taupō could continue for months or years and not result in an eruption," he said.
"If there was increased unrest, then we would see more substantial impacts on the local area."
That might include larger quakes, more ground deformation, or heightened geothermal activity beneath or around the lake area.
Haven't we just seen a period of unrest at Taupo?
Interestingly, this year still hasn't proven as shaky as 2019 for Taupō.
Over that year, GeoNet reported more than 1100 earthquakes, particularly beneath the area of the volcano covered by the lake - far more than the yearly average of several hundred earthquakes.
While most were small, one recorded on September 4, 2019, and centred in the middle of the lake near Motutaiko Island, registered a magnitude of 5.0.
That was enough to cause shaking from Turangi to Taupō, knocking items off shelves and causing highway slips, and was swiftly followed by another quake, measuring 4.5.
This year, quakes have been recorded at a rate of about 30 to 40 a week – one of the largest being a 4.2 quake on September 9 that more than 1200 people reported feeling.
The next two largest earthquakes – on May 17 and July 11 - were magnitude 3.6.
In more than 20 years, only seven quakes larger than magnitude 4 have been recorded beneath the lake – and the one that struck this month happened just 8km northeast of the 5.0 quake nearly three years earlier.
Before the 2019 quake, the last comparable one was more than a decade earlier, in 2008.
Fournier said it wasn't clear why bouts tended to play out in 10-year intervals – or why this latest one appeared to have broken the trend.
"It's certainly one of the things that we are taking into consideration as we are changing the alert level to one," he said.
"We don't think that it's necessarily worrying, but it's something that we have to acknowledge as it seems to be a little bit unusual, in terms of the timing."
Is there something wider going on?
Scientists often point out that activity at any two volcanoes experiencing unrest can occur independently – think two houses connected to the same sewer lines, but coincidentally having issues with the plumbing in their kitchen sink.
That's why we shouldn't jump to link this activity to recent eruptions, such as Whakaari/White Island's in 2019.
Still, our volcanism – and the 15,000 earthquakes large enough to be recorded each year – owes to the fact our country straddles two tectonic plates locked in a perpetual geological scrum.
In the past, slow-burning quakes along the plate boundary – called "slow slip events" - have coincided with volcanic activity.
One recently funded study, led by Victoria University volcano seismologist Dr Finn Illsley-Kemp, is looking at this relationship much more closely.
"For example, the unrest at Taupō in 2019 occurred at the same time as a large slow-slip event off the East Coast and we can see from GPS measurements that the ground around Taupō moved in response to that slow-slip event," he earlier told the Herald.
Following the rise in Volcanic Alert Level at Taupō Volcano today, we have a video update from @gnsscience Volcanology Team Leader Nico Fournier who explains this change and the current volcanic activity at Taupō: https://t.co/glSdAN1pM8
— GeoNet (@geonet) September 20, 2022
"So, we really want to investigate this possible connection at all volcanoes in New Zealand."
This time, the connection wasn't so clear, Fournier said. No slow-slip events have been observed since last year.
"At the moment, there is nothing that points to any large processes that could be driving this from afar," he said.
"But it's certainly one of the things that we've been wondering about for some of the previous events."
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