Product at Asian Sari-sari store is flung from the shelves as a 5.3 magnitude earthquake strikes Taupō.
Video / JhuVal Gabayan
It was one of the strangest natural events in New Zealand’s history: a midnight tsunami in Lake Taupō that destroyed boats and damaged the town’s foreshore.
Now, scientists can explain the surprising way in which it unfolded, amid a long bout of unrest within the supervolcano lying beneath the lake.
That 13-month event at Taupō, which ended last May, marked the first time that scientists had ever raised the hidden caldera system’s GeoNet alert level above zero – and it came with a record number of linked earthquakes.
Among those 1700-odd quakes was a magnitude 5.6 event that struck just before midnight on November 30, 2022 – jolting residents awake and sending a small surge of water towards the shore.
At Four Mile Bay, at the south end of the town, the lake tsunami made it about 20m to 30m up the beach, destroying two boats belonging to Taupō Pedal Boats and ripping wooden bollards from a park area.
Residents survey signs of a tsunami at Taupō's Four Mile Bay, the morning after the November 30 event. Photo / Dan Hutchinson
GNS Science volcano seismologist Dr Oliver Lamb said the quake, centred about 20km southwest of Taupō, was one of the largest detected beneath the lake for at least 50 years.
When he and colleagues investigated further, they found more curious factors that set it apart.
Their just-published study into earthquake activity surrounding the year-long period pin-pointed four distinct phases, each marked by varying numbers of quakes and magnitudes.
Lamb said the analysis essentially refined the GeoNet-mapped locations of all the quakes to get a more accurate picture of what was happening under the lake.
“We also calculated the orientation of the faults responsible for the largest earthquakes in the unrest episode.”
Most of the quakes were found to have happened in an area beneath the lake with multiple over-lapping caldera features, along with an active geothermal structure.
Those locations matched up tidily with where scientists suspect hidden faults lie – backing their belief that the faults were set off by new magma being squeezed into the volcano system from deep below.
The study also yielded some unexpected findings.
“The biggest surprise was how the largest earthquake actually ruptured,” Lamb said of the 5.6 event.
“Instead of a classic mechanism - where the two sides of the fault simply slide past each other - we found it was probably more like a ‘trap-door’ mechanism, where a significant section of ground under the lake moved upwards in one go,” he said.
“We think this might have triggered some of the tsunami waves recorded in the lake.”
Lamb said every unrest episode at Taupō offered new insights about the structure of the volcano, which last erupted in a 232AD event that obliterated the surrounding landscape.
The latest periods enabled scientists to confirm that unrest could be directly linked to quakes - and that these could create tsunamis.
A trove of quake data gathered from seismometers similarly revealed the existence of a magma chamber hidden beneath the lake’s Horomātangi Reefs, which one study estimated to be at least 20% molten, and roughly 250 cu km in volume.
“These events also allow us to assess how effective our current monitoring systems are and our tools for carefully analysing the activity,” Lamb said.
“We found that the current monitoring network was coping relatively well, but even so, we added two new permanent seismometer stations during the unrest to help bolster our monitoring efforts.
Source / GNS Science
“Altogether, this allows GNS Science and the GeoNet programme to be better prepared for the next unrest episode.”
The new findings, published in the scientific journal Seismica, come after scientists recently showed how 2016′s massive Kaikōura earthquake appeared to trigger a short-lived unrest period at Taupō.
That raised the possibility that earthquakes could indeed help trigger volcanic eruptions – a question long on the minds of geoscientists – and this may have happened in New Zealand’s past.
In another study last month, scientists published evidence to show the volcanic region Taupō sits in was being stoked with buried fluids released from within the Hikurangi Subduction Zone – our largest natural hazard.
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.
