Scientists monitoring Mount Ruapehu say the constant tremor beneath the mountain has strengthened over the past fortnight to a level not recorded in at least two years, though almost every other sign they watch has stayed flat and the volcano remains on the lowest rung of unrest.
GeoNet published a volcanic activity bulletin on Monday afternoon confirming the change. Volcanic tremor has “gradually increased from weak to moderate levels” since 22 July, duty volcanologist Michael Rosenberg wrote, after close to two years of minimal activity. The Volcanic Alert Level stays at 1 and the aviation colour code remains green.
Rosenberg told RNZ the shift was worth watching without being cause for alarm. “It doesn’t, by itself, mean that the volcano is going to erupt,” he said. “It’s more like a possible early warning signal.”
Volcanic tremor is a continuous shaking of the ground, quite unlike the sharp jolt of an earthquake. It is produced by fluids underground, gas, hot water and in some cases magma, forcing a path through cracks in the rock. Because it tracks what fluids are doing rather than what rock is breaking, it is one of the more useful early indicators available to volcanologists. It is also one of the harder ones to read, because plenty of tremor episodes come and go without anything else happening.
What makes this episode notable is how little else has moved. The temperature of Crater Lake, known by its Māori name Te Wai ā-moe, has “remained stable for the past four months” in a band between 18 and 23 degrees, with the variation inside that band put down to weather and the daily heating cycle. Output of volcanic sulphur dioxide and hydrogen sulphide remains low to moderate measured against the long-run trend. Persistent upwelling is visible on the surface, sulphur slicks trail across the grey water and a small overflow continues through the outlet channel, none of which is out of the ordinary for the lake.
Taken together, GeoNet said, the readings point to “a dynamic hydrothermal system and intermittent input of volcanic gas and heat” and sit “within the range expected during minor volcanic unrest”.
Rosenberg also ruled out a connection many people will have reached for. New Zealand has had a run of noticeable earthquakes over the past fortnight, including a magnitude 5.7 near Te Araroa on Sunday and a magnitude 4.6 near Porirua on Saturday. He said there was no link between that heightened seismic activity and the increase in tremor beneath Ruapehu.
Ruapehu has a long record of getting people’s attention and then quietly settling down again. In 2022 the mountain spent months at Volcanic Alert Level 2 after Crater Lake heated to 41 degrees and tremor ran at strong levels. Modelling at the time suggested roughly 230 megawatts of heat was needed to hold the lake that warm. No eruption followed, and the alert level eventually came back down. The same pattern has repeated several times since the 1990s.
The other side of the ledger is that Ruapehu can also move without much notice at all. On the evening of 25 September 2007 a gas-driven explosion lasted less than a minute, throwing rock and water across the summit and sending slurry lahars down two valleys, one of them into the Whakapapa ski area. Two climbers sheltering about 600 metres from Crater Lake were caught in the blast and one was severely injured. GeoNet’s own account of that night is blunt about the lesson, noting that this type of eruption occurs with little or no warning. The volcano was not at a heightened alert level beforehand.
That is the tension sitting behind Monday’s bulletin. Warning signals are genuinely useful, and the monitoring network on Ruapehu is among the densest on any New Zealand volcano, but the events most likely to hurt someone standing near the summit are the ones least likely to announce themselves. It is the reason the mountain carries hardware rather than relying on forecasts alone.
Two systems came out of earlier disasters. The Eastern Ruapehu Lahar Alarm and Warning System watches the Whangaehu Valley, the route of the mountain’s largest and most frequent lahars, and pages police, council staff, road and rail operators and duty scientists when sensor thresholds are crossed. A separate eruption detection system sounds sirens and a recorded voice message at exposed parts of the Whakapapa ski area within seconds of a qualifying volcanic earthquake, telling people to get out of the valley floors and onto higher ground.
Both exist because of what the Whangaehu has done before. On Christmas Eve 1953 a collapsing tephra dam at Crater Lake sent a lahar down the valley that took out the rail bridge at Tangiwai moments before the Wellington to Auckland express reached it. One hundred and fifty-one people died, in what remains New Zealand’s worst rail disaster. When a comparable dam break happened on 18 March 2007, sending an estimated 1.29 million cubic metres down the same valley, the alarm system worked, roads and rail were closed in time and nobody was hurt.
For now nothing about access to the mountain has changed. Monday’s bulletin announced no new restrictions and did not shift the alert level, which has sat at 1 through the current ski season. Level 1 describes minor unrest, the state Ruapehu spends much of its time in, and the summit hazard zone advice that applies in normal conditions still stands.
GeoNet said monitoring would continue and the situation would be reassessed if anything changed. Alert levels move in both directions, and on this mountain they have moved back down far more often than they have moved up. The honest position, for the moment, is that something beneath Ruapehu has picked up and nobody can yet say whether it means anything.
Have you been on the mountain this season, or do you remember 1995 or 2007? Tell us what you make of the latest update in the comments below.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂
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