'Unusual' geothermal activity at Lake Rotorua - GNS

Lake Rotorua on a quieter day (Getty)
Lake Rotorua on a quieter day (Getty)

The central North Island is known for its hydrothermal activity, but GNS Science is calling the latest movements on Lake Rotorua "unusual".

Deep rumbling and geysers of up to 30 metres saw some Ohinemutu locals head for the hills on Monday morning, believing the lake was about to blow.

The activity followed slow slip earthquakes in Kapiti and Manawatu, and a 4.5 magnitude quake of Paraparaumu on Sunday.

GNS volcanologist Brad Scott says the activity is unexpected as there hasn't been anything for 15 years, but it's hard to say if it's related to the quakes.

"Hydrothermal eruptions are pretty significant. They're quite violent, they're quite hot," he says.

"This one was fortuitously was offshore - it wasn't in the built-up area. They can be damaging and they can be dangerous."

He told Paul Henry the slow slip earthquakes have been going on for thousands of years.

"The slow-slip events happen every two or three years up the East Coast and we don't often see those always triggering things off - but it's not the sort of thing we can discount either."

The Rotorua Lakes Council said the geyser was "spectacular but not a concern". Mr Scott says it's almost impossible to say whether the activity is a precursor for a bigger explosion.

"Sometimes there's subtle behaviours at hot springs... but these things just sort of happen."

"In the 1960s, 70s and 80s, we used to have several hydrothermal eruptions a year occur. Since the bore closure the number of eruptions has declined, and we haven't had any since 2001."

Ohinemutu resident Tawhanga Nopera was woken by a loud rumbling sound on around 4am on Monday.

"I heard my sister get up, then she came in and said: 'There's a big geyser outside, we need to get out of the house.'"

The rumbling continued in patches for up to 15 minutes, he said.

"It was weird. It was kinda scary not knowing what it was. It was like a really deep, earthly rumble. My cousin across the road thought someone was playing really loud subwoofers."

The Papakura Geyser in Rotorua's Te Whakarewarewa Valley bubbled up for the first time in 35 years in 2013.

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