Mt Ruapehu earthquake swarm detected, not unusual says volcanologist

A swarm of earthquakes has been detected under Mt Ruapehu.

GNS Science duty volcanologist Yannik Behr said an earthquake swarm was detected beneath Mt Ruapehu’s north-eastern flank early Tuesday morning.

“The sequence is ongoing with earthquakes occurring roughly every six to seven minutes,” Behr said.

“The earthquakes are very small, less than magnitude 1, at 15-30 km depth.”

Behr said by Tuesday afternoon there had been about 90 quakes detected and they were ongoing this morning.

Small earthquakes are occurring every six to seven minutes under Mt Ruapehu, seen here from Ohakune.

When asked if the swarm could precede a volcanic eruption, Behr said swarms were not unusual in the central North Island where volcanic fluids interacted with pre-existing faults.

“The current sequence of earthquakes is possibly related to fluid movement at depth,” he said.

 “Apart from earthquake detections, we also measure the amount of gas and heat coming into and through Te Wai ā-moe/Ruapehu Crater Lake.

“Both remain, over the past few days, within the range of values we have measured in the last month.”

Behr said volcanic tremor - an important indicator of fluid movement closer to the surface - remained low.

The Dome Shelter near Mt Ruapehu's crater lake was almost submerged in a 2007 phreatic eruption that cost Auckland man William Pike his right leg. Photo / Alan Gibson

The mountain is currently at Volcanic Alert Level 1 which indicated minor volcanic unrest.

Behr said Mt Ruapehu was an active volcano and has the potential to erupt with little or no warning when in a state of minor volcanic unrest.

The last time it erupted was on September 25, 2007, which caused a seven-minute-long earthquake, two lahars and flying rocks - one that crushed the leg of primary school teacher William Pike when it landed on Dome Shelter near the crater.

The volcano also went through a period of noticeable activity in 1995 and 1996, most notably on June 17, 1996 when a large eruption produced giant clouds of smoke and ash.

Eruptions continued for weeks and damaged crops and killed livestock. The resulting ash cloud disrupted flights and caused delays across the country.