The volcanic alert level has been lifted for Mt Tongariro following a series of earthquakes.
Small earthquakes were recorded beneath the central North Island mountain on July 13.
They died down but flared up again on July 18 and increased in frequency yesterday and today.
GNS volcanologist Brad Scott said the small earthquakes were all below magnitude 2.5, but they had increased monitoring due to the frequency.
"Our historic seismic data has shown that these small volcanic earthquakes are common at Tongariro, but usually only occur at an average rate of 2 per year. We have recorded more than 20 since July 13.''
Seismic activity indicating unrest was last detected at Tongariro during 2001.
"Our routine volcano monitoring also includes the chemistry of the lakes, springs and fumaroles on Tongariro.''
The most recent samples collected in May showed no anomalies, Mr Scott said.
GNS Science had upgraded the volcanic alert level from zero to one, meaning activity had risen from typical background surface activity.
Level two signals minor eruptive activity and level five indicates a large hazardous eruption in progress.
Mt Ruapehu and White Island each have an alert level of one.
The aviation colour code was also upgraded for Mt Tongariro from green to yellow, meaning the volcano is experiencing signs of elevated unrest above known background levels.
This month's quakes were clustered in a zone between Emerald Crater and the Te Māri craters at a depth of between two and seven kilometres.
Volcanologists will set up portable seismic recorders around the eipcentres of the quakes and sample hot springs, crater lakes and fumarole in the area.
Tongariro is made up of multiple volcanic cones constructed over a period of 275,000 years. There have been five reported eruptions from Te Māri craters and Red Crater between 1855 and 1896 and these have since been dormant.
- The Dominion Post
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