GNS Science has raised the alert level on the volcano Mount Tongariro from 0 to 1, and changed the Aviation Colour Code to Yellow due of an ongoing series of earthquakes.

The sequence of quakes started on July 13, but soon declined, restarting on July 18 and increasing in number on July 19 and 20.

A series of earthquakes under Mount Tongariro has resulted in the increase of the alert status.

'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,” says volcanologist Brad Scott.

'We have recorded more than 20 since July 13.”

These indicate unrest at Tongariro and are the reason why the Volcanic Alert Level is changed to Level 1 and the Aviation Colour Code from Green to Yellow.

As part of GNS Science's routine monitoring of seismic activity they have recorded several volcanic earthquakes at Mount Tongariro since July 13.

The earthquakes are small, less than magnitude 2.5 and have only been well recorded by a few of the seismometers in GNS Science's permanent network. The earthquakes cluster in a zone between Emerald Crater and the Te Māri craters at 2-7km depth.

'To better understand the significance of these earthquakes we are planning to deploy portable seismic recorders around the epicentres of the earthquakes, conduct sampling of selected hot springs, crater lakes and fumaroles in the area,” says Brad.

Seismic activity indicating unrest was last detected at Tongariro during 2001. Routine monitoring also includes the chemistry of the lakes, springs and fumaroles on Tongariro. These were last sampled in May 2012, and showed no anomalies.

Mount Tongariro is a volcanic complex that lies to the north of Ngauruhoe. It consists of numerous craters and vents.

Te Māri craters lie about two kilometres east of Ketetahi hot springs on the north side of Mount Tongariro. The Te Māri craters are the last craters to be active on Tongariro. Ash eruptions have been recorded from Tongariro from 1855 to 1897, as well as unconfirmed activity in 1926-27.