Taupō volcanic unrest subsides, alert level returns to zero

Taupō has retuned to its normal tranquil self after a year of minor unrest. Photo / Merv Richdale

Taupō Volcano has settled enough for GNS Science to reduce the alert level to zero.

Monitoring has shown ground deformation and earthquake activity has returned to normal, ending a period of unrest stretching back to May last year.

The Volcanic Alert Level was raised to 1 in September in response to the minor unrest.

On November 30, a magnitude 5.7 earthquake happened 9km beneath the lake.

Duty volcanologist Agnes Mazot said the earthquake was widely felt and was accompanied by significant ground movement at Horomatangi Reef.

“We measured 180mm of uplift and a shift to the southeast of 160mm. This movement of the lakebed created a small tsunami in the lake while the ground shaking set off a submarine landslide near Wharewaka Pt. This created a second local tsunami that flooded the foreshore area.”

More than 820 aftershocks were recorded by February and then another moderate earthquake occurred on March 5.

Mazot said a combination of magma movement at depth, faults slipping and hydrothermal fluids had likely been driving the earthquakes and ground deformation.

“We have seen several processes at play during the volcanic unrest over the last year at Taupō Volcano. In the initial phase starting in early May, we saw an increase in the number of earthquakes per month and by June there was an indication of uplift at Horomatangi Reef.

“By September both trends were well established with the monthly number of earthquakes ranging between 20 and 90.”

Instruments around the lake recorded outward movement around the lake and Horomatangi Reef and showed uplift at a rate of 60mm per year, plus or minus a 20mm margin of error.

“These were within our expectations for Taupō volcano at Volcanic Alert Level 1.”

Between September and November earthquake activity slowed, until the magnitude 5.7 shake on November 30.

In total, over the past year, there have been just over 1800 earthquakes under Lake Taupō.

Although it was the first time the Volcanic Alert Level for Taupō has been raised, it was just the most recent of 18 historical unrest episodes in the past 150 years.

“Several of these were more severe than what we have observed over the last year. None of those episodes, or the many other episodes which would have occurred over the past 1800 years before written records were kept, ended in an eruption,” Mazot said.

The last eruption at Taupō volcano was in 232AD, plus or minus 10 years and the chance of an eruption remains very low in any one year, she said.

Volcanic Alert Level 0 is mostly associated with environmental hazards.

“However, earthquake activity can and will still occur at background levels and at areas near to Taupō, such as around the geothermal systems and in the south and western ends of the lake.

“We continue to actively monitor the volcano and, when we have more information, will share this.”

The Volcanic Alert Level reflected the current level of volcanic unrest or activity and was not a forecast of future activity.