Ruapehu Alert Downgrade

GNS Science downgraded the alert status for Mt. Ruapehu yesterday, following the absence of eruptive activity since September 25th.

In a bulletin issued at 10:00 a.m. on Tuesday 9th October 2007, GNS Science Duty Volcanologist Brad Scott announced that the status of Mt Ruapehu had been reduced from Alert Level 2 (minor eruptive activity) to Alert Level 1 (some signs of unrest).

The alert at the volcano was raised to 2 on the evening of September 25th, when a hydrothermal eruption ejected about 500,000 cubic metres of water, lake floor sediment and blocks up to a metre across from the crater lake. The eruption caused two small lahar flows, one into the top of the Whakapapa skifield, the other down the Whangaehu Valley. The eruption was similar to events in 1969, 1975, 1988 and 2006.

No further eruptive activity has occurred at the volcano since September 25th, but other activity has been recorded. The lake temperature, which had cooled to unusually low levels, has again started rising from 13 ºC and is now at 19 ºC, which is still cool for Ruapehu’s lake.

A volcanic earthquake at 11:05 p.m. on September 29th was shorter than that recorded during the eruption, lasting only 3 minutes. Two further periods of weak volcanic tremor of 5-10 minutes’ duration followed later the same night.

Seismographs indicate that another two periods of minor volcanic tremor occurred at Mt. Ruapehu and Mt. Ngauruhoe a few days later. The first period of tremor commenced at 5:31 and ran until 6:12 a.m. on Friday 5th October 2007. The second period began at 6:19 a.m. and faded at Mt. Ngauruhoe at 6:30 a.m. and at Mt. Ruapehu at 6:35 a.m. Neither of these two events has been mentioned in the GNS Science bulletins.

The small increase in volcanic tremor is thought to be consistent with the volcano’s hydrothermal system responding to an eruption through it. The possibility of further activity remains while the lake’s system stabilises, but the absence of further eruptive activity was sufficient to allow a lowering of the alert status.

[Compiled from bulletins issued by GNS Science and data observed on the GeoNet website.]

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