Sep 27 1995, 10:00 am - Ruapehu Volcano

Situation Summary

As of 09:30 the following new information is available:

• Seismic activity, including volcanic tremor, continued at a low level until about 23:00, then there was an increase in tremor to moderate levels until 02:00, a decrease from 02:00 to 04:00, then an increase back to moderate levels which has been sustained to the time of writing. Individual volcanic earthquakes have resumed as well; one small example around 01:10, then many associated with the up-tempo of tremor levels from 04:00 onwards. All our seismographs (including, remarkably, that at the Dome Shelter) continue to operate and are providing valuable data.

• After the quieter period yesterday, visual observations this morning from 06:00 this morning confirmed a new period of moderately vigorous activity was occurring at the same time as the elevated seismic levels. Our observation flight has confirmed that the lake still exists but is greatly reduced in size. No water was present in the upper Whangaehu valley at this time, suggesting that the floods related to the activity of 06:00 to 07:00 will have peaked. Our observers have recorded a further small lahar deposit in the northern Whakapapaiti channel. As of 9:30 am, the size of eruptions was greatly reduced relative to Monday and earlier this morning. Indications are that the plume had risen to about 8 - 10 km during more vigorous activity, but the timing of individual explosions was at longer intervals (typically 10 - 45 minutes) than on Monday.

• Institute scientists (with the valued assistance of staff from Ruapehu Alpine Lifts) have cleared the snow and taken measurements from a survey areas on the north slopes of the mountain above Whakapapa skifield and 2.5 km from the crater. Preliminary results indicate that there has been no tilt of this area since May 1994. We use measurements like this to monitor whether intrusion of magma has occurred; from our previous work any intrusion is known to be accompanied by some upwards bulging and tilting of the ground surface.

• Seismologists from the Institute's Wellington office have joined us and have installed an additional 8 seismometers in an arc around the west, south and east sides of the mountain about 20 km from the crater. This has two aims. One, to provide us with back-up seismic information in case our existing equipment breaks down. Second, to provide more accurate estimates of places below the volcano where the volcanic tremor and earthquakes are centred, so that we can follow any changes in position and thus detect movement of the magma.

Conclusion and Alert Status

The eruption involving new magma is continuing at Ruapehu. In the light of our observations and the seismic data available to us, we advise that the Alert Level for Ruapehu remains at Level 4.

C.J.N. Wilson
Volcanology Programme Leader