Ruapehu - IGNS - Science Alert Bulletin V96/03

April 22 (Monday)  1000 NZST (UT +12)

Ruapehu Volcano

Situation Summary

At about 1310hr on Sunday April 21 personal working at Tukino
Skifield became aware of a large flood in the Whangaehu River.
They were initially alerted by the noise which was described as a
roar, and was later described as a flood of mud and boulders in
the river valley. At that time the Department of Conservation was
notified, which in turn informed GNS.

Examination of the seismic records at Wairakei Research Centre
confirmed a seismic event starting at 1257hr, lasting for several
minutes. Although there was considerable energy release recorded
at the Dome there was very little at Chateau and Ngauruhoe. The
dominant frequency of the records were also higher than the
typical eruption events at Crater Lake. This suggested a very
shallow source, possibly a landslide.  A GNS field party got away
about 1400hr with the intention of visiting the Whangaehu valley.
Upon arrival at Turangi to met with DOC it was decided to fly as
the weather was rapidly improving.  An observation flight was
made between 1600 and 1700hr.

Observations on the flight confirmed a portion of the north east
rim of the lake basin had collapsed into the Whangaehu Glacier
and flowed down into the Whangaehu River valley.  The collapse
had also deposited debris into the lake basin and lake.  There
was also evidence of head wall erosion back up the Whangaehu
Glacier towards the Dome.  The water level of the lake appeared
to be higher and the lava dome is no longer visible.  The failed
area is in a region thought to have been weakened during the 1995
eruptions and the resultant debris will most likely consist of a
mixture of 1995 eruption and weakened crater wall material.  This
type of failure has been a possibility in this area since
September 1995 and further failures could be expected, especially
during periods of high rainfall.

We advise that the Alert Level remains at Level 2

BJ Scott for
CJN Wilson  (Volcanology Programme Leader)