Ruapehu - IGNS - Science Alert Bulletin V95/33

Saturday 7 October 1995
19.00 hours NZDT (UT +13)
Ruapehu Volcanic Activity

Situation Summary

As of 1900 hours the situation is as follows.

*    In the last 24 hours we have recorded 2 significant eruption
earthquakes, at the following times:
7 October 1114: ashfall and small lahar into the Whangaehu catchment
7 October 1503: ballistic blocks to 1km, ashfall and lahars into 
                Whangaehu catchment.

*    At about 10.38 hours, a very small eruption earthquake was recorded
on the Dome seismograph and observers on the mountain reported activity in
Crater Lake. Eruption slugs rose 50-100m above the lake, and a tall
eruption column rose much higher above the mountain.  The level of
volcanic tremor on the Dome and Chateau recorders showed marked but steady
increases after that event until the 15.03 event.

*    A  moderate eruption earthquake (M2.7) was recorded at 11.14 hours and
this event produced ashfall to the north east across the summit plateau
and a small lahar down the Whangaehu River catchment.

*    At 15.03 hours today there occurred the largest eruption earthquake
to be recorded since the recent eruption sequence of Mt Ruapehu
commenced.  Seismicity built up over a period of 4.5-5 minutes and then
remained at a "peak level" for  8 minutes.  Sustained tremor is continuing
at an elevated level which is just lower than on Monday Sept 25, but of
larger amplitude than at any other time during the September/October
eruption sequence.  The accompanying eruption produced a spectacular
eruption column that rose rapidly to over 25,000ft  (7.5km) and was
clearly visible under fine weather conditions.  The eruption expelled lava
blocks, water, lake-floor muds and volcanic ash from Crater Lake.  The
lava blocks were thrown over 1 km from the crater, while the water and
lake floor muds generated lahars and floods into the Whangaehu catchment.
No lahars were generated into the Mangaturuturu and Whakapapa catchments.
Ash fell from the plume to the north east of the volcano.

*    Weather conditions have been favourable today allowing two inspection
flights to the crater area today. Both  flights have revealed a Crater
Lake is still present within the summit crater, although the lake is
somewhat reduced in size with respect to previous inspections. The morning
flight indicated that the eruptions of 6 October  were accompanied by
ballistic blocks that had been thrown at least 400m from the lake. Small
lahars had also been produced during the 6 October eruptions into the
Whakapapaiti and Whangaehu catchments, and further ashfall had accumulated
on the eastern flanks of the volcano.  The afternoon flight showed that
the 15.03 hours eruption had little effect on either the size of Crater
Lake or the stability of the crater walls.


The level of eruptive activity seen today is, in itself, little different
to that observed over the last few days, ie 2-3 eruptions per 24 hours
that produce tall, ash-bearing eruption plumes above the volcano,
accompanied by small lahars. However the level of background seismicity
(volcanic tremor) has increased significantly following the last 2
eruption earthquakes. Presently, background seismicity is at levels just
below the highest levels seen during this eruption sequence, BUT there is
no sustained eruptive activity, as seen on Monday 25 September when the
tremor was last at this level.  The fundamental criterion for raising the
alert level of the volcano would be enhanced and sustained eruptive
activity coupled with this level of tremor.  At this time the level of
eruptive activity remains low, but tremor is high and indicates that an
escalation to Alert Level 4 could  occur relatively rapidly, i.e.  within
a period of hours.

Conclusions and Alert status

Intermittent explosive eruptions are continuing from the volcano, during a
period of increasing levels of volcanic tremor.  We therefore advise that
the Alert Level for Ruapehu remains at 3, with the important proviso that
a rapid escalation to Level 4 could ocur with little warning.

Dr C J N Wilson, Programme Leader, Volcanology.