Ruapehu - IGNS - Science Alert Bulletin V95/17

27 September 1995
10.00 hours
Ruapehu Crater Lake
 
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 hours 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
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 W, S
and E 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.
 
For further information contact:
Dr C J N Wilson, Programme Leader, Volcanology.
 
Ph: (07) 374 8211; Fax (07) 374 8199