Ruapehu - IGNS - Science Alert Bulletin V95/48

Wednesday 18 October 1995
1700 hours NZDT (UT +13)
Ruapehu Volcanic Activity

Situation Summary

As of 1700 hours, the following information is available.

*     During the last 24 hours, volcanic tremor at Ruapehu has reduced to
background levels, being replaced by intermittent long period earthquakes
with magnitudes between 1 and 2, and interpreted as occurring in
association with magma at depth in the volcano.  These events are
frequently followed after 1 to 3 minutes by bursts of higher frequency
tremor which at times today have been correlated with relatively small
eruptions jetting ash and scoria up to 200-300 m above the summit.

*     Eruptions have been occurring from two main vents, in the northern
and southern parts of the ash-filled crater floor, which is estimated at
between 100 and 150 m  below the level of the previous Crater Lake
surface.  The crater does not appear to have been significantly deepened
as a result of the  large ash eruption  on Saturday 14 October.  This
suggests that magma has risen in the vent to replace that removedby
eruption.

*      A dark ash plume has been drifting to north of the volcano at
relatively  low elevation (~10 000 feet) during the day.  Dark ash falls
cover the northern slopes of Ruapehu, including the Whakapapa skifields.
At least two small lahars have been observed in the Whangaehu River during
today.   The source of these lahars has not yet been established.  They
are not originating within the crater, and may form by release of bodies
of snow/ice melt water trapped on upper slopes within the Whangaehu
catchment.

*     GNS staff in the field have been overflying the volcano; examining
recent lahar and ash deposits; carrying out EDM deformation surveys at
Tukino, Turoa and Whakapapa, and servicing seismic instrumentation.   A
successful COSPEC flight was made yesterday, to determine the output of
SO2 volcanic gas from the crater.

Conclusions and Alert Status

The present eruption episode is continuing at a reduced level, with weak
semi-continuous ash emissions.  We  advise that the Alert Level remains at
3 for Ruapehu volcano.

IA Nairn  for 
Dr CJN Wilson, Programme Leader, Volcanology