Ruapehu - IGNS - Science Alert Bulletin V95/42

Saturday 14 October 1995
2230 hours NZDT (UT +13)
Ruapehu Volcanic Activity

Situation Summary

Update:  As at 2230 hours the situation is as follows:

*     Volcanic tremor continued at fluctuating but generally low levels
during most of the 24 hours prior to 1630 hours today.  The tremor record
from the Dome (summit) instrument was strongly banded into intervals of
moderate and low tremor. Two small (magnitude1-1.5) volcanic earthquakes
were recorded at 0434 and 0634 hours.  A period of elevated tremor
commenced at 1550 hours, and increased to intense levels by 1630 hours.
This intense tremor has been accompanied by voluminous ash emission.  Ash
plumes have risen to more than 35 000 feet and a continuous ash cloud has
been reported on 2000 hour weather satellite imagery, extending downwind
at 30 000 feet to cross the coast between Napier and Waipukurau. Heavy ash
falls have been reported between Waiouru and the east coast.  The Desert
Road has been closed by ashfall.

*     Ash emission appears to have declined after about 2030 h,
accompanied by decreasing tremor levels.  Tremor has been replaced by
occurrence of numerous discrete volcanic earthquakes, the largest of
magnitude about 3.  At least one of these events (at 2200 hours) produced
a loud boom heard by IGNS staff collecting ash at the Wahianoa  Aqueduct
on the Desert Road.  The volcanic earthquakes appear to record individual
explosions in the summit crater.

*     Bad weather has prevented ground observations of Ruapehu today.

*     Observations of Ruapehu crater yesterday afternoon showed
intermittent ash eruptions from the deep southern vent in the now dry
crater basin, with mostly steam and gas emitted from the northern vent.
This was the reverse of the situation observed during an overflight
yesterday morning, when ash was being emitted from the northern vent, and
steam/gas from the southern vent.

Summary and Alert Status

The present eruption episode at Ruapehu is continuing. From 1630 hours to
2030 hours today volcanic  tremor was more intense than at any other time
during this eruption sequence, accompanying voluminous ash emission.  We
advise that the Alert Level  remains at 3.

Dr IA Nairn for
Dr CJN Wilson, Programme Leader, Volcanology.