Ruapehu - IGNS - Science Alert Bulletin V95/39

Thursday 12 October 1995
1700 hours NZDT (UT +13)
Ruapehu Volcanic Activity

Situation Summary

As of 1700 hours, the following information is available.

*     Tremor levels rose to high levels from around 2100 hours on 11
October until 0500 hours on 12 October, accompanying a sustained explosive
eruption. Tremor levels were low for over 4 hours, then rose sharply at
roughly 0915, accompanying another eruption, then declined back to low to
moderate levels from 1100 hours.  The levels of tremor have been similar
to those seen during the two peaks of explosive activity to date, on 25
September and 7 October,  but have not been accompanied by any large
explosion earthquakes.

*     The seismic activity between 2100 and 0500 hours was accompanied by
a continuous explosive eruption, producing an eruption plume which reached
to about 8-10 km altitude when visible in the early morning.  This plume
was blown to the northeast by strong southwesterly winds, and ash was
dropped from it to produce a significant fall at least as far away as
Gisborne.  Tentative maximum ash fall thicknesses have been reported as
follows:

• 10-20 centimetres at the Tukino ski field,
• "a few centimetres" on the Desert Road,
• 1-2 centimetres on the North Arm airstrip (Wai-o-tupuritia Stream valley),
• 2-3 millimetres at Tarawera (on Highway 5 between Taupo and Napier),
• 2 millimetres at Tuai (Waikaremoana),
• 1-2 mm at Wairoa.

Traces of ash were also reported from Opotiki, but none at Whakatane,
Rotorua, Taupo or Turangi.  Although data are tentative, preliminary
indications are that the volume is between 0.01 and 0.1 cubic kilometres,
so making this the biggest volcanic ash fall deposit erupted in New
Zealand since the 1945 Ruapehu eruption.

*     The seismic tremor from 0915 hours this morning also accompanied a
vigorous ash-rich plume which rose to a maximum height of about 5 km and
was blown more to the east than the overnight plume.  Ash fall thicknesses
are not available for this event.

*     The style of activity at Ruapehu and its seismic signature have
changed during the course of last night.  When compared with previous
vigorous explosive eruption episodes, there was a marked lack of large
volcanic earthquakes accompanying the very  strong tremor levels.  The
limited eye-witness accounts of the eruption, and subsequent field
observations by Institute scientists, suggest that  water from the  crater
lake played a much less important role in the activity.  Incandescent
material was reaching the surface, and eruptions last night and this
morning were sustained for longer times.  Although the eruption plumes
themselves are not reaching greater heights than their counterparts
earlier in the eruption sequence, the plumes (at least when visible in
daylight) were carrying  more ash to greater heights than the steam-rich
plumes seen in the last 3 weeks.

Summary and Alert Status

The present eruption episode is continuing at an elevated level.  We are
now recording alternating periods of extended near-continuous vigorous ash
eruptions, betwen quieter periods of low to moderate tremor and minor ash
eruption.  Pending  examination of the summit area when the weather
conditions improve, we advise that the Alert Level remains at 3.

Dr CJN Wilson, Programme Leader, Volcanology