Ruapehu - IGNS - Science Alert Bulletin V95/28 Alert Status Change

Change In Alert Status From Level 4 To Level 3

Key parameters

The key parameters used in this evaluation are as follows.

(i)   Volcanic tremor and volcanic earthquakes have declined to levels
similar to those which were seen prior to this eruption episode.

(ii)  SO2 levels in the plume, as measured by the COSPEC instrument, have
declined from 2600 tonnes per day on 25 September to 170-300 tonnes per
day on 1 October.

(iii) Observations of eruption plumes show they are weak steam pulses,
indicating hydrothermal activity that is not accompanied by ash emission.

(iv)  There have been no lahar-forming eruptions since 08.40 hours on 29
September (83 hours).

Hazard zones at Level 3

The change in Alert Status must not be interpreted to mean either that the
present eruption has ended or that there are no longer hazardous areas on
the volcano.  Isolated explosive eruptions can still occur without useful
warning signs.  The accompanying map details some ongoing hazard zones
that apply at level 3.  These are as follows:

(i)   A zone within 2 km radius of the centre of Crater Lake where there
remains a high risk of impact from ballistic blocks, inclined eruption
jets and surges (Red Zone).

(ii)  A zone of continued high risk from lahars in the Whangaehu Valley
(Red Zone).

(iii) A zone of lower risk from lahars in the Mangaturuturu, Whakapapaiti
and Whakapapanui valleys, the specific level of risk varying with wind
velocity (Orange Zone).

(iv)  A zone between 2 and 3 km radius of the centre of Crater Lake of
lesser risk from ballistic blocks, inclined eruption jets and surges,
which would require discrete explosions of a size larger than any observed
to date in the1995 eruption (Yellow Zone).

Other catchments may be affected by lahars under conditions of uncommon
wind speed and direction.

Other hazards

Certain secondary hazards are largely unrelated to discrete explosions.
They do not, therefore, fall within the scope of this change in alert
status and are not included on the map.

Specifically these are:

(1)   secondary remobilisation of primary ash by rain and stream action;

(2)   slumping and collapse of portions of the walls of Crater Lake basin,
and the channels of the recent lahar paths;

(3)   aggrading streams over-topping their normal channels and
contaminating adjacent catchments.

Significant hazards from these processes will continue to be present for a
period of weeks to months, particularly in times of high intensity rain