Ruapehu Volcanic Activity

The level of volcanic activity at Mt Ruapehu has declined in the last week to ten days to levels that warrant lowering of the scientific Alert Level from 3 to 2. However this does not mean that eruptive activity has completely stopped. At level 2 the volcano is still very much alive, but is not erupting as vigorously as it previously was. The Institute of Geological and Nuclear Sciences will be decreasing the level of surveillance to a level which is commensurate with activity at level 2. As part of the process of lowering the Alert Level we have also reassessed the hazards and presented below is an assessment of the present hazards.

Today the volcanic tremor has remained at a similar level to that observed for the last few days, with the mean seismic ground velocity about 3-4 -m/s. High frequency volcano-tectonic earthquakes continued to register on The Dome seismograph at the rate of 10-12 per day. These levels are similar to those which existed at Mt Ruapehu when it was last at level 2 (mid-September 1995).

A COSPEC flight was made yesterday afternoon and again obtained a SO2 output value of about 1000 t/d which is well down from the highs which have exceeded 10 000 t/d. Visual observations on the COSPEC flight and from Dome this morning confirm that the lakes have enlarged and are now interconnected by a channel in the topographic low between them.

Activity at Ruapehu volcano has been at a low level for some days now and a detailed evaluation of the surveillance data available at this time indicates that the Alert level should be lowered from level 3 to level 2. The change in Alert Status must not be interpreted to mean either that the present eruption has ended or that there are no hazardous areas on the volcano. Isolated explosive eruptions can still occur without useful warning signs. Note that the September 23 explosive eruption which sent lahars into the Whakapapa Skifield occurred from an Alert level 2 status.

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. The seismic ground velocity at Dome (which has ranged up to the 110-m/s limit before overloading) has averaged 3-4 -m/s during the last week and has not exceeded 10 -m/s since 27 October.

(ii) SO2 levels in the gas plume, as measured by the COSPEC instrument, reached over 10 000 t/d in October, but have consistently fallen from over 6000 t/d on November 3 to less than 1000 t/d by November 23. Observations of eruption plumes show they are weak steam pulses, indicating hydrothermal activity that is not accompanied by ash emission.

(iii) Ground surface deformation measurements around and on the volcanic cone have not detected any significant deformation.

(iv) There have been no significant ash rich eruption columns produced since October 14 and no ash emissions since the end of October.

These changes to the Hazard Zones must not be interpreted to mean either that the present eruption has ended or that there are no hazardous areas on the volcano. Isolated explosive eruptions can still occur without useful warning signals. The accompanying map shows hazard zones that apply now that the scientific alert level is set at 2. We have subdivided the hazards in the RED and ORANGE zones into two sub-zones. Sub-zone 1 reflects processes associated with explosive eruptions and gas emissions from the active crater, while sub-zone 2 reflects the lahar and debris flow hazards. In some places both can exist together.

The hazard zones are as follows:
(i) RED 1 is a zone within 800 metres radius of the centre of the active summit crater. Within this zone there remains a high risk of ballistic block impact, and the possibility of pyroclastic fall and surge activity. Also, within this zone there is a significant hazard from volcanic gas.

RED 2 is based on the Whangaehu catchment and reflects the lahar hazard in that area. This hazard is related to expulsion of the re-forming lakes and/or slumping of material erupted into this catchment during earlier phases of the eruption.

(ii) ORANGE 1 reflects a zone of lesser hazard due to ballistic block impact, pyroclastic fall and gas hazard about the active summit crater.

ORANGE 2 reflects a continued high risk from secondary lahars and debris flows which now exist in the Wahianoa and Mangatoetoenui catchments. This hazard is especially severe on the glaciers in these catchments.

(iii) YELLOW zones represent hazards from the thawing out of erupted ejecta, the specific level of risk varying with ice and snow volumes, slope and thickness of volcanic debris. Minor secondary lahars and debris flows could occur in the Mangaturuturu, Whakapapaiti, Whakapapanui catchments and on the steep slopes above Turoa.

Other areas may be affected by ashfall under conditions of uncommon wind speed and direction. Areas affected by the volcanic gas hazard are very dependent on wind direction and velocity. The hazard zone could extend beyond the mapped area under favourable conditions. Caution should be exercised in all sectors down wind of the active summit crater. Certain secondary hazards are largely unrelated to discrete explosions from the active crater and will continue to be the dominant hazards for some time. Formation of a new crater lake will modify hazards further, particularly with regard to gas emissions, and the situation may change rapidly.

BJ Scott for CJN Wilson (Volcanology Programme Leader)