Nov 15 1995, 12:00 pm - Ruapehu Volcano

Introduction

On October 2, 1995 a Hazard Map for Ruapehu Volcano was released (see Science Alert RUA-95/28). Since then the eruption style has changed significantly, and some of the parameters which were previously considered in evaluating hazards associated with the volcano have changed. Due to these changes, aspects of the hazard map released on October 2, 1995 are now no longer valid.

Key parameters

The key parameters used in compiling this new map are as follows:

1. Crater Lake was initially destroyed by the eruptions and replaced by a shallow crater, with two active vents on its floor. These vents are now starting to fill with water to reform lakes.
2. SO₂ levels in the plume, as measured by the COSPEC instrument, have remained high (2,000 - 5,000 tons per day) since late October.
3. Volcanic tremor and volcanic earthquakes have declined to levels which are lower than those earlier in the eruption sequence, but still remain elevated by comparison to pre-eruption events. Earthquakes related to small eruptions are being recorded.
4. Observations of eruption plumes show they are primarily weak steam pulses that are not accompanied by any significant ash emission.
5. There has been considerable deposition of volcanic ejecta onto the steeper upper slopes of the volcano. Much of the volcanic ejecta froze shortly after falling, and stayed intact. As the spring thaw advances, numerous secondary lahars and debris flows are occurring due to melting of ice and snow. Many of the streams draining off the mountain are discoloured due to the sediment loads of volcanic material.

Revised Hazard Zones at Level 3

These changes to the Hazard Zones 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 signals. The scientific alert level remains at 3. We have subdivided the hazards in the RED and ORANGE zones into two sub-zones. Sub zone one (1) reflects processes associated with explosive eruptions from the active crater, and gas emissions, while sub zone two (2) reflects the lahar and debris flow hazards. In some places both can exist together.

The hazard zones are as follows:

1. RED 1 is a zone within 1.5 km radius of the centre of the active summit crater. Within this zone there remains a high risk of impact from ballistic blocks, 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.
2. ORANGE 1 reflects a zone of lesser hazard due to ballistic blocks, 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.
3. YELLOW zones represent hazards from the thawing of erupted ejecta, the specific level of risk varying with ice and snow volumes, slope and thickness of volcanic debris. 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 dependant 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 that are largely unrelated to discrete explosions from the active crater 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.

B.J. Scott (for C.J.N. Wilson)
Volcanology Programme Leader