Ruapehu's Crater Lake has cooled from 32.5°C in late April to 11°C in early October. The temperatures recorded since late August were the lowest since detailed measurements began. Periods of low Crater Lake temperature preceded strong eruptions in June 1969 and April 1975.

The following report, from J. H. Latter, describes recent seismicity.

"The period of declining temperatures up to 7 September was seismically quiet, apart from minor volcanic tremor and associated B-type earthquakes that ended on 6 July, a short swarm of roof rock shocks from 11 to 13 July, and less than an hour of strong volcanic tremor on 24 August. In particular, there were no large B-type earthquakes, as after the April 1975 eruption, which might have suggested activity at the volcanic focus that was unable to break out at the surface. This might be interpreted as evidence that the low lake temperatures were due either to a blockage deeper than the volcanic focus (thought to be about 1 km below Crater Lake), or that heat transfer had declined or stopped, for some different reason.

"Beginning gradually on 7 September, a swarm of roof rock earthquakes marked the end of seismic quiescence. The swarm was unusually prolonged, ending on 2 October with an earthquake of magnitude 2.4 ML (i.e. quite small), which was not a true roof rock earthquake. This was well-recorded on the Tongariro National Park network and could be accurately located at 2 km below Crater Lake, which, in the model used, marks the top of the Tertiary sedimentary rocks below the volcano. Rather than being a roof rock earthquake, it was therefore a "wall rock," or perhaps even "floor rock" earthquake, for the known volcanic focus; although it is possible that it may represent a roof rock earthquake for a deeper volcanic focus (magma chamber?) below the Tertiary sedimentary rocks.

"A train of weak B-type earthquakes preceded the 2 October earthquake by about 54 hours. Volcanic tremor began about 8 hours later but lasted only a few hours. Tremor did not become common on the records until after the 2 October earthquake. Since then it has built up steadily on the records, becoming quite strong and almost continuous since 14 October. Clearly the roof rock swarm and terminating earthquake took place in response to stress caused by blockage of some kind and had the effect of removing the blockage. Since 2 October, more typical volcano-seismic activity, as shown by the tremor, has resumed.

"An unusual feature of the volcanic tremor has been that some has been of a much higher frequency than measured previously at Ruapehu. In the past, 3 Hz tremor has been thought to have been correlated with eruptions. On 7 October, tremor with dominant frequencies as high as 4.5 Hz was recorded, probably because of intrusion at shallow levels high in the volcano in very restricted or narrow spaces [see also 28 October in 6:12].

"In spite of the interesting type of recent seismic activity, it is stressed that the amount of energy released is small. Roof rock activity has peaked at a level well below that reached in November 1976 and March 1977. Furthermore, B-type earthquakes have all been small (the largest ML 2.3), and the volcanic tremor, which peaked on 18 October at about 1.9 x 10⁶ J [and again on 25 October (~3.5 x 10⁶ J)], was exceeded in November 1976, October 1977, October 1978, and August and October 1979. It is far below the level that prevailed before the 1971 eruptions.

"It seems likely that the present moderately high level of tremor will be associated with reheating of Crater Lake, and that eruptions will take place. But the volcano can perhaps be considered in a less dangerous state than during its recent period of low temperatures and marked roof rock activity."

Geological Summary. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: J. Latter, DSIR, Wellington.