Increased seismicity and higher crater lake temperatures preceded a small ash eruption in late October. About 3 weeks later, a second small explosion from the crater lake ejected tephra that may have included fresh magma. Minor explosive activity was continuing in late December. Volcanic tremor may indicate shallow intrusions of magma beneath the summit crater lake, or lava extrusion onto the lake bottom.

Volcanic tremor started to increase in early October, and had become quite strong and almost continuous by the 14th. For the next 2 weeks, seismographs recorded moderately strong tremor with frequencies between 1.1 and 2.7 Hz, a normal range for Ruapehu during periods of activity [but see 6:10]. Tremor declined on 28 October, stopping completely for 5.5 hours. Tremor resumed at 2230 on 28 October, at the highest frequency (5 Hz) recorded at Ruapehu since a seismograph was installed near the summit in May 1976. The high-frequency tremor continued for about 10 hours and was interpreted by J.H. Latter as indicating shallow intrusive activity. Between 29 October and 14 November, seismographs recorded occasional normal-frequency (2.0-2.8 Hz) tremor and a few weak low-frequency (B-type) volcanic earthquakes, the strongest a magnitude 2.4 event on 6 November. A 3-hour episode of high-frequency (3-3.5 Hz) tremor was recorded late 17 November, and 3-5 Hz tremor that started late the next day lasted about 10 hours.

Park Ranger Pat Sheridan observed ash on snow in the Crater Lake area just before noon on 19 November. However, an Air New Zealand pilot flew over the volcano 4 hours later and saw no evidence of a recent eruption. Geologists have not been able to resolve the conflicting observations, but their overflight the next morning revealed dark gray mud extending about 150 m SW of the lake. NZGS personnel visited the summit area 24 November and saw dark gray ash to 700-800 m down the valley of the Whangaehu River, which flows down the E flank from Crater Lake. The maximum thickness of the deposit was less than 1 cm. C. P. Wood analyzed the tephra, primarily precipitated lake sediment (including yellow sulfur spheres to 1 mm in diameter) and altered andesite, but containing many angular chips of fresh-looking dense black glassy andesite, particularly in the coarser size fraction. Wood suggested that the glassy andesite may have been ejected directly as fresh magma or may have been fragments of lava extruded very recently onto the crater lake floor. A [very small (M 1.7)] B-type earthquake at 0123 on 20 November, at the end of an episode of high-frequency tremor, seemed the seismic event most likely to have accompanied the tephra eruption. The crater lake temperature was 42°C on 24 November, up from 36.5° three days earlier and 24° during the last visit by geologists on 4 November. Tilt stations were reoccupied on 24 November, but no significant changes had occurred since measurements 20 days before.

Between 18 and 27 November (the last day for which detailed seismic records were available at press time), 10-20 hours of volcanic tremor were recorded on most days, at frequencies of 4.5-5 Hz until 22 November, 3.5-5 Hz until the 24th, and 3-3.5 Hz thereafter. There were only about 2 hours of tremor on the 24th and 25th. None of the tremor was strong, but the highest amplitudes were recorded 27 November. Small low-frequency volcanic earthquakes began early 23 November, apparently centered less than 1 km below the crater lake, at about the level where roof rock events normally occur (06:10). Latter noted that this suggests magma has intruded the roof rock beneath the lake. Low-amplitude tremor was continuing as of late December.

Geologists returned to the volcano 28 December. They observed three large vapor plumes during their climb to the summit, and while at the crater lake saw a vigorous explosion that produced a 500-m steam column and waves more than 2 m high. No ashfall was noted although the initial jets of water were darkened by tephra. A nearby seismograph recorded a 3 Hz signal during the explosion. The only indications of recent tephra emission were small 1- to 2-mm-thick lobes of dark gray ash and sulfur that extended about 100 m from the lake. The temperature of the lake had risen further since the previous measurement 24 November, to nearly 47°C. Reoccupation of tilt stations showed about 7 µrad of inflation since 24 November, but there had still been a net deflation of 4 µrad since 13 October.

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; B. Scott, I. Nairn, and C. Wood, NZGS, Rotorua.