J.H. Latter reports that the [8 December] eruption was preceded by nearly continuous [weak, high-frequency (3-4 Hz)] volcanic tremor that began gradually on 2 December. At 0806 on the morning of the eruption, high-frequency tectonic earthquakes began [in the roof- and wall-rocks surrounding the source of volcanic earthquakes below Crater Lake]. Volcanic earthquake sequences were recorded 0844-0850 and 0903-0911 (before the eruption) and at 1010 (1-2 minutes after the eruption). Events of the main sequence at 1010 were initially relatively deep, but migrated upwards in < 10 seconds to within a short distance of the surface, just N of the lake. Tremor was strongest after the eruption from 1100 to 1500 and continued until 16 December. After about 18 December, seismicity gradually decreased to background levels. However, at about noon on 10 January, a series of sporadic, powerful, tremor episodes began, suggesting renewed gas movement.

Field observations 16 December showed little change in lake temperature or chemistry (Mg and Cl contents) since the eruption; however, effervescing rapids at the outflow area suggested a relatively high CO₂ content. Slight convection in the lake on the morning of the 16th had increased to a slow continuous upwelling from the central vent by afternoon. The lake temperature was 14°C.

On 11 January, the N vent area was convecting with unusual strength, and six upwelling cells produced yellow sulfur slicks. At about 1400, strong convection began over the central vent. Lake temperature had increased to 27°C. Mg/Cl had increased by 0.007 since the 8 December eruption. This increase suggested that deeper, hotter fluids had been discharged into the lake, or that upper-level vent rocks had been exposed to more reactive fluids. Deformation remained low. On 13 January at 1230, ski lift personnel saw steam rising above the volcano. Geologists noted that on eight occasions since 1980, small phreatic/hydrothermal eruptions have followed within 3-7 weeks of episodes of rapid lake heating (at 15-20°C/month). The most recent occurred July-August 1987 and March-April 1988.

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 Geophysics, Wellington; B. Christenson and S. Sherburn, DSIR Wairakei; P. Otway, NZGS Wairakei.