Prior to November 2001, the last notable seismic activity at Ruapehu was moderately elevated tremor during February 2001 (BGVN 26:07). The Institute of Geological & Nuclear Sciences (IGNS) reported that on 21 November at 0218 a moderate-to-large volcanic earthquake was recorded at Ruapehu. The earthquake was followed by ~1 hour of moderate-to-strong volcanic tremor. IGNS did not believe an eruption occurred because no air waves were recorded, and there were no reports of unusual activity. As of 23 November, seismicity continued to be at higher-than-normal levels and weak tremor continued. Clouds obscured the view of Ruapehu.

Ruapehu's crater lake was sampled on 11 September. The lake's temperature was 21°C and its color was blue-green. Further observations during September and November confirmed the color and revealed that the lake had cooled over the previous months.

On 25 November seismic activity decreased to background levels and observations revealed that there were no signs of eruptive activity. Upwelling sediment caused Ruapehu's crater lake to change from its normal blue-green color to dark gray. The temperature of the lake was still relatively low (22°C), which further supported the theory that no eruptive activity occurred after the earthquake.

On 7 December, IGNS reported that two small, long-period earthquakes were recorded beneath the volcano. A visit to the crater lake on 8 January revealed that the temperature had increased since late November 2001 to 36-38°C. There was evidence of convection over the S-central vent area, including minor sulphur slicks, upwelling, and light steaming. Minor volcanic tremor was also recorded.

Steam plumes were emitted from Ruapehu on 11 and 12 January. This activity was believed to be associated with hydrothermal activity in the crater lake. Ruapehu remained at Alert Level 1 ("Initial signs of possible volcano unrest") on a scale of 0-5.

General Reference. Houghton, B.F., Latter, J.H., and Hackett, W.R., 1987, Volcanic hazard assessment for Ruapehu composite volcano, Taupo Volcanic Zone, New Zealand: BV, v. 49, p. 737-751.

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: Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).