GeoNet reported that a period of increased seismicity was detected at Ruapehu during 22-23 February. The series of volcanic earthquakes began with the largest event, a M 1.3, which was followed by smaller events. The data showed that the seismicity originated from a source 3 km below the summit. The amplitude of the volcanic tremor declined during 24-28 February and then again increased around 29 February-1 March.

In response to the 22-23 February sequence, volcanologists collected lake water samples and measured gas emissions the week after. The temperature of the lake water suggested slow heating from 24 to 30 degrees Celsius. They observed upwelling of gray sediment and sulfur slicks on the surface of the water. Carbon dioxide and sulfur gas emissions had increased since last measured on 7 February. GeoNet noted that gas-flux and the increased seismicity remained within the normal ranges for Ruapehu; the Volcanic Alert Level remained at 1 (minor volcanic unrest) and the Aviation Color Code remained at Green.

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.

Source: GeoNet