GeoNet reported that 40 small earthquakes at Ruapehu was recorded by the seismic network during 21-26 November. The earthquakes were 0.3-1.8 in magnitude and located at depths of 3-6 km, though most clustered at depths of 4-5 km. Volcanic tremor levels were low during 2023 and did not vary in response to the sequence. The temperature of the crater lake water was slowly rising, starting in mid-October. During an overflight on 22 November, scientists observed that the color of the lake had changed to blue-green instead of the typical gray color, consistent with less gas flux disturbing lake-bottom sediment. No upwelling was observed and sulfur slicks on the water’s surface were visible. Water was overflowing at the lake’s outlet. Overall, activity was low. The Volcanic Alert Level remained at 1 (on a scale from 0-5) and the Aviation Color Code remained at Green (the lowest level on a four-color scale).

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