Report on Ruapehu (New Zealand) — March 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 3 (March 1983)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Deflation and B-type earthquakes
Please cite this report as:
Global Volcanism Program, 1983. Report on Ruapehu (New Zealand) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 8:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198303-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Viewing Crater Lake from the air, pilot K. Newton had reported the color was gray on 5 March, but was reverting to blue-green 3 days later. When NZGS personnel visited Ruapehu on 17 March, Crater Lake was gray. They found no evidence of recent eruptions; neither ash deposits nor surge marks. Upwelling over the N vent area was slight. Over the central vent no upwelling was visible, but thin black sulfur strands appeared in midafternoon.
Lake water temperature measured at the outlet was 23°C, 4° lower than on their last visit, 28 February. Concentrations of both chlorine and magnesium had risen slightly; the Mg/Cl ratio remained 0.104.
The horizontal deformation survey showed that the distance between 2 stations on opposite sides of the crater had decreased an additional 8 mm since 28 February, for a total contraction of 22 mm since 10 February.
Since seismicity increased 23 February, there have been 9 B-type earthquake sequences, the three reported last month plus others on 4, 7, 8, 9, 12, and 14 March. These sequences typically began with a high-frequency roof rock (tectonic) earthquake of about M 2 at a relatively shallow depth. Within a minute or so, this was followed by a deeper B-type (volcanic) earthquake of magnitude 2.9-3.4, at a depth between the focus of normal magmatic events (about 1 km depth) and those in the roof rock. No significant volcanic tremor has occurred since the earthquake series began. The lake's color changes appeared to correlate with the earthquake series.
The largest B-type earthquake in the series, [ML 3.25], occurred at 1406 on 12 March. According to J.H. Latter, earthquakes at Ruapehu have not in the past exceeded this magnitude in a closed-vent situation, as this appears to be, without an accompanying eruption.
The seismicity and deflation were tentatively interpreted by the NZGS as indicating a decreasing magmatic or gas pressure at a deep level below the N vents (or, less likely, intrusion occurring beyond the crater, resulting in compression of the crater rim). As long as the present seismicity persists, they consider the probability of eruption to remain higher than usual.
Geologic Background. 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 110 km3 dominantly andesitic volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake, is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.
Information Contacts: P. Otway, NZGS, Wairakei; J. Latter, DSIR, Wellington.