Frequent explosions from Crater Lake continued through late January. The lake temperature continued to rise, and volcanic tremor and shallow volcanic earthquakes were frequent. NZGS personnel carried out field work in the summit area 12, 18, and 21 January. Eruptive Activity, Temperature, and Ion Concentration of Lake Water. During the roughly 12-hour visit on 12 January, 23 small hydrothermal explosions from the crater lake were noted, separated by quiet periods of 4-127 minutes. Water jets from these events rose as much as 70 m, and two generated large steam columns. A dark-colored deposit on snow extended from Crater Lake onto the lower flanks. The phreatic explosion that produced this deposit was not observed, but possibly accompanied a moderate B-type volcanic earthquake recorded the previous day. The crater lake temperature was about 57°C, 10° hotter than when it was last measured, on 28 December. Geologists were hampered by poor weather 18 January, but observed 4 hydrothermal explosions, similar to those of 6 days earlier, during brief periods of good visibility. During better weather on 21 January, 20 hydrothermal explosions were seen in 7 hours, again similar to those of 12 January. The temperature of the lake remained at 57°C. Depth soundings at two points in the center of Crater Lake indicated that no major lava dome growth has occurred, but did not rule out the extrusion of small quantities of lava onto the lake floor. Concentrations of Mg and Cl, and the Mg/Cl ratio of lake water have both shown large increases after falling during early 1981. The NZGS noted that these increases demonstrate that interaction between fresh rock and lake water has been occurring, but it is uncertain whether the fresh rock is new magma or older rock exposed to lake water for the first time because of explosive activity.

Deformation. Tilt surveys in the summit dome area measured only 3 µrad of apparent deflation between 28 December and 12 January, and no additional deflation was detected on 18 January. Since inflation peaked 25 June, very little change in tilt has been detected in this area. However, horizontal deformation measurements showed that 16 mm of expansion had occurred 13 October and 21 January along a 600-m line across the N side of the crater. Horizontal movements across the dome during this period did not exceed 10 mm.

Seismic activity. The increased seismic activity that started 18 November peaked 30 November-2 December, then declined. [Although reported in considerable detail here, November-December seismicity was substantially weaker than that associated with October's minor eruptive activity.] High-frequency (3.5 Hz) tremor, strongest 28 November, was succeeded by low-frequency (1-2 Hz) tremor of deeper origin that peaked 30 November and declined 1 December. A magnitude 2.3 B-type (low-frequency volcanic) earthquake was recorded on 1 December and a magnitude 2.4 low-frequency volcanic event occurred the next day at 1221, centered in the roof rock overlying the usual focus of B-type shocks. Until the first roof rock event of clearly intrusive or magmatic character occurred on 23 November, all of the roof rock seismicity had been tectonic and of high frequency. Volcanic tremor remained at a low level until 26 December, except for a minor peak on the 3rd. However, tremor was detected every day through this period, with dominant frequencies of 3.5-4.5 Hz through 15 December, and more normal frequencies (2-2.5 Hz) after that date.

A sequence of small earthquakes on 22 December was probably accompanied by an explosive eruption. A magnitude 2.0 B-type shock at 1208 was followed 2.5 minutes later by a M 2.2 low-frequency volcanic event located in the roof rock zone. A weak air wave, detected by a nearby microbarograph, had a probable origin time of 8 seconds before the second earthquake. Similarly, a small explosion witnessed by geologists on 28 December was accompanied by the onset of very weak low-frequency tremor, and was followed 9 and 12 seconds later by magnitude 1.25 and 1.5 volcanic events in the roof rock zone.

Tremor strengthened in late December and early January, accompanied by discrete low-frequency volcanic earthquakes, the strongest at 0722 on 24 December (in the roof rock zone, M 2.2), 1 January at 0613, and 2 January at 0104 (both slightly deeper, magnitude 2.3, B-type events). This period of increased seismicity was interpreted by J. H. Latter to suggest that high-level intrusion of magma was accelerating, probably accompanied by extrusion onto the crater lake floor.

High-frequency tremor continued until 5 January, then diminished by about a factor of 4. Tremor increased again on the 14th and reached a peak 17-18 January. Dominant frequencies ranged from 3 to 5 Hz, indicating that the activity continued to be very shallow. Few significant discrete volcanic earthquakes were recorded during the first half of January, but explosions observed on 12 January were accompanied by very small B-type events.

Portable seismographs were operated by NZGS geologists visiting the crater 18 and 21 January. On the 18th, they recorded weak high-frequency but detected no earthquakes accompanying the 4 observed hydrothermal explosions. Increases in tremor amplitude preceded some of the explosions observed 21 January by as much as 5 minutes, but again, none were accompanied by discrete earthquakes.

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, Wellington; D. Sheppard, Chemistry Division, Wellington; B. Scott, P. Otway, and I. Nairn, NZGS, Rotorua.