Seismic activity, Crater Lake temperature, and strength and frequency of the lake's hydrothermal eruptions declined in February and early March, but increased again in mid-March.

Summit-area monitoring by NZGS personnel 11 February showed little change since the visit 6 days earlier. Only 4 small explosions from Crater Lake were noted in 8.5 hours. The largest, lasting about a minute, ejected three 30 m-high columns of muddy black water, which collapsed onto the lake surface to form small base surges. The temperature of the lake water had risen slightly, from 49° to 50.5°C. Distance-measuring and tilt surveys showed no significant changes. The next visit by geologists, on 5 March, lasted 4 hours, but no explosions were observed nor was there any evidence of new ash around the lake. However, climbers saw two very small explosions the next day. The lake temperature had dropped almost 10°C, to 41°C, in about 3 weeks. Only minor tilt changes were observed.

Park rangers received a report of an eruption at about 1215 on 16 March that generated a steam cloud filling the entire crater area to an estimated height of 1 km. NZGS personnel saw one steam explosion during a 2.5-hour visit 18 March. Continuous steaming of Crater Lake was reported during the early morning of 20 March. Geologists returned 23 March and observed 5 explosions from Crater Lake in 10 hours. Four were relatively small, producing columns of water 5-30 m high. However, a larger explosion at about 1430 produced large waves, and jets of black water that rose more than 100 m above the lake surface. Lake temperature had increased 6° since 5 March, to 47°C. No significant tilt changes were detected during surveys 23 and 26 March. A single Crater Lake explosion was observed during 5 hours of NZGS fieldwork 26 March.

The following is from reports by J.H. Latter. [For Latter's definitive analysis of this activity, see New Zealand Volcanological Record, no. 12, p. 31-37].

A period of higher-amplitude volcanic tremor began about 1600 on 14 January, climaxed 26 January and ended 30 January. Since then, strong tremor has been recorded only during an 8-hour period 10-11 February. Through 25 January, the tremor was dominantly high-frequency (3-4 Hz), suggesting that its origin was very shallow, but since then the strongest tremor has been mainly low-frequency (1-2 Hz). The focus of activity has evidently moved down to a lower level within the volcano. Latter notes that this could either be due to a process of withdrawal of magma, which up to now has been standing at a high level, or to the arrival of fresh magma from greater depths at the normal volcanic focus about 1 km below Crater Lake.

Only small volcanic earthquakes occurred between mid January and the end of February. A marked swarm of low-frequency volcanic earthquakes (B-type) took place, at about the normal focus, 20-22 February; activity peaked about 1200 on 20 February with several magnitude 2.1 earthquakes. This magnitude was relatively low, and it was not known whether the events were accompanied by eruptions. Latter notes that it was likely that the B-type swarm represented a minor stoppage in the volcano's conduits, but that the stoppage must have been rather weak since it was evidently overcome by quite small-magnitude earthquakes. Similar but smaller events took place 21-22 January (when no eruptions took place), and 3 and 14 February.

Shallower seismic activity peaked 23-25 January, when high-frequency tremor was fairly strong, preceded by the largest magnitude volcanic earthquakes at this level since 24 December (the so-called C-types, two ML 2.0 events). A smaller C-type earthquake (ML 1.8) occurred 28 January; since then there have been few, the largest only ML 1.6 (on 26 February). During the declining stages of activity 24-25 January, 31 January, and 24-26 February (after the B-type swarm mentioned above), high-frequency roof rock earthquakes with magnitudes between 1.6 and 1.9 have been detected.

Latter notes that "the best fit for B-type earthquake data suggests a mean depth of origin of 0.77 km beneath the floor of Crater Lake. Adopting an explosion model for the earthquakes, and equating the travel time (origin time of earthquake minus observed eruption time) of 8.5 seconds with upward movement of gas from this depth, gives an average velocity of the gas column of about 90 m/s. Applying the same velocity to the onsets of C-type earthquakes yields a depth of origin of about 250 m below the floor of the lake. This estimate, though crude, is probably of the right order, and suggests that magma had risen during the increased activity (since September 1981) by about 500 m.

"The decline in seismic activity at the end of January, and the change to tremor of deeper origin, appears more likely to have been due to withdrawal of magma than to a major blockage of conduits within the volcano. Although lake temperature has declined, partly no doubt because of the accelerated melt around Crater Lake during the long spell of fine weather, the volcano still gives the impression of being 'open vent.' The small magnitude (ML 2.1) of the largest earthquakes occurring since activity declined suggests that only minor blockages have formed, and have been fairly quickly overcome."

High-level (high-frequency) tremor continued 1-23 March, although none was recorded 4 or 7-10 March. Tremor was strong 11-16 March, peaking on the 13th, but remained much weaker than in late January. Occasional episodes of low-frequency tremor were recorded during the first 3 weeks in March, some lasting for several hours. These were interpreted by Latter as indicating movement at the base of the magma column, at least 500 m tall, that may extend from 200-300 to 700-800 m below Crater Lake. A swarm of B- and C-type earthquakes began on 15 March, culminating in a 6-minute B-type sequence 21 March that reached a magnitude of 2.7, the largest volcanic earthquake at Ruapehu since 2 January. Clouds obscured the volcano 21 March, so it was impossible to determine if an eruption accompanied this event. The swarm was continuing as of 23 March.

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; I. Nairn and B. Scott, NZGS, Rotorua; P. Otway, NZGS, Wairakei; R. Beetham, NZGS, Turangi.