After 4 months of slow inflation, a new, though mild, eruptive phase occurred at Tavurvur in late December. Until mid-December, emissions from Tavurvur crater were mainly white vapor clouds of varying density. A weak glow was visible around the crater mouth on most nights. An explosion occurred on 7 December at 0636. A series of explosions occurred later on 21, 23, 24, 28, and 29 December, resulting in ash columns rising ~1,000-3,000 m above the summit. Explosions at night displayed glowing lava projections. Roaring and rumbling sounds were heard for days preceding and accompanying the explosions. Seismicity and ground deformation reflected the eruptive activity.

Low-frequency volcanic earthquakes from Tavurvur resumed on 2 December following a quiet period during September-November. The number of earthquakes was low initially (1-3/day) but increased suddenly on 22 December. By the end of the month the number had decreased again. The increase in earthquakes was accompanied by periods of low harmonic tremor with durations ranging from a few to 20 minutes.

Also in the first week of December, a series of 10 high-frequency earthquakes occurred. The earthquakes were too small to be located, but according to analysis of arrival times at recording stations they appeared to originate NE of Rabaul. The occurrence of these earthquakes, together with the increase in low-frequency earthquakes and summit activity, illustrate a long perceived theory that NE earthquakes precede an increase in volcanic activity at Tavurvur with the lead-time ranging from a few weeks to months. This particular period of eruption at Tavurvur was foretold by a reversal in tilt at the Sulphur Creek water tube tiltmeter (3.5 km NW) from the first day of December. A deflation of ~4 µrad E occurred during the month. The amount of material drained by this eruption was too small to cause a noticeable subsidence. Interestingly, the output of SO₂, measured daily by COSPEC, dropped early in the month from 600 metric tons/day to undetectable levels. Explosions on 7 and 24 December released 400-900 tons /day of SO₂. Soil CO₂ flux, integrated over 2 weeks, remained relatively low (<200 mg/m²/day).

Geological Summary. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: Ben Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.