A continuous glow was visible at nights throughout January 1998 at Tavurvur crater, and there was also a slow but steady inflation of the volcano during the month. An expected eruption began at Tavurvur on 3 February 1998.

The eruption began with emissions of pale to dark gray ash clouds typically 5-20 minutes apart. There was no noise associated with the emissions although small, low-frequency seismic events did accompany each event. Over the next few days roaring and rumbling could be heard down-wind (to the SE) of Tavurvur and seismic events became generally larger. Loud explosions were recorded once to 5 times daily. The explosions usually were accompanied by forceful emissions of dense gray to dark ash clouds that rose to 2000-3500 m above the crater. These were followed by moderate to small ash-cloud emissions lasting ~30 minutes. During the explosions lava fragments were ejected to heights of 200-300 m, showering the slopes 200-500 m from the base of the cone. Some small ash flows were also generated during explosions. During strong ash emissions at night, successive 5-minute projections of glowing lava fragments were observed. This pattern of eruptive activity lasted until the end of February.

Ash rose to 300 m above the crater (600 m a.s.l.) and was usually distributed to the SE, with occasional drifts to the N and W. Each ash emission produced light ash fall at Talwat village SE of Tavurvur near the base of the cone. There was also very light ash fall recorded elsewhere on New Britain, including at Tokua airport 20 km from Tavurvur.

Seismic activity was generally low. A slight increase in the frequency of volcanic earthquakes in early February reflected the increase in activity at the summit of Tavurvur. The increase was indicated on the 1- minute RSAM data as background values of 20 RSAM units increased to 100. Between 10 and 48 earthquakes were recorded daily. The average number per day was 27, but after 22 February they dropped to 9. Two high-frequency earthquakes recorded during February were located 20-30 km ESE of the caldera.

During the current phase of eruptive activity there has been no significant change in ground deformation compared to the inflationary trend prior to the eruption. A water-tube tiltmeter located 3.5 km NW of Tavurvur showed a slow yet steady rate of inflation: total accumulated tilt for February was 4 µrad. Real-time GPS measurement taken from a remote station on Matupit Island 2 km W of Tavurvur showed no significant change.

Although COSPEC SO₂ measurements lacked precursory signatures suggesting an eruption, a slightly higher SO₂ flux of ~350 metric tons/day was measured when the eruption started. After several days the flux decreased to a low level of ~190 tons/day. The low flux values attained during the month were partly due to a change in wind direction away from the fixed observation post.

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.