Activity at Tavurvur crater was at a low level during October 1997 despite a slow re-inflation of the shallow caldera reservoir. Weak fumarolic activity composed of white and blue vapor emissions occurred at the summit area; during mid-October the emission rate increased and roaring noises were heard. Output of SO₂ from the crater peaked at ~960 metric tons/day during mid-October; during the following week, the emission rate returned to a normal level of 400 tons/day. A similar peak occurred towards the end of the month.

Seismic activity remained low throughout October. Two high-frequency events, on 15 and 21 October, were recorded; however, neither was large enough to be located.

The water-tube tiltmeter at Sulphur Creek (3.5 km from Tavurvur) indicated an additional 8 µrad inflation of the central magma reservoir area during October. Inflation since the 17 August eruption (BGVN 22:08) totaled 15 µrad. Dry-tilt measurements around Greet Harbour showed an inflation of 2-5 µrad radial to the central magma reservoir. Leveling results confirmed the inflationary trend with a 3-cm uplift since March at the end of Matupit Island and a 3-5-cm uplift measured by sea shore survey since June. No changes in tilt were recorded on the Vulcan side.

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: B. Talai and H. Patia, Rabaul Volcano Observatory (RVO), P. O. Box 386, Rabaul, Papua New Guinea.