Mild eruptions continued at Tavurvur during September. Weak, white to pale-gray vapor-and-ash emissions took place at short irregular intervals, and plumes rose ~1,000 m above the crater. These emissions were occasionally accompanied by roaring sounds. On 2, 7, and 9-12 September, strong explosions sent ash clouds up to 4 km above the crater, resulting in light ashfall on Matupit Island and Rabaul town.

After the explosions on 26 August (BGVN 21:08), the release of SO₂ was at a low level of ~200 metric tons/day (t/d). However, the flux rate gradually increased and reached ~1,500 t/d on the night of the 11 September explosions. Seismicity showed variations similar to the SO₂ flux. The background seismicity level was 5-20 low-frequency events/hour and 30-100 RSAM (Real-time Seismic Amplitude Measurement) units. From 8 to 10 September, seismicity increased to ~40 low-frequency events/hour and 100-200 RSAM units. After the eruption on 11 September, seismicity returned to a normal level (3-15 events/hour and 25-100 RSAM units). Ground deformation was not evident around the mid-September eruptions.

After 18 September, seismic activity increased to medium levels (30-40 events/hour and 50-150 RSAM units). Likewise, the flux rates of SO₂ changed from 200-400 t/d to 1,000-1,500 t/d by the end of September. Beginning on 22 September, tiltmeters recorded deflation of the central caldera reservoir at a rate of up to 1 µrad/day. Following these anomalies, strong eruptions took place in early October, sending ash clouds to an altitude of 5.5 km.

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: C. McKee and B. Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.