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Report on Rabaul (Papua New Guinea) — September 1996


Bulletin of the Global Volcanism Network, vol. 21, no. 9 (September 1996)
Managing Editor: Richard Wunderman.

Rabaul (Papua New Guinea) Strong explosions produce ash clouds and ashfall

Please cite this report as:

Global Volcanism Program, 1996. Report on Rabaul (Papua New Guinea) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199609-252140


Papua New Guinea

4.271°S, 152.203°E; summit elev. 688 m

All times are local (unless otherwise noted)

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 688-m-high asymmetrical pyroclastic 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 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western 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.