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Report on Rabaul (Papua New Guinea) — May 1992

Bulletin of the Global Volcanism Network, vol. 17, no. 5 (May 1992)
Managing Editor: Lindsay McClelland.

Rabaul (Papua New Guinea) Seismic swarm; uplift over broad area

Please cite this report as:

Global Volcanism Program, 1992. Report on Rabaul (Papua New Guinea). In: McClelland, L. (ed.), Bulletin of the Global Volcanism Network, 17:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199205-252140.

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Rabaul

Papua New Guinea

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

All times are local (unless otherwise noted)


"Slow magmatic inflation continued in May, although an unusual swarm of seismic activity took place at the beginning of the month. Seismic activity in the usual annular seismic zone remained at a low level throughout May, with a total of 125 events. Starting on 2 May, however, an unusual swarm of earthquakes occurred 4.5-5 km under the N (older and inactive) rim of the caldera, slightly E of Rabaul township. Approximately 300 such events were recorded 2-19 May, with ~140 occurring on 3 May. A dozen were felt by residents. Five events were of ML >=3.0, the largest ML 4.2. Levelling measurements on 4 June indicated that uplift had occurred over a broad area of the caldera since the previous measurements on 11 May. This suggests a deeper source than usual. The biggest changes (20 mm) were recorded at the S end of Matupit Island."

Geologic Background. 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: P. de Saint-Ours and C. McKee, RVO.