Report on Rabaul (Papua New Guinea) — December 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 12 (December 1984)
Managing Editor: Lindsay McClelland.
Rabaul (Papua New Guinea) Seismicity decreases; moderate deformation persists
Please cite this report as:
Global Volcanism Program, 1984. Report on Rabaul (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 9:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198412-252140.
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"A further decrease in activity was evident in December. Seismicity reached its lowest level since the beginning of the current crisis period in August 1983. Seismic energy release for the month was [1.3 x 1015] ergs; 2,887 earthquakes were recorded. The largest earthquake for the month was an ML 2.5 event that was part of a small seismic swarm in the Vulcan area on 22 December. The month's seismicity was concentrated in the Vulcan-Matupit Island area. A mild rate of ground deformation persisted. The greatest changes were in the Greet Harbour area, where the maximum tilts were about 20 µrad on each side of the mouth of the harbour."
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: C. McKee, RVO.