Report on Rabaul (Papua New Guinea) — November 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 11 (November 1983)
Managing Editor: Lindsay McClelland.
Rabaul (Papua New Guinea) Increased seismicity prompts stage-2 alert implying possible eruption within a few months
Please cite this report as:
Global Volcanism Program, 1983. Report on Rabaul (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 8:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198311-252140.
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"A stage-2 volcano alert, implying a possible eruption within a few months, was declared by the RVO on 29 October, in response to the increased seismic activity and ground deformation in September and October.
"In November the seismic activity continued to intensify but the rate of ground deformation remained the same. The total number of caldera earthquakes for the month was 5748, an increase of 550 over October's total. Seismic crises became more common but less intense, occurring on 5, 9, 15, 17, 18, 26, 29, and 30 November. Daily totals of earthquakes on these days ranged from 220 to 538. During the month, there was a linear increase in background seismic activity from about 100-170 events per day. Early in November, earthquakes appeared to be concentrated on the NE part of the caldera, near Tavurvur, but the area around Vulcan, on the W side of the caldera, became active later in the month. The strongest earthquakes had magnitudes of about 3.5.
"The pattern of ground deformation was similar to that seen in the previous 2 months. Dry tilt stations on the NE side of the caldera continued to indicate that the centre of uplift is near the mouth of Greet Harbour. The maximum measured tilt was 43 µrad at a station on the coast of Tavurvur's SW flank. Tilts accumulated gradually during the month at most stations, although an offset of about 10 µrad was recorded at a station on Matupit Island after the seismic crisis of 5 November.
"EDM data from networks newly established in November showed that horizontal movements were near or within noise levels but that slight expansion may have been occurring within about 3 km of the inflationary centre on the NE side of the caldera."
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.