Report on Rabaul (Papua New Guinea) — June 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 6 (June 1984)
Managing Editor: Lindsay McClelland.
Rabaul (Papua New Guinea) Seismicity declines; deformation continues
Please cite this report as:
Global Volcanism Program, 1984. Report on Rabaul (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 9:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198406-252140.
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"Overall, June was a relatively quiet month. A further decline in seismicity took place. The total number of earthquakes for the month was 5,304, and average daily earthquake totals were steady at about 160. The only perturbation to the steady rate of seismic energy release was a swarm of earthquakes in the Greet Harbour area on 25 June. The total number of earthquakes that day was 450, and the largest event had a magnitude (ML) of 2.8. No significant ground deformation accompanied this seismicity.
"Uplift continued throughout June in the Matupit Island-Greet Harbour area, but at variable rates. Between 23 May and 6 June the S end of Matupit Island rose 43 mm. From 6 to 26 June uplift in the same area was only 10 mm. Inflationary tilt around Greet Harbour persisted but at a reduced rate, with maximum changes of 20 µrad at Sulphur Point and Rapindik. Slight deflationary tilt occurred in the Vulcan-Karavia Bay area, with a maximum change of 20 µrad at Karavia. Horizontal distance measurements across Greet Harbour indicated E-W expansion at the same rate as previously, but a decline in the rate of N-S expansion."
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.