Report on Rabaul (Papua New Guinea) — August 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 8 (August 1985)
Managing Editor: Lindsay McClelland.
Rabaul (Papua New Guinea) Seismicity declines to pre-crisis levels; tilt slows
Please cite this report as:
Global Volcanism Program, 1985. Report on Rabaul (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 10:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198508-252140.
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"Activity showed a further reduction in August, and appeared to be at pre-crisis levels. The total number of caldera earthquakes for the month was 236, as compared to 595 in July. The located earthquakes were mostly from the N part of the caldera seismic zone, and an unusual concentration of events was detected 1-2 km N of that zone. Tilt measurements generally showed a continuation of the exponentially decreasing tilts observed following the New Ireland earthquake of 3 July. Steady, slow dilation (up to 10 ppm per month), centred near the mouth of Greet Harbour, continued to be observed by horizontal distance measurements."
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.