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) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 10:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198508-252140
Papua New Guinea
4.2459°S, 152.1937°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."
Geological Summary. 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 asymmetrical 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 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W 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.