Report on Rabaul (Papua New Guinea) — February 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 2 (February 1985)
Managing Editor: Lindsay McClelland.
Rabaul (Papua New Guinea) Seismic activity, ground deformation at low levels
Please cite this report as:
Global Volcanism Program, 1985. Report on Rabaul (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 10:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198502-252140.
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"Activity at Rabaul remained at a low level in January and February. Monthly caldera earthquake totals were 1,297 and 1,672, and seismic energy outputs were about [4.4 x 1014] and [3.4 x 1015] ergs. In January the strongest earthquake was an ML [2.3] event on the 25th, and only six events had magnitudes exceeding 2.0. The strongest earthquake in February was an ML 3.0 event on the 24th, and 13 events had magnitudes exceeding 2.0. January's seismicity was concentrated in the Tavurvur and Sulphur Creek-Matupit Island areas, while in February, seismicity was strongest in the Vulcan-Matupit Island area and at the head of Greet Harbour.
"Mild ground deformation continued, with changes being above uncertainty levels only in the Greet Harbour area. The largest tilt and horizontal distance changes were of the order of 15 µrad and 10 ppm per month. Levelling completed at the end of January showed maximum uplift since early December of only 15 mm at the S end of Matupit Island. This is only slightly higher than the average pre-crisis uplift rate from 1971-83."
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.