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Report on Rabaul (Papua New Guinea) — September 1983


Scientific Event Alert Network Bulletin, vol. 8, no. 9 (September 1983)
Managing Editor: Lindsay McClelland.

Rabaul (Papua New Guinea) Earthquake swarms and uplift at intracaldera cone

Please cite this report as:

Global Volcanism Program, 1983. Report on Rabaul (Papua New Guinea) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 8:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198309-252140


Papua New Guinea

4.271°S, 152.203°E; summit elev. 688 m

All times are local (unless otherwise noted)

An exponential increase in seismic activity in Rabaul caldera began in late August and culminated in an intense crisis with 621 earthquakes on 19 September. The strongest event had an ML of 4.2. Since then seismicity has remained high at 40-120 events per day and has included several minor crises. The total number of caldera earthquakes in September was 2,135, a significant increase over the previous highest monthly totals of 1,170 and 1,079 in January and March 1982.

"The earthquakes have been concentrated at depths of 0-3 km near Tavurvur Volcano, a small post-caldera cone on the E section of the elliptical caldera bounding fault, but other sections of this fault have also been seismically active (07:8-9).

"Tilt measurements showed distinct uplift centered 1.5 km S of Tavurvur. Uplift commenced in early September in relation to increasing seismicity. A sharp tilt change of up to 49 µrad accompanied the seismic crisis of 19 September, but tilt rates have since returned to normal. The depth and increase in volume of the source of ground deformation are estimated to be about 1.7 km and 1.9 x 102 m3."

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 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.