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Report on Rabaul (Papua New Guinea) — September 1997

Bulletin of the Global Volcanism Network, vol. 22, no. 9 (September 1997)
Managing Editor: Richard Wunderman.

Rabaul (Papua New Guinea) Tilt recorded after a M 4.7 tectonic earthquake on 25 September

Please cite this report as:

Global Volcanism Program, 1997. Report on Rabaul (Papua New Guinea). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 22:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199709-252140.

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Rabaul

Papua New Guinea

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

All times are local (unless otherwise noted)


An earthquake of M ~4.7 on 25 September, centered ~40 km SSW of Rabaul, caused small offsets on tiltmeters throughout the caldera. A water-tube tiltmeter at Tavuliu on the SW caldera rim showed a large non- recovered offset of 20 µrad to the WSW, possibly due to a small rotational slip of the cliff in this area. The earthquake also seemed to trigger a 300 ppm rise in SO2 from Tavurvur, declining to pre-earthquake levels after a few days. Possibly also related to the shock on the 25th was the only low-frequency seismic event of the month, on 26 September, near Tavurvur. Since an airwave was produced, this event was probably an explosion at shallow levels within the conduit of Tavurvur.

In the immediate area of Rabaul Caldera, seismicity remained low. Only two high-frequency events occurred under or near the caldera. Striking at depths of 2 to 4 km, one occurred on the 20th and the other on the 29th.

Steady glow was observed at night from Tavurvur until 9 September. Roaring and rumbling sounds could be heard during 1-6, 14, 17, and 28 September. Fumes continued to be emitted throughout the month; in dry, warm conditions these appeared as a blue-gray haze and would sometimes cause a sulphurous odor and hazy conditions for several km downwind; in cooler and/or more humid conditions the fumes would appear as a thick white plume.

A water-tube tiltmeter at Sulphur Creek, 3.5 km from Tavurvur, showed a 3.5 µrad inflation of Tavurvur for September. This inflation has been continuing since the 20 µrad deflation caused by the 14 March eruption (BGVN 22:03). The eruptions of 12-13 April, 1 June, 11 July , and 17 August (BGVN 22:04-22:08), caused no significant deflation, and there has been a 16 µrad recovery.

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: B. Talai and H. Patia, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.