Report on Rabaul (Papua New Guinea) — May 1988
Scientific Event Alert Network Bulletin, vol. 13, no. 5 (May 1988)
Managing Editor: Lindsay McClelland.
Rabaul (Papua New Guinea) Seismicity increases slightly
Please cite this report as:
Global Volcanism Program, 1988. Report on Rabaul (Papua New Guinea). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 13:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198805-252140.
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"Seismicity increased somewhat in May, reaching a monthly total of 187 caldera earthquakes. This is the highest monthly earthquake count since late 1986. However, this total is within the range of normal inter-crisis levels and the events were all of small magnitude (ML <2). There were 25 locatable events in May, which were distributed in the N half of the caldera seismic zone.
"No significant changes were observed in tilt and electronic distance measurements. However, the trend of subsidence shown by data from tide gauges off Vulcan and Matupit Island may have flattened, and levelling results obtained on 6 and 19 May indicated slight uplift of Matupit Island (maximum of 14 mm since 10 March)."
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: H. Patia and P. Lowenstein, RVO.