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Report on Rabaul (Papua New Guinea) — January 1994


Bulletin of the Global Volcanism Network, vol. 19, no. 1 (January 1994)
Managing Editor: Richard Wunderman.

Rabaul (Papua New Guinea) Seismicity declines further; review of 1993 seismicity and deformation

Please cite this report as:

Global Volcanism Program, 1994. Report on Rabaul (Papua New Guinea) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 19:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199401-252140


Papua New Guinea

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

All times are local (unless otherwise noted)

"Seismic activity declined further in January, with 591 small earthquakes recorded . . . . A number of these earthquakes occurred in small swarms of 30-80 events at intervals of 1-2 weeks. Twenty-three of these earthquakes were located. As usual, they originated from the annular caldera seismic zone, mainly to the NE (Greet Harbour area) and W (Vulcan headland). Routine levelling on 4 January showed that the greatest change since the previous survey (6 December 1993) was only 4 mm uplift at the S end of Matupit Island. Other ground deformation measurements were inconclusive.

"A review of . . . 1993 shows that earthquakes occurred in all parts of the caldera seismic zone, but the W and NE parts were the most active (figure 13). Rates of seismicity and ground deformation were variable (figure 14). Seismicity showed two periods of higher level activity, March-May and October-November. Rates of uplift were low early in the year, but accelerated at the time of a seismic swarm in May. Higher rates of uplift then continued through several months of low seismicity (June-October), until the second period of increased seismicity ended in November. At the end of the year there were indications that the rate of uplift had declined. The greatest measured elevation change in 1993 (~12 cm) is consistent with the long-term rate of uplift between 1973 and 1983."

Figure (see Caption) Figure 13. Map showing seismicity in Rabaul Caldera during 1993. Earthquakes with location error (horizontal and vertical) of
Figure (see Caption) Figure 14. Seismicity and uplift at Rabaul Caldera during 1993. Lines show cumulative uplift (left scale) at deformation stations (inset map); solid bars show daily number of earthquakes (right scale). Courtesy of RVO.

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: Rabaul Volcano Observatory (RVO).