Report on Rabaul (Papua New Guinea) — January 1990
Bulletin of the Global Volcanism Network, vol. 15, no. 1 (January 1990)
Managing Editor: Lindsay McClelland.
Rabaul (Papua New Guinea) Seismicity and deformation decline; unrest since October summarized
Please cite this report as:
Global Volcanism Program, 1990. Report on Rabaul (Papua New Guinea) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199001-252140
Rabaul
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"January marked the end of a period of minor, short-term unrest that started in October 1989 and was comparable to August-November 1988 activity. Seismicity in January (401 recorded events) had declined markedly since December (table 2). The background level ranged from 1 to 15 events/day, compared to 5-40/day in December. Three small earthquake swarms occurred January 1-2 (69 events), 13 (39), and 23 (79) from the N (Greet Harbour) and E (Blanche Bay) sides of the caldera (table 2 and figure 11). The increased rate of ground deformation recorded in December apparently stabilized in January, although a complete survey is required to assess the amount of caldera-wide elevation and tilt change (table 2)."
Table 2a. Summary of monthly seismicity during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.
| Date | Events/Month | Swarms | Magnitude (ML) |
| Jan-Sep 1989 | 100-200 | 0 | less than 1.5 |
| Oct 1989 | 346 | 2 | less than 2.0 |
| Nov 1989 | 546 | 4 | 2.3 and 3.0 |
| Dec 1989 | 886 | 4 | 2.1 and 2.3 |
| Jan 1990 | 401 | 3 | 3.1 |
Table 2b. Summary of local earthquake swarms during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.
| Date | Number of Events (felt) | Location | Magnitude (ML) |
| 20-21 Oct 1989 | 67 | Greet Harbour | less than or = 2.0 |
| 24 Oct 1989 | 83 | Greet Harbour | less than or = 2.0 |
| 12 Nov 1989 | 36 | Greet Harbour | less than or = 2.0 |
| 17-18 Nov 1989 | 138 (5) | Sulphur Creek - Beehive | 2.3 |
| 20 Nov 1989 | 39 (3) | Karavia Bay and Blanche Bay | 3.0 |
| 24 Nov 1989 | 84 | Greet Harbour | less than or = 2.0 |
| 12 Dec 1989 | 52 | Vulcan | less than or = 2.0 |
| 13 Dec 1989 | 121 | Greet Harbour and Blanche Bay | less than or = 2.0 |
| 18 Dec 1989 | 45 (1) | Vulcan | 2.1 |
| 24 Dec 1989 | 76 (1) | Greet Harbour | 2.3 |
| 01-02 Jan 1990 | 69 (2) | Greet Harbour | 3.1 |
| 13 Jan 1990 | 39 | Greet Harbour | less than or = 2.0 |
| 23 Jan 1990 | 79 (1) | Greet Harbour and Blanche Bay | less than or = 2.5 |
Table 2c. Summary of ground deformation at at Matupit Island during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.
| Date | Location | Description |
| Jan-Jul 1989 | Matupit Island | less than or = 10 mm subsidence |
| Jul-Sep 1989 | Matupit Island | no change |
| Sep-Dec 1989 | Matupit Island | less than or = 10 mm uplift |
| Dec-Jan 1990 | Matupit Island | greater than or = 20 mm uplift |
Table 2d. Summary of tilt changes during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.
| Date | Location | Description |
| Jan-Sep 1989 | Greet Harbour | no significant changes |
| Jan-Sep 1989 | Vulcan | less than or = 20 µrads deflation |
| Oct-Nov 1989 | Matupit Island & Vulcan | less than or = 10 µrads inflation |
| Dec 1989 | Sulphur Point and Baluan | 10-20 µrads inflation |
| Jan 1990 | Greet Harbour | greater than or = 10 µrads inflation |
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Figure 11. Epicenters of seismic events at Rabaul, October 1989-January 1990. 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: I. Itikarai and P. de Saint-Ours, RVO.
