Report on Rabaul (Papua New Guinea) — January 1995
Bulletin of the Global Volcanism Network, vol. 20, no. 1 (January 1995)
Managing Editor: Richard Wunderman.
Rabaul (Papua New Guinea) Fumarolic activity with little seismicity or deformation
Please cite this report as:
Global Volcanism Program, 1995. Report on Rabaul (Papua New Guinea) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 20:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199501-252140
Papua New Guinea
4.271°S, 152.203°E; summit elev. 688 m
All times are local (unless otherwise noted)
"The two active cones . . . showed only fumarolic activity in January; the activity at Tavurvur declined during the month. Seismic activity was low throughout January, although small volcanic earthquakes continued to be recorded. Ground deformation was also low. The Stage IV alert for Rabaul was canceled on 5 January, and reverted to Stage I.
"There was no further explosive activity at Tavurvur following the last Vulcanian explosion on 23 December 1994. Only fumarolic activity has been observed since that date. The amount of vapour released declined gradually during January. Up until the middle of the month there was still a distinct plume rising to several hundred meters above the crater. By the end of the month, however, there was only intermittent vapour release, with an occasional small plume. No fumarole temperatures have been measured.
"Although the external shape of Tavurvur appears to be little different, the internal crater structure was totally changed by the 1994 eruption. Almost all of the crater features produced by eruptions in 1878, 1937, 1941, and 1942 were destroyed and replaced by a single shallow bowl-shaped crater. The low point of the crater is still on its W side, where the 1994 lava flow exits. Inside the main crater, slightly off-center to the SE and taking up perhaps a third of the crater floor, is a single cone, which was built up during the later stages of the 1994 eruption. The cone's crater is almost hemispherical and the vent is no longer visible due to the accumulation of debris on the crater floor. During January, as crater temperatures dropped, both the inner and main craters became very brightly colored with sulphur and other precipitates. Fumarolic activity was concentrated in the inner crater, although there were some fumarolic areas in the main crater. There was also a distinctive blue-vapour fumarole about halfway down the 1994 lava flow, rising through the flow. This appears to have been active since the very first minutes of the eruption. Vulcan continued to exhibit only very weak fumarolic activity from both the 1937 and 1994 craters.
"Earthquake activity in January consisted of both high- and low-frequency events. There were very few high-frequency earthquakes (28) continuing the pattern of low activity seen since the eruption. Located earthquakes tended to be in either of two distinct zones. The first is shallow (1 km) and to the S of Vulcan, extending into Karavia Bay. The second is deeper, 3-5 km, and is located under the NE edge of the seismic network, with epicenters between Namanula Hill and Nodup on the NE coast. Earthquakes in the first zone are undoubtably due to structural readjustment following the Vulcan eruption. The cause of the earthquakes in the other zone is not yet known; it is not part of the ring fault. The last occurrence of earthquakes in this region, May 1992, was followed by an increase in seismic activity and ground deformation.
"At the end of December the number of volcanic earthquakes associated with Tavurvur dropped to a very low level. Starting on 29 December, however, a new type of low-frequency earthquake was recorded. These were only seen on three seismic stations in the N part of the network and, because of their emergent onset and variable waveforms, it was impossible to locate them; the most likely locations are in the NE portion of the network, perhaps even outside it. These events continued through January at an average rate of 14/day although >25/day were recorded on 12-14 and 30 December. Only a few Tavurvur volcanic earthquakes were recorded during the month. This level of activity is very low compared to that during the eruption and is probably due to a minor readjustment in the caldera's plumbing system - there may be a connection with the Namanula/Nodup high-frequency earthquakes.
"Electronic tilt data showed that the amount of ground deformation declined in December. The station on Matupit Island, which seems to be the most stable, showed that deflation following the eruption has gradually declined to ~0.5 µrad/day in the second half of January, with the center of deflation to the S of Matupit. The peak deflation rate, shortly after the equipment was installed at the beginning of October, was >3 µrad/day. Other ground deformation data confirm this trend.
"Following a recommendation from the RVO, the East New Britain Disaster Committee declared on 5 January that the level of alert for Rabaul was being reduced from Stage IV to Stage I. This was based on the cessation of volcanic activity at both Tavurvur and Vulcan and the continuing decline in ground deformation and seismicity, indicating that no resumption of eruptive activity was likely. Although no longer a hazard to what remains of the town of Rabaul, the two cones are still off-limits to the general public.
"Rabaul has remained under a State of Emergency, with access to the town controlled, because mudflows and flooding are still perceived to be a serious hazard. To date the rainy season has been unusually dry without any persistent periods of heavy rainfall. Mudflows and flash floods have already caused some damage and the roads into Rabaul are washed out after even slight rain, but there is great potential for more damage. Large amounts of ash still remain in the high ground surrounding Rabaul and there are large areas of potentially unstable land that have been exposed by the destruction of vegetation. The rehabilitation of areas affected by the eruption continues in a haphazard fashion. With the end of explosive activity, ashfall stopped being a nuisance and this has accelerated the rehabilitation of the Nonga area on the N coast. In Rabaul itself, most of the activity consists of clearing-up operations. However, a number of businesses have been re-established, the port is open and taking large ships, and one of the hotels has re-opened."
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: H. Patia, R. Stewart, and B. Talai, RVO.