The previous report on the activity of the Tavurvur cone at Rabaul (BGVN 30:08) covered the period through 12 September 2005. According to the RVO report of 10 October 2005 Tavurvur continued to erupt with discrete ejections of light to dark gray ash clouds with high ash content. The ejections occurred at irregular, but sometimes frequent intervals. Discrete, convoluted explosion clouds were also observed. Ash plumes from the eruptive activity rose between 800 to 1,500 m before being blown variably to the E, W, and S during the beginning of the 3-9 October 2005 period and later in that period towards the NW. Ash fell in the downwind areas. Occasionally roaring and rumbling noises were heard. Projections of glowing lava fragments showering the flanks of Tavurvur were visible at night during strong explosions.

Seismic activity was at moderate-to-high levels with most earthquakes associated with ash emissions and explosions. No high frequency explosions were reported and ground deformation measurements showed a general trend towards a slight deflation.

The RVO report dated 28 November 2005 stated that as of 20 November ash emission from the volcano ceased with only very small traces of white vapor being released from the now silent vent and other spots on the summit area. During 21-27 November seismicity was very low and ground-deformation measurements showed some small degree of inflation.

The volcano remained quiet until the middle of January 2006 when activity resumed. Single ash emissions occurred at 0722 on 10 January, 0854 on the 11th, 1638 on the 12th, and 2100 on the 15th of the month (all local dates and times). The emissions consisted of thick gray ash clouds that rose more than 1.5 km above the summit and then drifted E. The ash emissions on the 12th and 15th lasted three minutes. Seismic activity was at a low level with small, low-frequency earthquakes beginning to occur on 7 January 2006. The daily totals fluctuated between zero and seven per day.

RVO reported that during 30 January to 15 February, Rabaul caldera's Tavurvur cone continued to be relatively quiet. Variable amounts of gas were emitted from an active fumarole at the summit area on the upper part of the W flank. An average sulfur-dioxide flux of 200 metric tons per day was recorded and seismicity was at low levels. According to the Darwin VAAC, ash from Rabaul was visible on satellite imagery at a height of ~ 3.7 km (12,100 ft) altitude on 17 February.

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 asymmetrical 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 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W 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: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; Andrew Tupper, Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).