The Rabaul Volcano Observatory (RVO) reported that a large, sustained Vulcanian eruption began at Rabaul at about 0845 on 7 October 2006 (BGVN 31:09). A further point regarding that eruption, absent from our previous report, was that some members of the Volcanic Clouds Group (a listserv discussion group) conducted significant observations and initial modeling of the 7 October eruption clouds, including mapping the cloud's sulfur dioxide content and making forecasts of their dispersion. In the Volcaniccloud listserv discussions of the 7 October clouds, Andrew Tupper noted the following: "The cloud was at 16 km (upper troposphere/lower stratosphere) when it passed over Manus on its way NW . . . . However, the north/northeastern parts were initially higher . . . , with the eastward bit clearly stratospheric. There were multiple flights under the cloud over Micronesia for [sic] that reported that was no ash or smell?this puts a lower boundary (~ 10 km) on the cloud, consistent with our view that the bits at cruising levels had gone to the SE."

Since that event and in reference to the time interval for this report, 4 November to early December 2006, RVO has noted that activity continued at Tavurvur at varying intensities. The largest event in the reporting interval took place at 0715 on 14 November 2006; Tavurvur produced a large explosion that rose several kilometers above the cone.

During 4-13 November, mild eruptive activity continued at Tavurvur, with occasional small-to-moderate ash emissions continuing and blowing to the SE. An emission on 11 November consisted of thick white vapor accompanied by occasional small-to-moderate ash clouds that drifted variably to the SE, S, and NW and resulting in fine ash fall downwind. On 12 November the emission was blown W and NW, and on the morning of 13 November the ash cloud drifted N of the volcano.

An explosion occurred at Tavurvur at 0715 on 14 November 2006, accompanied by a thick ash cloud that rose to about 2 km above the summit before drifting NW. The explosion showered the flanks of the volcano with lava fragments, some of which fell into the sea. Fine ash fall occurred at Rabaul Town areas and downwind to the Ratavul and Nonga areas. Continuous ash emission followed the explosion. Seismic activity continued at low levels; however, high-frequency earthquakes continued to occur within the Rabaul caldera. After the large explosion on 14 November, mild eruptive activity continued at Tavurvur, consisting of continuous thick white vapor accompanied by pale gray to gray ash clouds that rose ~ 1.5 km above the summit before drifting variably S and E of Tavurvur. During 16-17 November, continuous thick white vapor accompanied by pale gray ash clouds rose to about 2.5 km above the summit before drifting variably to the NW and E with fine ash falling on settlements downwind, including Rabaul Town. One high-frequency earthquake occurred on 16 November.

Mild eruptive activity continued at Tavurvur during 18-20 November. On 18 November and on the morning of 20 November continuous gray ash clouds rose less than 200 m above the summit before being blown N and NW. Fine ash continued to fall on villages downwind including Rabaul Town. Activity on 19 November consisted of emission of thick white vapor only, accompanied by roaring noises heard between 1130 and 1400.

Quiet generally prevailed at Tavurvur during 20-23 November. Emissions then consisted of thick white vapor accompanied by a small amount of pale gray ash clouds. On 21 November the emissions accumulated in the atmosphere around the caldera causing haze, and on 22 November the emissions rose less than 1,000 m above the summit before drifting W. Fine ash fell on villages downwind. On the morning of 23 November the emission consisted of white vapor rising more than a kilometer above the summit before drifting E.

On 26 and 27 November the activity consisted of gentle sporadic emission of subcontinuous, gray to pale gray ash clouds of varying thickness. The ash clouds drifted NW to W resulting in fine ash fall downwind. From November to 1 December the emission consisted of pale gray to dark gray ash clouds being released more forcefully. The ash clouds rose less than 200 m above the summit before drifting E. On the morning of 2 December the emission consisted thick white vapor and pale gray ash clouds that rose about 2 km before being blown ENE. On 3 December thick pale gray ash clouds that rose about 1 km above the summit were emitted. The ash clouds drifted NE in the morning and then slightly to the W in the afternoon. On the morning of 4 December the ash cloud rose about 2 km before drifting E. Fine ash fall occurred in downwind areas. There was no glow from the volcano visible at night. From late morning to the afternoon of 4 December the activity consisted of emission of thick pale gray ash clouds that rose about 500m above the summit before drifting NW. In the morning of 5 December the ash cloud rose 200 m before drifting E. By mid-morning the ash clouds were rising about 1 km above the summit before drifting NNW, and during the early afternoon the ash clouds drifted briefly to the E and then S before going back to the E by late afternoon. On the morning of 6 December the ash cloud rose about a km before drifting N-NW. The emission was accompanied by loud roaring noises. Fine ash fall occurred in downwind areas including Rabaul..

There was no significant deformation until 10 December. The RVO reported that loud and continual roaring was present from 8 December 2006 until the morning of 9 December, when the roaring became intermittent. The roaring ceased on 10 December and at that time parts of the caldera underwent a rapid ~ 1 cm uplift. On 11 December the volcano was quiet with very little fume. At 0400 on 12 December, a loud explosion occurred with an airwave which shook houses in Rabaul. This event generated a billowing gray column that rose to a maximum of 1,000 m before being blown to the E. Following the 12 December explosion subsidence returned the site's level to that of 9 December. Seismic activity continued at low levels. No high frequency earthquake was recorded.

Table 5 shows the MODIS thermal anomalies observed during 22 October-12 December 2006 (see BGVN 31:09 for earlier October anomalies).

Table 5. MODIS thermal Anomalies for Rabaul volcano for 24 October through 12 December 2006. Courtesy of the Hawai'i Institute of Geophysics and Planetology.

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: Steve Saunders and Herman Patia, Rabaul Volcanological Observatory (RVO), Department of Mining, Private Mail Bag, Port Moresby Post Office, National Capitol District, Papua, New Guinea; Andrew Tupper, Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Darwin, Australia; National Aeronautics and Space Administration Earth Observatory (URL: http://earthobservatory.nasa.gov/NaturalHazards/); HIGP MODIS Thermal Alert System, Hawai'i Institute of Geophysics and Planetology (HIGP), University of Hawaii at Manoa, 168 East-West Road, Post 602, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Volcanic Clouds Group (URL: http://groups.yahoo.com/group/volcanicclouds/).