The mild Vulcanian activity at Tavurvur continued at a low level through November and December. During 1-14 November the emissions from the 1941 vent consisted mainly of small volumes of thin white vapor. Occasionally, small-to-moderate volumes of pale gray ash clouds were produced. In the early hours of 15 November, the activity changed as continuous, forceful emissions of thick, light-to-dark gray ash clouds became paramount. This pattern of activity was sustained with minor fluctuations until the 18th, accompanied by low booming and roaring noises. Emissions came from the 1941 vent and from a smaller vent on the W flank of the 1995 vent, the latter apparently becoming active on the 15th after a long period of quiescence. Also during this three day period, NW winds blew the ash clouds SSE to heights of 1-1.5 km. After the 18th, except for a mild explosion on 30 November, the low-level activity noted at the beginning of November prevailed once again and continued throughout December until the 30th when several belches of dark gray ash clouds were produced. Once again, these were blown to the SE. The 1995 lava-producing vent remained quiet.

Seismic activity was low throughout both months. About 459 low-frequency (LF) earthquakes were recorded in November but more than half of these (244) were recorded during the anomalous period of 15-18 November. Only 64 LF earthquakes were recorded in December. Both months had lower counts than the 617 recorded in October. Most of the LF earthquakes were associated with Tavurvur's summit activity of ash emissions.

Five high-frequency earthquakes were recorded in November and 11 in December. They were too small to be located in December, but the sequence of arrival times from the few stations that recorded them indicated all but two were from the NE, with one each from the E and SE. Ground deformation measurements showed a slight deflation during November with no significant change in December.

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: I. Itikarai, H.Patia, and F. Taranu, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.