During May, the intracaldera cone Tavurvur continued Vulcanian eruptive activity with minor fluctuations. In the first few days of the month there was a sharp increase, from 15 to 200 per day, in the number of low-frequency seismic events related to ash emissions. However, activity soon returned to more normal levels; between the 11th and 18th there were an average 20 events per day.

Most ash plumes contained relatively low to moderate amounts of ash and rose to less than 1,000 m above sea level. Throughout the first week ash plumes were blown to the SE and SW resulting in fine ashfall at the abandoned village of Talwat and in the Kokopo area. For the remainder of the month winds shifted between N and W resulting in ashfall in villages on the W of the caldera and in Rabaul town were it continued to be a nuisance to inhabitants. There were larger explosions with dark-gray ash clouds that rose to 1.5-3.0 km. From 9 May until the end of the month occasional explosions and roaring noises ranging in intensity from weak to loud accompanied the ash emissions. A weak glow was observed above the crater rim throughout the nights of 7 and 8 May, and incandescent lava fragments were ejected during explosions on the 7th.

Activity continued with minor fluctuations from 19 May to the end of the month. The seismic system recorded a total of 3,265 low-frequency volcanic events during May, a significant increase over the 1,064 recorded during April. Five high-frequency events originating outside the caldera were also recorded. Only two of these were located: one W of the caldera on 13 May, and another to the NE on 29 May.

Ground-deformation data showed that the slow on-going inflationary trend associated with the current phase of the eruption temporarily stabilized in early May only to resume again at the end of the month. These data may indicate that Vulcanian activity is likely to continue.

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: Ben Talai and H. Patia, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.