A short eruption of ash and blocks occurred at Tavurvur during July. The build up prior to this eruption was similar to the two previous Strombolian phases (1 June and 12 April); those build ups were characterized by relatively low-pressure, low-ash emissions and occasional moderate-to-large explosions.

The eruption began on at 2318 on 11 July and peaked at about 0700 on 12 July with a corresponding RSAM value of 450 units. Activity then dropped and fluctuated between 90 and 240 RSAM units; later, at about 2230 on 12 July, a peak of 420 RSAM units occurred. After 0200 on 13 July activity declined to a background level of 30 RSAM units.

The more vigorous periods of eruption included explosions with gray ash clouds rising 2-3 km above the summit and ejected blocks thrown ~1 km from the vent. The ash plumes blew N and NE, and fine ash fell downwind. Later, during 14-31 July, Tavurvur issued continuous gentle emissions of thin white and blue vapor. No lava flow was emplaced during the 12 July eruption. As a result, the volume of material erupted was very small, ~0.3 x 10⁶ m³.

Seventy-five low-frequency earthquakes (mostly explosion events) were recorded during the month. Most of these occurred during the eruption on 11-12 July with daily counts of 29 and 43, respectively.

The electronic tiltmeter at Matupit (2 km W of Tavurvur) accumulated 12 µrad of WNW-down tilt from the beginning of July until the eruption on the 12th (i.e. radial to an inflation of the shallow caldera magma reservoir). After the eruption, the tilting pattern changed to WSW (i.e. radial to a possible inflation between Rapindik and the north of Tavurvur). The eruption itself caused virtually no significant tilting. No clear trends were shown by any of the other tiltmeters further away from Tavurvur. These small ground deformations appear in accord with the eruption's short duration, low energy, and small volume.

After technical problems, COSPEC measurements resumed and during the first four days of measurements, 2- 5 July, the SO₂ output was 660-1,380 metric tons/day (t/d). The SO₂ flux then decreased during 5-10 July (~200 t/d), increasing again on 11 July (420 t/d). It remained high until the eruption on 12 July (~1,000 t/d) and continued so during the next three days. After that it decreased to ~600 t/d where it remained until the end of the month.

In overview, the observations and measured parameters all indicated that the 11-12 July eruption was small compared to the six Strombolian phases since December 1995.

Further Reference. Lauer, S.E., 1995, Pumice and ash: a personal account of the 1994 Rabaul volcanic eruptions, Quality Plus Printers Pty. Ltd., Ballina, Australia, 80 p. (ISBN 0 646 26511 3).

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: B. Talai and H. Patia, Rabaul Volcano Observatory (RVO), P.O. Box 385, Rabaul, Papua New Guinea; Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin, NT 0801 Australia.