On 11 May a Strombolian eruption took place at Tavurvur. Until early in May weak to moderate explosions occurred every few minutes and generated pale to dark-gray ash-and-vapor clouds that rose ~ 400-1,000 m before drifting 15-20 km downwind (mostly SE, S, and SW). Large incandescent ejecta were observed at night and roaring noises were heard from as much as 15 km away (BGVN 21:04).

Visible eruptive activity began to change mid-afternoon on 9 May. Vapor emissions reached ~1,500 m and seismicity increased to a peak around 2200, when a series of strong explosions started. By about 0800 on 10 May, the emissions were sub-continuous and explosions sent ash clouds ~2-2.5 km above the vent. The activity declined through the afternoon. Later that day the emission column was ~1.2-1.5 km high, with occasional explosion clouds rising 1.9 km. Shortly before midnight, explosions were occurring at intervals of ~5 minutes.

A moderate increase in activity began at midnight on 11 May. By 0245 it changed to Strombolian mode as explosions were occurring every 30 seconds, with increasing frequency and strength. Large bolts of lightning flashed through the growing eruption column. Slabs of lava ~10-15 m in diameter were ejected hundreds of meters above the vent, and meter-sized blocks were landing on the shore ~1 km from the vent. By 0300 the explosions and the lightning were almost continuous. The eruption column was a constant stream of incandescent lava fragments rising at least 400 m. There was a spontaneous evacuation of some people from nearby Matupit Island. Strong air-shock waves from the explosions were felt within a few kilometers from the summit. Irregular and continuous tremors were recorded, but observers noted that the shaking was due to the blasts and not to earthquakes.

Seismicity peaked at 0315. Within minutes the activity declined, the streaming of ejecta stopped, and the time between explosions increased to 30 seconds. By 0400 the activity had returned to the level observed on 10 May. At 0438 the first of a series of strong explosions, at irregular intervals of 10-40 seconds, sent incandescent ballistic blocks 1.5 km from Tavurvur. The last explosion, at 0728, generated and ash cloud that rose ~2.3 km.

During the following day a few large explosions occurred, but their frequency and strength were declining. The emissions were commonly white and blue vapors with occasional ash. By the end of 12 May the explosions stopped and seismicity consisted of irregular tremor. This type of activity persisted for 2-3 days, until 15 May when explosive activity resumed.

Several phases of intensified activity took place during the following weeks, but these were considerably less intense than that of 10-11 May. Seismicity remained weaker than during the previous five months (figure 26).

Figure 26. Seismicity at Rabaul for the period December 1995-May 1996 with detail over the days of peak activity in May. Courtesy of RVO.

A new electronic tiltmeter was installed on 30 April at Matupit Island, ~2.5 km WSW of Tavurvur. It initially measured moderate WNW downward tilt. This continued until 3 May when the pattern reversed and ESE downward tilt began. On 8 May, after accumulating ~10 µrad of rotation, the tilting pattern again changed and the instrument recorded WNW downward tilt. The WNW downward tilt that started on 8 May was probably related to the 9 May activity. The WNW downward tilt continued until 20 May, with rotation reaching up to 16 µrad. From 20 to 30 May the downward tilt returned to ESE and gradually decreased to zero.

The available COSPEC measurements showed a decline in SO₂ emission rate from the range of ~500-900 metric tons/day (t/d) at the beginning of May to background values of a few hundred tons per day during 8-15 May. At the end of the month the emission rate increased to ~800 t/d. Although the 8-15 May data failed to portray any flux increases associated with the 10-11 May eruption, later, on 18 and 26 May, peaks in SO₂ emissions correlated with some less dramatic periods of enhanced eruptive activity.

A total of 3,993 explosion earthquakes was recorded during May. Episodes of volcanic tremor numbered 106; more than 90% of these tremors took place during the 10-11 May eruption. Four high-frequency earthquakes were recorded during the month. Two of these were within the zone of defined by 1994 caldera seismicity.

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: D. Lolok and C. McKee, Rabaul Volcano Observatory (RVO), P.O. Box 385, Rabaul, Papua New Guinea.