"Explosions at Tavurvur were mostly mild with emission clouds rising slowly to ~1 km above the crater at intervals of ~5-15 minutes. Seismic activity was slightly elevated on 1-2 March, but then decreased sharply in accord with weaker visible activity. The activity remained low for 24 hours then started to increase at a steady rate until it peaked on the 6th. Activity decayed the following day, but then began a gradual recovery that continued until 14 March. The explosions continued at intervals of ~5-15 minutes with ash emissions lasting 2-5 minutes. On 15 March a slight increase in seismic activity occurred as indicated by larger and more frequent explosion earthquakes, although visible activity appeared unchanged. Seismicity peaked on the 19th and then declined slightly over a period of ~48 hours. During the next 10 days the activity showed minor fluctuations but on average there were ~6 events/hour. On 30 March at 0805 and 2034 two strong explosions occurred. Dense ash clouds rose ~3 km above the crater and the flanks of Tavurvur were showered with lava fragments. These explosions signified a dramatic change in the pattern of activity as the frequency of explosions dropped markedly. The intervals between explosions sometimes lasted several hours.

"Aerial inspections of Tavurvur and Vulcan were conducted on 6, 13, and 21 March. The active crater at Tavurvur was bowl-shaped. On two occasions (6 and 21 March) there appeared to be an ash-mantled lava mound on the floor of the crater. At the NW and SE edges of the mound were a number of small vents (~1-2 m wide). These vents were aligned roughly in two arcs, which might represent small fissures. Between eruptions some vents emitted blue vapour. When inspected on 14 March, three rubble-covered vent areas were noted on the S, E, and NE parts of the crater floor. Low ridges of ash separated these vents. Weak fumaroles were present on parts of Tavurvur's main crater, especially on the N Wall. Fumarolic activity was also noted on the 1994 lava flow.

"Apart from the seismic activity related to events at Tavurvur, which were basically low-frequency explosion earthquakes, overall seismic activity of Rabaul Caldera was very low. Only five well-located high-frequency earthquakes were recorded (compared to 4 in February and 28 in January). Three occurred outside the caldera and the other two were under Tavurvur. The electronic tiltmeter at Matupit Island continued to show a trend of slow deflation of the caldera.

"Vulcan continued to exhibit only weak fumarolic activity at the W base of the 1994 crater. Hot springs along the N shore yielded temperatures of ~100°C. Rabaul continued to be under a State of Emergency with access to severely affected areas being controlled because of the risk of mud flows and flooding. Since the eruption started in September 1994, only one death was reported related to flooding."

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, RVO.