Tavurvur's two-month-long eruption continued in February with weak to moderate explosions every few minutes. At close range, roaring and detonation sounds could be heard. Pale to dark gray ash and vapor clouds rose ~400-1,000 m above the crater rim and formed a plume 10-15 km long. The plume generally trended SE over the sea, but occasionally it moved NW over Rabaul Town. At times, ballistic blocks were ejected as far as the outer slopes of Tavurvur's low cone. Sprays of incandescent lava were occasionally seen at night. There were no emissions from Vulcan.

During February, seismicity reached its highest level since the current phase of eruptive activity began on 28 November, 1995 (BGVN 20:11/12). A total of 5,212 eruption-related seismic events were recorded in February, which compares to 3,850 in January, and 1,283 in December. Seismicity peaked in mid-February, declining slightly during the second half of the month. February earthquakes consisted of 4 short-duration volcanic tremors, 5,187 explosion earthquakes, and 21 high-frequency earthquakes. The first two groups of events were directly associated with Tavurur's eruptive activity; 707 of the explosion earthquakes had a distinct air-wave phase recorded at distant seismic stations.

High-frequency earthquakes chiefly occurred in two main time intervals of dissimilar duration. The first interval included 10 events and occurred during 5 minutes on the 10th. The largest event had a magnitude (M_L) of 3.1 and was felt in Rabaul Town with a Modified Mercalli intensity of III. The second interval included nine events and occurred over four consecutive days. Except for one earthquake on the W side of the caldera seismic zone, all others were located immediately NE of the caldera.

Ground deformation measurements indicated slight inflation. Between 1 February and 1 March, just W of Tavurvur (Greet Harbor area), tilt amounted to ~15 µrad. In the second half of February, on the opposite side of the caldera (the SW, in vicinity of Vulcan), tilt amounted to ~5 µrad.

Reference. Lauer, S., Pumice and ash: a personal account of the 1994 Rabaul volcanic eruptions, Quality Plus Printers, Ltd., Ballina, NSW, Australia, 1995.

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: Ima Itikarai and Ben Talai, Rabaul Volcano Observatory, P.O. Box 386, Rabaul, Papua New Guinea.