"Seismicity in February was steady at a level similar to January. The total number of recorded caldera earthquakes was 580 (compared to 591 in January). Two small earthquake swarms were detected during the month. A swarm of 53 events on 9 February was tightly clustered immediately E of Vulcan Cone. Another swarm, consisting of 173 events on 23 February, was more diffuse, with earthquakes scattered at the entrance of Greet Harbour. Altogether, 54 earthquakes in February (~9% of the total) could be located. Earthquakes, other than those of the swarms, occurred in the N and NW parts of the annular caldera seismic zone. Routine levelling on 24 February to the S end of Matupit Island showed uplift of ~20 mm since the previous measurement on 4 January. This rate of uplift is consistent with the normal uplift trend for the central part of the caldera."

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 688-m-high asymmetrical pyroclastic 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 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western 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 C. McKee, RVO.