"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 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 C. McKee, RVO.