"Seismic activity declined in December when 817 earthquakes were detected . . . . Normal background level is 300-400 earthquakes/month. Sixteen of these earthquakes were located, eight of them with errors (both horizontal and vertical) of <1 km. Locations were all in the Greet Harbour area (NE part of the caldera seismic zone), with the majority at depths <2 km. The percentage of located earthquakes is lower than usual, probably due to the location of the activity. Signals from Greet Harbour were recorded on the 3 or 4 stations nearby, but generally not on enough other stations to allow locations to be calculated. Small earthquake swarms occurred throughout the first half of the month, with the largest on 6 December. The rest of the month had a more normal level of activity, with the largest swarm on 23 December. None of the earthquakes were felt; the largest being a M 1.0 event."

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: C. McKee and R. Stewart, RVO.