"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 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: C. McKee and R. Stewart, RVO.