"Daily totals of caldera earthquakes ranged from 4-163. There was only one seismic swarm during the month, on 25 November, which included 144 recorded earthquakes. The strongest event in the swarm had an estimated magnitude of 2.4 and was felt with an intensity MM III. Most of the earthquakes for the month occurred as discreet events, and on 1, 2, 3, and 16 November, >100 earthquakes were registered. The total number of earthquakes was 1,467 in November . . . . There were 72 locatable earthquakes; most originated from the NW and NE parts of the caldera seismic zone, but a few were located in its SE and SW parts. Levelling measurements on 6 December indicated that the S end of Matupit Island . . . rose ~16 mm since the previous survey on 1 November."

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: I. Itikarai and C. McKee, RVO.