"Seismicity increased slightly in October. A total of 346 events was recorded, compared to monthly totals of 100-200 in the last 10 months. The largest daily event total was 83, recorded on the 24th, when earthquakes occurred in rapid succession for ~2 hours. A similar swarm with a total of 67 events occurred 20-21 October, all with M_L <2. No events were felt, and only five were large enough be recorded by the entire network and located. Two events occurred in the NW and one in the S part of the caldera seismic zone. The remaining two events appeared to originate from the E part of the region bounded by the caldera fault. Ground deformation readings in October showed no significant change."

Geologic Background. 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, RVO.