"Seismic activity . . . remained low during October, when 588 earthquakes were recorded . . . . The highest daily total was on the 22nd, when a brief swarm of 149 events was recorded. Three or four of the largest were felt in Rabaul. Over half of the 24 earthquakes located in October were from the swarm, 1-2 km NE of Vulcan on the W side of the caldera ring-fault system. The rest of the located events were scattered around the NW and E sides of the ring fault. Levelling measurements between 8 August and 16 October showed a small amount of uplift (4 mm) at the S tip of Matupit Island. Another round of levelling measurements on 9 November indicated uplift of 13-16 mm along the SE coast of Matupit Island. This uplift may correlate with the seismicity on the 22nd. No significant changes were shown by EDM or dry-tilt measurements."

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: I. Itakarai, R.C. Stewart, and C. McKee, RVO.