Seismicity remained at a low level during September with 69 events recorded. The four located events were in the Beehives area and N Blanche Bay (E part of the caldera). Measurements on the Matupit Island level line showed subsidence in the Sulphur Creek and Matupit Island areas (N and central-NE caldera) from 23 June to 3 September. The greatest subsidence (11 mm) was recorded at the causeway leading to the island. EDM data did not show any significant horizontal changes during the month.

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: J. Mori, C. McKee, and P. Lowenstein, RVO.