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