Seismicity declined to its lowest level since the 1983-85 unrest; only 34 caldera earthquakes were recorded in August, compared to 80-150/month in the last seven months. There were periods of several consecutive days with no events and only three small events were recorded 13-20 August. The six located earthquakes were in the Greet Harbour and Karavia Bay areas (NE and E parts of the caldera).

Dry tilt readings on Vulcan Headland . . . showed a steady deflation of 20 µrad since May. In Greet Harbour, usually the most active area, tilt readings remained virtually unchanged although levelling revealed a slight subsidence of 10 mm at Matupit Island since the end of June. This slight deflationary trend was consistent with EDM results.

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: C. McKee, P. de Saint-Ours and P. Lowenstein, RVO.