"Activity remained at a low level throughout October, with 205 recorded small-magnitude earthquakes. The daily occurrence rate varied between 0 and 18 events. Only 12 were large enough to be located, distributed in the Vulcan and Greet Harbour areas.

"Tilt and horizontal distance measurements showed insignificant inflation rates. Levelling measurements showed a maximum of 7 mm uplift at Matupit Island from August to October, and 13 mm uplift at the foot of Tavurvur Cone from July to October."

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