"Seismicity continued to decrease in December to a total of 131 recorded earthquakes. Events averaged 4/day with highest counts at 12 on the 22nd and 23rd. Only three events could be located; two at the SW tip of Matupit Island and one near the S part of Blanche Bay's entrance. Ground deformation rates remained low. At Greet Harbour's entrance, EDM data showed continued slight expansion (8 ppm) and tilt stations in this area showed inflationary changes of up to 6 µrad."

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: H. Patia and P. Lowenstein, RVO.