The volcanoes at Rabaul Caldera continued to remain quiet in October. Tavurvur's summit area released bluish white vapors at very low rates; however, the emission rates rose during rainy days at the end of the month. No emissions came from Vulcan.

Only 19 earthquakes were recorded in October. Two of the 13 low-frequency earthquakes originated from Tavurvur while the rest came from either within or just outside the caldera's N sector. The six high-frequency earthquakes took place on the 20th (2 earthquakes), 23rd (2), 26th (1), and 29th (1). Most of these high-frequency earthquakes occurred in the caldera's NE sector (Namanula area). One high-frequency earthquake (ML 1.9, on the 23rd) originated near Tavurvur at about 1 km depth. October ground deformation remained very low.

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: Ben Talai, RVO.