"Activity showed a further reduction in August, and appeared to be at pre-crisis levels. The total number of caldera earthquakes for the month was 236, as compared to 595 in July. The located earthquakes were mostly from the N part of the caldera seismic zone, and an unusual concentration of events was detected 1-2 km N of that zone. Tilt measurements generally showed a continuation of the exponentially decreasing tilts observed following the New Ireland earthquake of 3 July. Steady, slow dilation (up to 10 ppm per month), centred near the mouth of Greet Harbour, continued to be observed by horizontal distance measurements."

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: C. McKee, RVO.