"A very low level of activity prevailed in April. The total number of caldera earthquakes in April (1041) is the lowest monthly total for the whole crisis period (starting September 1983). The seismic energy output was only about 6 x 1013 ergs and the strongest caldera earthquake was only M_L 2.1. Seismicity was concentrated in three areas within the caldera seismic zone: at the entrance to Blanche Bay, around the Vulcan headland, and in the NE part of Greet Harbour.

"Rates of ground tilt were barely above noise levels. The biggest tilt changes, about 6 µrad, were recorded at stations around Greet Harbour. Maximum rates of horizontal deformation continued to be about 10 ppm per month, near the entrance to Greet Harbour."

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.