"Seismic activity in May was again relatively high, as 1,525 earthquakes were recorded . . . . Normal background level since the 1983-85 unrest has been around 250-350 earthquakes/month. The earthquakes occurred as discrete events and in swarms on 18 and 20 May. The 20 May swarm contained ~680 events, of which 73 were located. Some of the earthquakes were felt, with MM III-IV, and the four largest earthquakes in this swarm all had magnitudes of 3.8. Located earthquakes were equally distributed around the N part of the ring fault. The swarm on 20 May was located near the mouth of Blanche Bay, an area that has not been very active since the 1983-85 crisis period.

"Routine monthly levelling showed an uplift of 21 mm at the S end of Matupit Island between 27 April and 25 May. This levelling was complemented by dry-tilt measurements that showed inflation of the caldera's central portion. During the 20 May earthquake swarm, data from the tide gauge network showed an uplift of 50-60 mm at a station in the centre of the caldera, near the area of inferred maximum deformation."

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: I. Itikarai, P. de Saint-Ours, R. Stewart, and C. McKee, RVO.