"Seismicity continued to intensify in March. The total number of caldera earthquakes was 8,729, as compared to 8,339 in February. More significant than actual numbers of earthquakes, however, was the continued increase in the proportion of stronger earthquakes. This change appears to be related to the increased incidence of somewhat deeper (2-4 km), more energetic earthquakes in the Vulcan area. The remainder of the caldera seismic zone continued to be active, highlighted by the usual strong concentration of very shallow, relatively low-energy events under the W flank of Tavurvur.

"Major seismic crises took place on 3 and 25 March. The totals of caldera earthquakes on those days were 932 and 726, respectively. The crisis of 3 March involved the E part of the caldera seismic zone, at the mouth of Blanche Bay, and included an event of M_L 5.1. Strong ground deformation was associated with this crisis. Tilts of up to about 50 µrad indicated inflation centred near Sulphur Point at the mouth of Greet Harbour. [Horizontal distance measurements near the time of the crisis were affected by movement of one of the caldera rim base stations.] After this crisis, expansion of Greet Harbour resumed at the same rate as before, about 25 microstrain per month. The crisis of 25 March was centred immediately NE of Vulcan, and the strongest earthquake was an M_L 3.7. Water spouts up to about 3 m high were observed briefly near the NE shore of Vulcan during this crisis. No significant ground deformation accompanied the seismicity.

"Five levelling surveys carried out between late November 1983 and mid-March 1984 showed that the area of maximum measured uplift in the caldera is at the S end of Matupit Island. The rate of uplift in this period was steady at about [50 mm] per month. This compares with an uplift rate of about [8 mm/month] for the period 1973-1983."

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.