"Generally low levels of volcano seismicity and ground deformation have prevailed since the 3 March crisis. However, two recent major regional earthquakes, both magnitude 7.1 (M_L), caused strong ground movements at Rabaul: intensities were MM V on 10 May and MM VI-VII on 3 July. The 10 May event triggered a swarm of caldera earthquakes. During the following 5-hours, over 100 caldera earthquakes were recorded, mainly from the Vulcan area. The largest was a magnitude 3.7 (M_L) earthquake about 1 hour after the regional event. There was little measurable ground deformation associated with this seismic swarm. No comparable caldera seismic response was detected following the 3 July regional earthquake, suggesting that rates of stress accumulation in the caldera are low. However, a close watch will be maintained for any long-term effects of the present intensified regional seismicity on the Rabaul volcano.

"Monthly caldera earthquake totals were 710 in May and 644 in June. In May, the pattern of seismicity was dominated by a broad zone of activity in the Vulcan area with a clustering of events at the E extremity of the Vulcan Headland. In June, events were scattered around the caldera seismic zone with no strong concentrations. The caldera seismic energy output for the two-month period was about [6 x 10¹⁶] ergs, most of which was related to the seismic swarm induced by the central New Britain earthquake of 10 May.

"Horizontal distance measurements have continued to show gradual rates of change, the maximum about 10 ppm per month in the Greet Harbour area. Levelling measurements carried out between 8 and 10 May showed that uplift rates were low since the previous survey in mid-March. The largest change in this two-month interval was 13 mm at the S end of Matupit Island. Since the March crisis, maximum monthly tilt rates have been generally low, ranging from about 6-17 µrad per month. The area of biggest tilt changes has continued to be around Greet Harbour. In June a higher tilt rate (30 µrad per month) was recorded at the NW shore of Greet Harbour, but the significance of this is uncertain at present."

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.