"Seismicity remained at a relatively high level in June (1,570 earthquakes) although there were indications of a decline in activity in late June and early July. Ground deformation measurements in June indicated low rates of deformation.

"Despite the relatively high totals of caldera earthquakes in June, daily earthquake totals appeared to be declining during the last week of the month. During the first 3 weeks, daily totals averaged ~40-50 events, but from the 22nd, the daily average was ~20 events.

"Several small swarms of events occurred, notably in the Greet Harbour and Vulcan-Matupit Island areas. For the month as a whole, the pattern of earthquake locations shows concentrations in these areas. The strongest caldera earthquake in June was an M_L 1.9 event in the Greet Harbour swarm on the 20th.

"Tilt and horizontal distance measurements indicated that small inflationary changes were continuing in the Greet Harbour area in June. The largest tilt changes, 7 and 5 µrad, took place at the N and SE shores of the harbour. The rate of horizontal deformation was 5-10 microstrain/month.

"Levelling measurements carried out on 25 June from a traditionally stable area near the E caldera wall to the eastern shore of Greet Harbour showed maximum uplift of 18 mm (at Sulphur Point) since 8 January 1986. This indicated relatively low rates of uplift (~3 mm/month) for this period. By comparison the maximum uplift rate on Matupit Island from late January to late May was ~10 mm/month, approximately equivalent to pre-crisis rates.

"Tilt measurements for the period late January to early July in the Greet Harbour area showed maximum inflationary tilts of 28 and 11 µrad at Sulphur Point and Matupit Island respectively. These tilt changes indicate rates of tilting reasonably consistent with the uplift rates."

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.