"Seismic activity declined in June, when 480 earthquakes were detected, compared to >1,000 in each of the previous three months. This represents a return to normal background levels of activity. Of these earthquakes, 25 were located, 16 with horizontal and vertical errors <1 km. Located events were centered mainly around the N and NW parts of the caldera ring fault. The only significant swarm activity was 2 June, with 121 earthquakes detected; 11 were located. Located events were scattered around the NW part of the ring fault, with the majority between Vulcan and Matupit Island. The largest earthquakes were felt in Rabaul with intensity II-III. In the two weeks after this swarm, most of the earthquakes were in the region of Greet Harbour or Matupit Island. By the middle of the month, activity had declined to almost insignificant levels. Small earthquake swarms in Greet Harbour during 25-28 June were not recorded on enough stations to be located.

"Routine monthly levelling on 6 July showed that uplift of as much as 20 mm had taken place at the S end of Matupit Island since the 26 May survey. Uplift of 21 mm had been measured between the previous two surveys (27 April-26 May). Tilt measurements showed changes of 15-20 µrad at the S tip of Matupit Island and at Sulphur Point on the E side of 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: D. Lolok and C. McKee, RVO.