"Seismicity remained at a relatively high level in May with 1,532 events. As in April, all of the earthquakes were quite small, the largest being an M_L 2.4. In April many shallow events occurred in the middle of the caldera, but May locations show a return to the normal pattern of ring-fault seismicity at depths of 1-3 km.

"Levelling measurements showed 9 mm of uplift at the SE tip of Matupit Island 7-28 May, about half the uplift rate measured in April. Tilt measurements showed small inflationary changes (0-10 µrad) in the Greet Harbour area. Electronic distance measurements showed no significant changes."

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: P. Lowenstein, RVO.