"Overall, a substantial decline in seismicity took place in May. The total number of earthquakes was 8,939, compared to 13,794 in April.

"Following the intense seismic and deformation crisis of 21-22 April, seismicity decayed exponentially and by mid-May the daily earthquake totals were averaging about 215. Small swarms of volcanic earthquakes that occurred on 4 and 27 May were followed by a major seismic and deformation crisis on 29 May. The earthquake swarms affected the NW part of the caldera seismic zone on the 4th and the Greet Harbour area on the 27th, but no significant ground deformation accompanied them.

"The seismic crisis of the 29th was centered near Vulcan; the main event had a magnitude (M_L) of 3.8. An unusual feature of this crisis was that deflation of up to about 30 µrad accompanied the seismicity in the Vulcan area and that apparently aseismic inflation of the same magnitude took place at the head of Greet Harbour. Levelling measurements about 1 week after the crisis indicated maximum uplift of 43 mm at the S end of Matupit Island.

"The main features of ground deformation before the crisis of the 29th were mild steady inflation (10-20 µrad) immediately SE of Vulcan, deflation (up to 40 µrad) and slight subsidence (a few mm) near Rapindik at the head of Greet Harbour, and uplift of Matupit Island (about 41 mm at the SE coast). Horizontal deformation continued unchanged throughout May.

"The effects of the crisis of the 29th suggested direct interplay between different parts of the caldera. It is noteworthy that eruptions occurred simultaneously at Vulcan and Tavurvur in 1878 and 1937, and that disturbances along a line connecting these two centres were also observed at the same time."

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.