"Seismicity was at a moderately high level in July with 1,312 events. A seismic swarm in the Sulphur Creek/Beehives/Northern Vulcan area on 12-13 July, included the largest earthquake of the month, an M_L 2.5 event. The 227 events counted on the l2th yielded the highest daily total since May 1985. There was also a small swarm on the 8th, in the Beehives/Vulcan area. Toward the end of the month the seismicity appeared to be declining again with 10-20 events counted/day, compared to 20-100/day for the first half of the month.

"Levelling measurements on 29 July showed that Matupit Island continues to rise slowly. Results indicate that the southern part of the island has been uplifted ~15 mm since the last survey on 28 May. This uplift rate of ~8 mm/month is less than that measured during April and May and is approximately equivalent to pre-crisis rates. Horizontal distance measurements showed small inflationary changes of 5-10 microstrain in July. There were no significant tilt changes measured for the month. In summary, ground deformation rates in July remained low (at pre-crisis levels) and seismicity was declining."

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: J. Mori and C. McKee, RVO.