"The state of unrest continued in August at about the same level as in July. The total number of earthquakes for the month was 5,285, compared to 8,938 in May, 5,304 in June, and 4,404 in July. But the total [seismic energy] release was about [2.4 x 10¹⁶] ergs, compared to [1.4 x 10¹⁷] in May, [1.9 x 10¹⁵] in June, and [1.3 x 10¹⁷] in July. [These and subsequent energy release values for Rabaul have been corrected by RVO.]

"Seismicity in August was concentrated in the N half of the caldera seismic zone with a crisis consisting of 628 earthquakes, including a magnitude 3.4 event, on 3 August. Six smaller earthquake swarms took place in the Greet Harbour area, one on 11 August, and five in the period 25-27 August.

"None of the seismic activity was accompanied by any sudden ground deformation changes. Ground deformation measurements showed continuing slow inflation mainly in the Matupit-Greet Harbour area at a similar rate to that in July. The maximum measured vertical uplift at the SE end of Matupit Island was about 35 mm, and expansion across Greet Harbour amounted to about 10-20 ppm.

"Most of the evidence obtained in the past 2 months suggests that the volcano has settled into a fairly steady and linear rate of progress toward the anticipated eruption. In the absence of any unexpected changes, the situation could continue at the present rate for several months to a few years before an eruption occurs."

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.