"Although the monthly total number of earthquakes was higher than in any of the previous months, the percentage of stronger earthquakes (those with sufficient energy to be recorded by five or more of the nine harbour seismic stations) was lower than in any of the previous months (table 1). The distribution of earthquakes was similar to that in November with the highest concentration in the NE part of the caldera near Tavurvur, but most of these were small, recorded by only the three closest seismic stations.

Table 1. Earthquakes at Rabaul, September-December 1983. 'Stronger' events were recorded by five or more of the nine harbour stations.

"The maximum measured tilt was 17 µrad, by the station on the coast, on Tavurvur's SW flank. Tilts again accumulated gradually throughout the month, and no offsets were associated with any of the minor swarms of harbour earthquakes. Ground deformation rates were less than half those in November, and indications are that there was considerably less energy release in December than in any previous month during the current crisis.

"It is still too early to determine whether the decrease of unrest in the Rabaul Caldera in December is just temporary, or marks the onset of a longer-term improvement in the situation."

Further Reference. McKee, C.O., Lowenstein, P.L., de St. Ours, P., Talai, B., Itikarai, I., and Mori, J., 1984, Seismic and ground deformation crises at Rabaul Caldera: prelude to an eruption?: BV, v. 47, p. 397-411.

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.