The activity at Tavurvur continued as in previous months, with regular Vulcanian eruptions mainly emitting dust with few blocks. These events occurred at intervals of ten minutes to one hour; the longer the preceding interval, the more powerful the eruption.

The overall trend of seismic activity remained low, although short periods of increased activity were observed. During the first two weeks, on 5, 6, 8, and 10 September, bands of discontinuous non-harmonic low-amplitude tremor lasted from a few minutes to about an hour. This activity was coupled with a daily average of 10 discrete low-frequency earthquakes. From 13 September, an increase in low-frequency events became more apparent, with the highest number of 128 recorded on the 18th. This increase continued until 23 September, after which the activity declined to previous levels. Event counts recorded at the KPT seismic station, ~1.5 km W from Tavurvur crater, showed an increase during the month. The total number of events was about 675 compared to about 154 in August. RSAM values also showed a general increase. A few high-frequency earthquakes on 3 September were too small to be located, only seismic stations to the N of the Rabaul Harbor Network recorded them.

A water-tube tiltmeter at Sulphur Creek (3.5 km from Tavurvur) showed a 3.5-mm inflation of Tavurvur for the month. This inflation has been continuing ever since a 20-µrad deflation associated with an eruption on 14 March 1997. In other words, eruptions after 14 March 1997 have lacked significant deflation, and since then cumulative inflation has totaled ~30 µrad.

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: Patrice de Saint-Ours, Steve Saunders, and Ben Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.