During all of August and until 17 September the mild activity that has occurred at Tavurvur cone since November 1998 continued, consisting only of weak, pale gray ash emissions. These emissions, usually several hours apart, had slightly increased frequency and ash/vapor volume for two periods around 11 August and at the very end of that month. During the reporting interval the volume of ash was small and the ash plumes rose only a few hundred meters above the summit. Three mild explosions occurred on 5, 13, and 15 September and sent dark gray ash clouds to ~1,000 m before they were blown to the NW.

However, between 0915 and 1015 on 17 September, a vent that presumably last erupted in 1941 opened. For about eight consecutive days the new vent produced continuous emissions of dark gray ash clouds, sometimes in puffs. Associated with the ash emissions was the strong odor of sulfur gas. Night observations on 17-18 September showed weak red glow at the mouth of the vent, seemingly associated with the puffs, and weak deep roaring noises were heard. Subsequent to the eight days, the emissions changed to pale gray in color and occasionally to white vapor for short intervals. Three mild explosions occurred on 20, 26, and 28 September; the resulting ash columns rose ~1,500 m and caused significant ashfall on Rabaul Town.

Fluctuating volumes of continuous emissions took place in October from the old 1941 vent that re-opened on 17 September 1999. The emissions fluctuated from very thin white to thick white-gray vapor, and occasionally dark gray ash clouds. The ash clouds rose several hundred meters above the summit and were later blown to the N, NW, and W between 1 and 17 October by variable winds, and primarily to the S and SE after 18 October. Two moderate dark gray ash clouds on 28 and 31 October were released forcefully and rose about 1,500 m above the summit before they were blown NW, resulting in light ashfalls.

The vent that produced the lava flows in 1996 and 1997 (South Vent) showed declining vapor/ash emissions during October. Early in October the vent produced about 4-5 distinct daily ash emissions, but almost none during the last 2-3 weeks. Three mild explosions of dark gray ash columns on 7, 11, and 18 October rose ~1,000 m above the summit before they were blown to the N and NW, causing fine ashfalls. The odor of sulfur was at a very reduced level compared to that during the second half of September, and by 16 October was almost unnoticeable.

The number of low-frequency earthquakes increased to 235 in September over the 165 in August, exceeding the numbers in May (159), June (35), and July (89). A total of 40 high-frequency events were recorded in August, the highest for any month this year, whereas only four occurred in September, and only two of which had epicenters NE of the caldera. Twelve explosions were detected seismically in August compared with three each in June and July. There were a total of 617 low-frequency events, a further increase, in October. All these events originated from the summit of Tavurvur. The bigger events were associated with summit emissions, as observed in the past. Only four high-frequency events were recorded. None of them were big enough to be located, but from the sequence of arrivals from stations that recorded them, they appeared to originate from the NE, outside of the caldera.

Measured deformation of the ground surface within the caldera remained slight as it has since February, although ~5 µrad of tilt accumulated at the water-tube tiltmeter at Sulphur Creek, before the two periods of increased activity in August. Overall, ground deformation remained low with indications of very low levels of long-term caldera resurgence.

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: Ima Itikarai, Kila Mulina, and Steve Saunders, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.