This report covers activity at Rabaul from June to October 2001. Repeated ash columns appeared during June over Tavurvur, ejected both noiselessly and associated with loud roaring and rumbling noises. The activity was dominated by strong discrete explosions during 1-2 June that showered rock fragments on Tavurvur's flanks. At night, red incandescent lava fragments were visible. The resulting dark, billowing ash columns rose rapidly to 1-1.5 km above the summit before strong winds dispersed them NNW. Significant downwind ashfall affected surrounding areas, including the town of Rabaul. Shock waves from the larger explosions caused windows and doors to rattle.

For several subsequent days, activity was characterized by occasional ash cloud emissions and a handful of moderate explosions that sent dark billowing ash clouds rising ~1 km above the summit. On 6 June, the activity changed to frequent pale-ash clouds, but the roaring and rumbling noises resumed. The noises ceased around 14 June but discrete, moderate explosions continued to send thick, dark ash columns ~1 km above the summit. The larger explosions again showered lava fragments on the N flanks. During 18-19 June, the activity changed to sub-continuous ash eruptions and fewer discrete explosions.

On 20 June a quiet period began, marked by emissions of thin, white vapor. This quiet continued during July and most of August. Mild ash eruptions began on 28 August following an increase in the number and magnitude of low-frequency earthquakes a few days earlier. Subsequently, activity remained low throughout September and October. Small-to-moderate amounts of steam were emitted from the active vent, and low-frequency earthquakes continued to occur, but their number and size were relatively low. No significant ground deformation was detected.

The seismicity associated with the June activity (575 low-frequency events) was about three times less than that of May (over 2,000) (BGVN 26:06). The explosive pattern in June was similar to May, but at a reduced level. Collectively, 27 explosions occurred on 1, 4, 14-17, and 19 June, but three-fourths of these occurred during 14-17 June. Volcanic tremors were recorded on 8, 12, and 15 June. Four high-frequency earthquakes were recorded during the month; one was located outside the caldera while the other three were too small to be located. Ground-deformation measurements were relatively low throughout June.

Reference. Lauer, S.E., 1995, Pumice and ash: a personal account of the 1994 Rabaul volcanic eruptions: Quality Plus Printers Pty. Ltd., Ballina, Australia, 80 p.

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 and F. Taranu, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.