"About 2 months of steady mild activity consisting of weak-moderate white vapour emission and low seismicity ended abruptly with a very intense seismic crisis 3-8 November.

"Weak discontinuous tremor began at 1900 on 3 November but showed a dramatic change at 2300 when frequent volcanic earthquakes of large amplitude started. No change was noted in the summit crater emissions. Numerous volcanic earthquakes continued over the next 2 days and a stage-1 alert warning of an increased risk of an eruption was issued on 5 November. At approximately 0200 on 6 November a new period of continuous large-amplitude earthquakes occurred and a stage-2 alert was issued, warning that an eruption was likely in the near future. A series of large events was recorded between 0800 and 1000 while observers near the base of the volcano reported dark summit emissions; one report suggested that they contained incandescent material. A stage-3 volcano alert was issued, warning of the possible development of a full-scale eruption later in the day, and the authorities stood by in case people in the most dangerous areas had to be moved.

"Reports from passing aircraft late in the day indicated that tephra emissions may have recurred in early or mid-afternoon. An aerial inspection at 1700 by an RVO volcanologist failed to confirm the presence of new ejecta on the volcano's flanks, but faint haziness possibly due to earlier ejection of fine dust was noted downwind. The emission plume at the summit was found to be white and normal in size.

"At 0530 on 7 November, shock waves were seen radiating from the summit crater through the emissions, indicating further mild explosive activity. The plume was larger than normal (about 300 m high) and extended 5-10 km from the summit. Seismic activity and emissions remained at high levels until 8 November when they abruptly declined to normal again, allowing a return to a stage-1 alert.

"At the peak of the crisis, over 3,000 volcanic earthquakes per day were recorded, declining to about 1,300 per day between 8-15 November, and 700 at the end of the month. Four A-type events were recorded on 22 November."

Geological Summary. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: C. McKee, RVO.