"Unrest appeared related to the amount of rainfall on the volcano. Heavy rainfall started 26 February and continued until 7 March. After ~6 weeks of very low-level activity (following phreatomagmatic eruptions in January; 14:01) volcanic seismicity strengthened on 28 February, with periods of irregular tremor that increased progressively in amplitude. Following the maximum daily rainfall of 142 mm on 3 March, a strong white vapour plume was observed above the summit crater the next day. On the 5th, the plume had become a large ash-laden column rising to 1,000 m, while seismicity reached 8 times 'normal' amplitude levels. A large grey plume with weak red glow was observed on the nights of 5, 6, and 9 March. Seismicity subsided 6-8 March but returned to a moderately high level during the following week, though with a change in pattern; the tremor periods being replaced with a succession of discrete, emergent events.

"Following another period of strong rainfall (63 mm on the 15th), the moderately strong vapour plume again became ash-laden (grey) until the 22nd. The seismicity showed a different response; it suddenly declined from medium intensity to virtually nothing on the 17th, after a period of strong tremor lasting ~10 minutes. Subsequently, seismicity re-intensified progressively until the 20th when it rapidly rose from medium to high, but dropped suddenly again to virtually nothing on the 22nd.

"At the end of the month, the summit crater was gently releasing a weak to moderate white vapour plume and seismicity was at a very low level. On the 30th, an Ms 5 regional earthquake, 75 km away, triggered continuous tremor lasting the following 2 1/2 days.

"The thick dark column and night glows at the beginning of the month caused some degree of alert amung the local population. An on-site inspection and survey on the 6th showed no significant ground deformation, allowing the release of an information bulletin forecasting phreatic or phreatomagmatic activity similar to early January."

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: P. de Saint-Ours and C. McKee, RVO.