Variable amounts of emergent vapor and minor debris flows at Ulawun were reported during January-March 2003 (BGVN 28:03). Rabaul Volcanological Observatory (RVO) reports, covering much of the period 14 April-5 October 2003, indicated the volcano remained quiet over this time, without emissions from the N-valley vent.

The main summit crater continued to release weak to moderate volumes of white (occasionally white-gray) vapor during 14-29 April, 7-27 May, and 11-18 June. Seismicity was low except for an episode of volcanic tremor between 15 and 19 April. Gas effervescence was reported close offshore of Ulamona Jetty in the second half of April. A slight increase in seismicity was noted between 18 and 23 May.

The period 25 June-22 July was quiet, with no audible noise or night-time glow, and weak to moderate volumes of vapor from the main summit crater. The Volcanic Ash Advisory Center in Darwin reported these plumes as being visible on weather satellite imagery. The plumes appeared white-gray on occasions and were unusually strong bluish white gray over the last three days of the period. Volcanic seismicity was low, with several strongly felt tectonic earthquakes on the night of 3-4 July. A large regional earthquake centered 45 km N of Rabaul affected the area on 16 July, leading to a large tiltmeter offset, which slowly recovered over the following days.

Reports for the period 12 September-5 October indicated that the main summit continued to release weak to moderate volumes of white vapor, with occasional white-gray emissions. Seismicity was low with no significant ground movements.

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: Ima Itikarai, Rabaul Volcanological Observatory, P.O. Box 386, Rabaul, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).