"The increase in activity . . . continued through June. During the first half of the month emissions from the summit crater were moderate to strong, consisting of thick white vapour. The emissions increased somewhat to strong thick white vapour in the second half of the month. Grey and blue emissions were also reported on 6, 15, and 21 June. Steady weak red summit crater glow was visible on 6 and 16 June only, compared with consistent steady glow in May until the 23rd. Weak rumbling noises were heard between 2000 and 2300 on 15 June, but these may have been distant thunder.

"Seismic activity in June consisted mainly of sub-continuous low-frequency tremor, with an occasional larger low-frequency earthquake. The RSAM monitoring showed that the seismicity level remained fairly steady throughout the month, with a slight dip in the middle. On a number of occasions, most notably on 9, 20, and 30 June, the activity almost totally stopped for short periods of usually less than an hour. The cause of this is not known. A small number of local high-frequency earthquakes continued to be recorded, although the rate declined during the month.

"On 20 and 21 June there were a number of high-frequency earthquakes with longer S-P times, around 3.5 seconds. Their signals look very similar to those from the earthquake swarms located near Bamus volcano (16 km SW) in 1990 (BGVN 15:2-5)."

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: D. Lolok, R. Stewart, I. Itikarai, P. de Saint-Ours, and C. McKee, RVO.