"Intriguing seismicity, possibly indicating an eruption in the near future, continued in April and included periods of volcanic tremor. Amplitudes of discrete events were generally low, although a degree of cyclicity in amplitudes was apparent, with a period of about 8-11 days. Daily earthquake totals increased from about 600 to about 1,500.

"After the 21-23 March seismic crisis, Ulawun's seismicity showed a fairly steady decay, reaching a very low level in early April. One clear A-type event was recorded on 7 April. A new seismic crisis, preceded by a lull about 2 hours long, began on 10 April at about 0310. The initial, strong continuous tremor changed to discontinuous tremor within a few hours. The entire period of tremor lasted about 6 hours. After this crisis, a steady decline was evident until 17 April. Small A-type events were recorded 11-16 April.

"On 17 April, 5 periods of tremor occurred. After about 3 hours of very low seismicity, the first began at 1645. A distinct lull also preceded the third period. Tremor was mostly continuous, with total duration of about 280 minutes. Individual periods lasted about 29-106 minutes, and were followed by about 5 hours of frequent discrete shocks and discontinuous tremor. Beginning 18 April a gradual decay in amplitude and frequency of occurrence of the shocks was recorded. Possible small A-type events were recorded 20-29 April.

"No unusual visible activity directly accompanied the seismic crisis. However, ejection of one or more smoke rings, seen to rise rapidly to about 500 m above the summit, was reported 11-18 April. Blue vapour emission was seen 14 April. Ulawun's usual white vapour emissions were moderate to strong throughout the month, but increased toward the end."

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.