The Rabaul Volcano Observatory (RVO) reported that at 0800 on 27 March a dense, dark-gray ash plume rose 2-3 km above the Ulawun’s summit and drifted S. Two seismic stations, located 10 km NW and 6 km from the volcano, indicated that seismicity remained low, though small, low-frequency earthquakes within background noise were detected by both stations. Increase seismicity was detected at the station 10 km NW in Real-time Seismic-Amplitude Measurement (RSAM) data 1-2 hours before the eruptive event, peaked about 30 minutes after the event, and then decreased to near-background levels about three hours later. RVO noted that the ash event was likely over, though seismic data suggested that there was a low probability that small ash plumes could sometimes occur. Continuous low rumbling was heard on 28 March. During 28 March-1 April activity was low and diffuse plumes contained ash but over the week changed to only white steam plumes.

Operations were restored at the third, and primary, seismic station (UULA) 2.8 km from the summit, on the lower WSW flank, on 28 March. The station immediately began recording small low- and high-frequency volcanic earthquakes, with low-frequency earthquakes being the dominant signal. Both the number and magnitude of both types of earthquakes increased on 1 April.

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.

Source: Rabaul Volcano Observatory (RVO)