The mild Vulcanian activity continuing since November 1998 continued through June 1999. With time, the eruption's from Rabaul's Tavurvur cone appeared to be progressively waning in intensity. Still, during May and June several moderate explosions occurred.

Some May and June explosions sent ash clouds 1 km above the summit. The ash clouds drifted NW, some resulted in light ashfall over Rabaul Town. The mild ash-bearing outbursts in June occurred with very long intervals (sometimes 24 hours) between them. Notable outbursts took place on 9 days during the month (3, 5, 6, 9, 13, 15, 16, 17, 19, 23 and 25 June); although many only lasted 2-10 minutes, the last one of the set prevailed for 25 minutes. Typical plumes rose to 500 m high; SE winds typically blew these plumes and fine ash fell, including some on Rabaul Town.

In accord with these visual observations, both deformation and caldera seismicity remained low. Although during May, April, and March there had been 150, 142, and 120 low-frequency earthquakes, respectively, during June there occurred only 38 such earthquakes. The two located earthquakes appeared to the NE of the main ring fault. Anomalously, during the past 2 years there has been an absence of recorded high-frequency earthquakes on the ring fault. Instead, located earthquakes have consistently struck NE of the ring-fault system.

On the 16th a regional earthquake directly E of Wide Bay triggered a 31-cm tsunami. Since then, more than 20 regional events have occurred within a 20 km radius of the initial quake.

Geological Summary. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: Ben Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.