During March, the frequency of explosions at Tavurvur rose progressively to about one weak ash plume per minute (BGVN 23:03). On 3 April, the volcano began producing 1-2 large explosions per day but continued emitting about 20 weak ash explosions per hour. Larger explosions showered the volcano's flanks with blocks and sent dark ash clouds to an altitude of ~1 km.

After fours days with short bands of harmonic tremor, activity changed on 11 April, when at 0730 visible and recorded activity dropped suddenly to background level. After 1030 large explosions occurred every 2-12 hours and sent dark ash clouds to altitudes of 1.5-3 km. The largest explosion threw blocks beyond the cone's base and Sulphur Point into the sea and produced short ash flows down every flank. Between explosions only weak white and blue vapor wisps were visible. This pattern persisted with minor fluctuations until the end of April. In total, 1,064 explosions were detected during April; 6 high-frequency events also occurred on the caldera's NE rim on 13 and 25 April.

Trade winds shifted SE on 23 April, causing dense ashfalls in the now partly re-occupied Rabaul town. Ashfalls affected the city throughout the 1996 dry season (BGVN 21:07 and 21:09); scientists predicted that ashfalls would beleaguer the town through 1998 unless emissions cease.

Ground deformation data showed ongoing inflation; they did not suggest an end to the current eruptive phase. Since a March 1997 eruption produced a 4 x 10⁶ m³-lava flow (BGVN 22:03), the central part of the caldera has steadily risen ~12 cm, indicating the slow accumulation of 10-20 x 10⁶ m³ of magma in the shallow central reservoir. Ongoing Vulcanian activity at Tavurvur has not significantly modified the inflation process. Rates of uplift and tilt were only moderately slowed by eruptions during April, June, July, August, and December 1997, and February-March 1998 (BGVN 22:04, 22:05, 22:07, 22:08, 22:12, 23:02, and 23:03). Since the current phase of activity began in February 1998, inflation has appeared radial to the central magma reservoir rather than to Tavurvur. Petrological analysis of samples erupted from mid-1997 to February 1998 indicated that the shallow resident dacitic reservoir received new influxes of basaltic magma.

Scientists stated that despite the "healthy" appearance of the volcanic system, the re-inflation has accumulated only 5% of the volume erupted in September 1994 (BGVN 19:08 and 19:09). They predicted that a dangerous eruption was not imminent, but was a possibility in the long term.

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.