The mild upsurge of activity in late December 1997 (BGVN 22:12) decreased in January. Tavurvur Crater returned to normal activity with weak to moderate white vapor emissions that were thicker following rainfalls. Loud roaring and rumbling noises were heard on 4 January. Weak to bright red glow was seen at the summit almost every night in January.

Seismity was at a low level. Low-frequency volcanic earthquakes occurred on 1, 4, and 6 January; 11 high-frequency earthquakes were recorded during January. All of the high-frequency events occurred outside the caldera, eight to the NE, and three to the E and SE.

Ground-deformation instruments recorded the continuous, slow inflation that has been occurring since May 1997. In agreement with sea-survey measurements, GPS data since November 1997 revealed an uplift of ~1.5 cm on Matupit Island. The Sulphur Creek water-tube tiltmeter (3-5 km NW of Tavurvur) showed ~6 µrad of January inflation.

During January, COSPEC measurements of SO₂ registered ~200 metric tons/day (t/d) compared to ~450-750 t/d during November-December 1997. The low readings were due in part to a change in wind direction that blew the plume away from the observation point.

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 688-m-high asymmetrical pyroclastic 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 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western 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.