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 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.