The very slow-moving supply of magma into Rabaul Caldera resulted in mild, intermittent Vulcanian activity at Tavurvur in January. Emissions occurred at irregular intervals a few minutes to several hours apart. Longer intervals between emissions usually resulted in more powerful and voluminous explosions.

As in previous months intermittent small-to-moderate pale gray ash emissions continued, most of them without accompanying sounds. However, on 2, 4, 6-7, 21-23, and 25-27 January, loud vulcanian explosions occurred accompanied by dense ash clouds rising approximately 2,000-3,000 m above the summit. Large ballistic blocks littered the slopes of Tavurvur and small, hot pyroclastic flows traveled down the slopes. There was a shift in wind direction resulting in light ash falls in populated areas.

The recorded seismicity was lower in January than in December (1,413 events, compared to 2,843). There were 18 vulcanian explosions during January. Twenty-eight high-frequency earthquakes occurred located outside the caldera along a NE-trending line extending ~20 km offshore. Most of these events happened during the first two week of January.

Ground-deformation measurements continued to show a slow, inflationary trend. Survey measurement from the center of Rabaul to the tip of Matupit Island (2 km to the W of Tavurvur) showed a maximum inflation of 6 mm for the month. A continually recording GPS site at Matupit showed no significant deformation within the instrument accuracy.

A dry tiltmeter at Matupit showed upward tilting toward Tavurvur of about 12 µrad, while the water-tube tiltmeter at Sulphur Creek (3 km NW of Tavurvur) showed 7 µrad of radial uptilting. Other dry-tilt sites farther away from the center of the caldera showed no significant change.

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: Herman Patia, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.