Activity in August was restricted to Tavurvur crater while Vulcan cone and the rest of the caldera floor remained quiet. During the first two weeks of the month Tavurvur released continuous gentle weak emissions of white and blue vapors, occasionally accompanied by loud to low roaring. Night glow around the crater mouth occurred on some days.

On 17 August at 0327 a loud explosion was immediately followed by forceful emissions of dark ash-laden clouds rising to ~3 km altitude. Glowing lava fragments were observed within the eruption columns. Eruptive activity increased gradually until it peaked at 0500 with a corresponding Real-time Seismic Amplitude Measurement (RSAM) value of 740. During the 15 minutes of peak activity there was loud sub-continuous lava fountaining. Glowing lava fragments projected ~500 m above the crater rim fell on the S and E slopes of Tavurvur; some fell on the E caldera wall. At 0515 eruptive activity dropped sharply and by 0700 (RSAM = 180) roaring and explosion sounds only occurred ~3 times/minute. The intervals between sounds and ash projections became longer and at about 1300 (RSAM = 64) the eruption stopped. No lava flows were produced during the eruption.

Two vents were active during the eruption, one on the E crater rim and the other just W of the rim. The E vent was the more active of the two. The eruption clouds were pale gray (during daytime) at the height of the eruption but were paler (whitish) as the level of eruption decreased. The emissions rose to ~3,000 m altitude during the eruptive period but dropped to ~400 m when the eruption stopped. The emissions were blown NE and N with continuous light to moderate ash falls in the Matalau area and light ashfall on the E part of Rabaul Town. The eruption was mild, short-lived (~10 hours) and smaller than the last eruption on 11 July (BGVN 22:07). It was the smallest of the last seven Strombolian phases of activity (10-11 May and 4-5 October 1996, 9 January, 14 March, 12-13 April, 1 June, and 11-12 July 1997). The last two eruptions (July and August) were more Strombo- Vulcanian in character.

On 16-17 August there were 173 low-frequency earthquakes recorded associated with explosions at Tavurvur. Only four high-frequency earthquakes were recorded during the month; one was located to the NE (offshore) outside the caldera, and another ~1 km S of Matupit Island. From 18 August until the end of the month Tavurvur returned to its normal gentle white emissions with blue vapors accompanied by occasional (sometimes hours apart) loud to low roaring sounds. The emissions rose as high as 400 m and then were blown N. Steady weak red glow at night around the crater mouth occurred throughout the month.

Electronic tilt at Matupit Island started to show a NW-down tilt beginning on 5 August. This continued until the 13th when the E-W component of the tilt plateaued; the N-S component continued its steady N-down trend until the 17 August eruption, when that component also plateaued. This cessation of ground tilt continued until 24 August when both components simultaneously resumed their steady NW-down tilt. It is thought that this cessation of tilt for 6 days indicated small-scale relaxation of tumescent stress in Greet Harbour caused by the eruption; no deflation occurred and the inflationary trend on the 24th was at the same rate and direction as before the 13th.

Dry tilt around the caldera showed no overall pattern in response to the eruption. The closest regularly occupied dry tilt station to Tavurvur (Transmitter, on Matupit Island) showed a slight inflationary trend in relation to the deformation center in the mouth of Greet Harbour before the 17 August eruption, and a 20 µrad NW offset after the eruption. This offset had not been recovered by the end of the month.

The water-tube tiltmeter at Sulphur Creek (~3.5 km NW of Tavurvur) showed a steady NW-down tilt of ~4 µrad (away from Tavurvur) seven days before eruption happened. One day before the eruption, an increase of 6 µrad was recorded, but deflation (SE-down tilt) with a similar magnitude occurred during the eruption. Sulphur Creek resumed its steady NW-down tilt trend after the eruption. A GPS network along the old Rabaul Airstrip (eight stations) and around the N part of the caldera showed few significant horizontal movements during August, although post-eruption surveys seem to indicate a slight NW movement of most stations (up to 4 cm at some stations at the E end of the runway).

COSPEC measurements showed fluctuations in SO₂ emissions during the month. A high SO₂ emission level of 1,080 tons was recorded on the 12th. Output dropped on the following two days (to ~500 tons) and was on a slow steady increase when the eruption occurred (at ~800 tons). During the eruption SO₂ output continued to rise, reaching a peak on the 19th (at 1,050 tons). It decreased on the 20th to low levels before resuming an upward trend, reaching the highest peak of 1,200 tons on 28 August before it dropped once more.

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.