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Report on Rabaul (Papua New Guinea) — August 1998


Bulletin of the Global Volcanism Network, vol. 23, no. 8 (August 1998)
Managing Editor: Richard Wunderman.

Rabaul (Papua New Guinea) Scattered ash emissions during July and August

Please cite this report as:

Global Volcanism Program, 1998. Report on Rabaul (Papua New Guinea) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 23:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199808-252140


Papua New Guinea

4.2459°S, 152.1937°E; summit elev. 688 m

All times are local (unless otherwise noted)

Tavurvur's ash emissions diminished during July and parts of August, but there was a larger-than-usual outburst on 20 August. Caldera deformation continued in July and August.

Ash emissions from Tavurvur continued in July although at a reduced level compared to previous months. During the first two weeks of July emissions were associated with discrete low-frequency seismic events. Occasionally (e.g. on 3, 7, 12, and 16 July) there were moderate explosions producing pale-gray ash plumes that rose ~1,500 m above the summit. Some of these explosions ejected lava fragments onto the flanks. Following lessened activity during 17-24 July, the pattern of emissions changed slightly: in addition to discrete ash emissions, there were periods of continuous emission lasting from a few minutes to a maximum of ten minutes. Ash from these emissions, like that from the explosions earlier in the month, fell NW of Tavurvur in Rabaul Town.

During the first three days of August, intermittent ash emissions generally were pale-gray and poor in ash content. Only one emission, on the 1st, was accompanied by a moderate explosion. A loud roaring and rumbling sound without an ash emission was heard on the following day. There were no ash emissions from 4 to 11 August. Following 12 August, intermittent moderate emissions of pale-gray ash resumed, although there were periods of continuous ash emissions lasting from a few minutes to about an hour. Later in the month, long intervals (12-20 hours) of relative calm were followed by large emissions that generated hot ash flows down the W, S, and E slopes of Tavurvur. On 20 August a violent explosion produced a dense dark-gray ash-laden cloud that rose ~2,500 m above the summit. The explosion ejected many lava fragments high above the summit that showered the slopes of the volcano. Some fragments landed 1 km from the base of the cone.

The reduced level of activity in July was accompanied by a drop in the general level of seismicity as recorded at the KPT seismic station, which is ~1.8 km from Tavurvur's summit. Real-time Seismic Amplitude Measurement (RSAM) values dropped from 55 to 20 units and the hourly event counts dropped from 10 to 2. During July a total of ~200 low-frequency volcanic events was recorded, the lowest count since February 1998. Only four high-frequency earthquakes were recorded; two were located outside of the caldera to the E. Seismicity remained low in August despite a minor increase during the second week of the month associated with ash emissions. These emissions were recorded as discontinuous non-harmonic tremors lasting from a few minutes to about an hour. There were no significant changes from July in either RSAM values or hourly event counts. The total number of low-frequency earthquakes for August was 152, but the count may have been affected by the intermittent functioning of one of the triggering stations. The two high-frequency earthquakes recorded in August were too small to be located.

Ground-deformation measurements continued to show a slow, ongoing inflationary trend through July and August. During each month an inflation of about 2-3 µrad towards Tavurvur was recorded at the Sulphur Creek water-filled tiltmeter (3.5 km NW of Tavurvur). Sea-shore surveys in Rabaul Harbour showed ~1 cm of uplift in the Greet Harbour area. A lower amount of uplift also occurred around the Vulcan area. GPS measurements confirmed an increase in altitude at Matupit Island. The inflation trend had been accompanied by a SW-directed horizontal movement of about 2.5 cm in the previous two months. On the W of the harbour a GPS site on the SE of Vulcan Island showed no significant inflation; however the horizontal components showed a significant deformation of 4 cm towards to NW in the preceding 6 months.

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.