Smithsonian Institution
Global Volcanism Network Bulletin v. 19, no. 11, November 1994
Rabaul (Papua New Guinea)  Explosions from Tavurvur show steady
      decrease in frequency

New Britain, Papua New Guinea (4.27S, 152.20E)
"The eruption that started on 19 September (see Bulletin v. 19,
nos. 8-10) continued through November. Tavurvur exhibited moderate
Vulcanian activity that declined slowly in strength, while Vulcan
remained quiet. Vulcan exhibited only weak fumarolic activity from
four small vents filled with bubbling water at the base of the new
"Activity at Tavurvur consisted mainly of discrete explosive
pulses. The ash content was generally low, producing a pale-grey
emission column. The size of, and timing between, explosions was
variable, but there was a general decline in activity during
November. At the beginning of the month, explosions were 1-4
minutes apart and the emission columns rose forcefully to ~1.5 km.
By the 6th, the intervals were 1-10 minutes and the crater was
sometimes clear of emissions. Blue vapours were seen around the
active vent at the bottom of a 50-m-high tephra cone. There were,
however, large explosions on the 5th, 6th, and 9th which showered
the flanks of Tavurvur with blocks and bombs, and produced a large
billowing cloud up to 2 km high. From 9 to 19 November, emissions
were mainly of white vapour with occasional explosion clouds up to
1 km. The eruption was mainly silent, except for rumbling and
roaring noises on the 10th and 11th.
"The Tavurvur crater was never freely open during this phase of the
eruption, but was clogged up with a mass of rubble, welded together
and sometimes glowing. The dark ash-laden billowing clouds that
suddenly rushed out of the vent every few minutes seemed to
percolate through the rubble. A lava mound, 10 m in diameter and 2
m thick, formed over the vent on the 15th but was destroyed by a
large explosion the next day. A new lava mound had formed by the
18th, this time 20 m across and 4 m thick, possibly consisting of
two lobes and fractured into four main blocks. The intermittent
ash-laden emissions were then hissing out from under the sides of
the mound. Details of the crater could not be seen again until the
25th, when all traces of the lava mound had disappeared from the
base of the bowl-shaped crater, presumably blown out by the large
explosions heard at intervals of 1-4 hours on the 19th.
"From the 19th until the end of the month explosions were generally
mild. Large explosions, however, occurred on 20-22, 26, and 29
November. At night, these explosions resulted in a shower of
incandescent blocks on the flanks of the volcano. Sizeable blocks
were occasionally found in the Talwat road that goes around the
base of the cone.
"Seismic activity in the caldera was lower in November than in
October. It was dominated by shallow explosive and low-frequency
earthquakes associated with the eruptive activity at Tavurvur. RSAM
amplitudes and event counts showed a marked decline between 29
October and 2 November (figure 7). Throughout the rest of the
month, the data were dominated by diurnal meteorological effects,
although a gradual decline could still be seen. Data captured on
the seismic data-acquisition system showed an average of ~6.5
low-frequency and explosive events per day, compared to almost 26
per day in the second half of October.
"Before the eruption, seismic activity in Rabaul was dominated by
high-frequency earthquakes located on the caldera ring-fault
system. Since the eruption, there have been few high-frequency
earthquakes detected (58 in October and 37 in November, compared to
normal pre-eruption levels of 200-300/month) and most of these were
located away from the ring fault or in previously inactive regions
of it. The level of seismicity cannot be easily compared to earlier
pre-eruption levels because totally different seismic detection
systems were used. However, it is believed that the level is much
lower than before the eruption. This, and the fact that the
majority of the epicenters are away from the ring-fault system that
previously contained almost all of the seismicity, suggest that the
caldera is no longer in a highly pressurized state.
"Ground deformation determined from electronic tilt meters and
dry-tilt measurements indicate a reduction in the rate of deflation
of the caldera since the onset of the eruption. This change is
illustrated by an offshore pylon near the centre of deformation, 2
km S of Tavurvur, which subsided by 8 cm in November, compared to
18 cm in October and at least 45 cm in the last 10 days of
Information Contacts: B. Talai, R. Stewart, and P. de Saint-Ours,
Rabaul Volcano Observatory, P.O. Box 386, Rabaul, Papua New Guinea.
Figure 7. Seismicity at Rabaul (stations KPTH and TALH),
October-November 1994. Courtesy of RVO.