Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 2, February 1995

Rabaul
New Britain Island, Papua New Guinea
4.27S, 152.20E; summit elev. 688 m
All times are local (= GMT + 10 hours)

"Eruptive activity resumed at Tavurvur on 13 February after one and
a half months of quiescence; no precursory activity was detected.
Following the end of the 1994 eruption on 23 December (Bulletin v.
19, no. 12), Tavurvur had exhibited only fumarolic activity. The
amount of vapour released declined during January and emissions
became intermittent in the first half of February. Seismicity was
low, although some volcanic earthquakes continued to be recorded.
The deflation rate of the caldera was also extremely low. In
consequence, the alert level for Rabaul was reduced from Stage IV
to Stage I on 5 January.

"After about 0100 on 13 February, small explosions started from
Tavurvur's 1994 crater. Activity increased during the early hours
of the morning, and large explosions occurred at 0328, 0857, 0919,
0937, 1012, 1100, and 1230. Each of these lasted 2-3 minutes and
generated ash clouds that rose 1,500-3,000 m above the crater. Some
of the ash clouds were laced with lightning. Ballistic blocks were
seen falling onto the flanks of the cone and into the sea around
Tavurvur. Between the larger explosions, emissions were less
energetic or in "puffs" over periods of 5 minutes or more. After
the first day, the emissions generally rose 500-1,000 m above the
crater and were blown SE, producing a 10-km-long discontinuous,
diffuse, pale-grey plume.

"Each of the explosions was accompanied by a distinctive explosive
or low-frequency earthquake whose amplitude corresponded to the
size of the explosion. Changes in the eruptive activity could
therefore be tracked using RSAM data from station KPTH on Matupit
Island (figure 4). An analysis of RSAM 1-minute data produced the
event counts and mean amplitudes shown in figure 4. These showed
that after a few hours of large events, at an average rate of
~10/hour, the activity was dominated by smaller explosions that
peaked after about a day and a half on 14 February, at an average
rate of 15/hour. The number of explosions and their amplitude then
declined over the next 2-3 days. On 17 and 18 February, however,
the activity increased again, perhaps associated with heavy rain on
the 16th and 17th. The event count stayed fairly constant until the
end of the month, although event amplitudes exhibited a slowly
increasing trend.

"Apart from the low-frequency explosive events associated with the
Tavurvur eruption, earthquake activity at Rabaul was very low in
February. There were only four small high-frequency earthquakes
recorded, compared to 28 in January. Two were located at shallow
depths near Vulcan and the other two were outside the seismic
network to the NE.

"Throughout the first part of February, ground deformation data
continued to show the slowing deflationary trend seen since
September 1994, with the deflation centered S of Matupit Island.
Electronic tilt data from station MPT on Matupit Island showed
deflation of ~0.5 śrad/day during this period (figure 4). Seashore
survey measurements around Greet Harbour were in good agreement,
with subsidence of <1 cm/month. Following the renewal of activity
at Tavurvur, ground deformation rates seem to have decreased, with
only 3 śrad of tilt at MPT in 3 weeks, and no measurable changes in
seashore levelling data. The gap in the tilt data on figure 4 was
because the battery at MPT was stolen the day before the explosive
activity began.

"There were three aerial inspections of Tavurvur during this
period. On the morning of 13 February, before the large explosions
took place, there was no marked change in the configuration of the
bowl-shaped crater compared to the previous inspection in January.
There also was no open vent, although the explosive emissions rose
through the central part of the crater floor, which was covered
with ash and rubble. On 20 February, emissions were seen rising
from an obstructed vent in the SE part of the crater, while a
strong fumarole was active on the W side of the crater (at the head
of the 1994 lava flow). A small mound of lava seen on the 27th at
the base of the crater was 20-30 m wide, only a few meters high,
and was partly mantled with ash. Emissions were released through
cracks in the lava or from between blocks near the edges.

"Throughout February, Vulcan continued to exhibit only very weak
fumarolic activity from diffuse sources around the edge of the
floors of both the 1937 and 1994 craters. At some time in late
January or February, hot steaming springs appeared along the N
shore of the Vulcan headland. Measured temperatures were
consistently around 100 deg C.

"The Gazelle Peninsula has remained under a State of Emergency,
with access to Rabaul controlled because of the risk from mudflows
and flooding. Although the rainy season has been unusually mild so
far, mudflows and flash floods are causing much damage to the roads
into Rabaul and are flooding the remaining buildings in the town
and in nearby villages."

Information Contacts: same as for Manam.

Figure 4. Rabaul tilt and seismicity measured at stations MPT and
KPTH on Matupit Island, 1 February-10 March. Positive N and W tilts
indicate deflation of the caldera. Note that times are GMT (= local
time - 10 hours). Courtesy of Rabaul Volcano Observatory.