Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 1, January 1995

Rabaul (Papua New Guinea)  Fumarolic activity with little
       seismicity or deformation

Rabaul
New Britain Island, Papua New Guinea
4.27 S, 152.20 E; summit elev. 688 m

"The two active cones in Rabaul showed only fumarolic activity in
January; the activity at Tavurvur declined during the month.
Seismicity was low throughout January, although small volcanic
earthquakes continued. Ground deformation was also low. The Stage
IV alert was canceled on 5 January, and reverted to Stage I.

"There was no further explosive activity at Tavurvur following the
last Vulcanian explosion on 23 December 1994. Only fumarolic
activity has been observed since that date. The amount of vapour
released declined gradually during January. Up until the middle of
the month there was still a distinct plume rising to several
hundred meters above the crater. By the end of the month, however,
there was only intermittent vapour release, with an occasional
small plume. No fumarole temperatures have been measured.

"Although the external shape of Tavurvur appears to be little
different, the internal crater structure was totally changed by the
1994 eruption. Almost all of the crater features produced by
eruptions in 1878, 1937, 1941, and 1942 were destroyed and replaced
by a single shallow bowl-shaped crater. The low point of the crater
is still on its W side, where the 1994 lava flow exits. Inside the
main crater, slightly off-center to the SE and taking up perhaps a
third of the crater floor, is a single cone, which was built up
during the later stages of the 1994 eruption. This cone's crater is
almost hemispherical and the vent is no longer visible due to the
accumulation of debris on the crater floor. During January, as
crater temperatures dropped, both the inner and main craters became
very brightly colored with sulphur and other precipitates.
Fumarolic activity was concentrated in the inner crater, although
there were some fumarolic areas in the main crater. There was also
a distinctive blue-vapour fumarole about halfway down the 1994 lava
flow, rising through the flow. This appears to have been active
since the very first minutes of the eruption. Vulcan continued to
exhibit only very weak fumarolic activity from both the 1937 and
1994 craters.

"Earthquake activity in January consisted of both high- and
low-frequency events. There were very few high-frequency
earthquakes (28) continuing the pattern of low activity seen since
the eruption. Located earthquakes tended to be in either of two
distinct zones. The first is shallow (1 km) and to the S of Vulcan,
extending into Karavia Bay. The second is deeper, 3-5 km, and is
located under the NE edge of the seismic network, with epicenters
between Namanula Hill and Nodup on the NE coast. Earthquakes in the
first zone are undoubtably due to structural readjustment following
the Vulcan eruption. The cause of the earthquakes in the other zone
is not yet known; it is not part of the ring fault. The last
occurrence of earthquakes in this region, May 1992, was followed by
an increase in seismic activity and ground deformation.

"At the end of December the number of volcanic earthquakes
associated with Tavurvur dropped to a very low level. Starting on
29 December, however, a new type of low-frequency earthquake was
recorded. These were only seen on three seismic stations in the N
part of the network and, because of their emergent onset and
variable waveforms, it was impossible to locate them; the most
likely locations are in the NE portion of the network, perhaps even
outside it. These events continued through January at an average
rate of 14/day although >25/day were recorded on 12-14 and 30
December. Only a few Tavurvur volcanic earthquakes were recorded
during the month. This level of activity is very low compared to
that during the eruption and is probably due to a minor
readjustment in the caldera's plumbing system--there may be a
connection with the Namanula/Nodup high-frequency earthquakes.

"Electronic tilt data showed that ground deformation declined in
December. The station on Matupit Island, which seems to be the most
stable, showed that deflation following the eruption gradually
declined to ~0.5 śrad/day in the second half of January, with the
center of deflation to the S of Matupit. The peak deflation rate,
shortly after the equipment was installed at the beginning of
October, was >3 śrad/day. Other ground deformation data confirm
this trend.

"Following a recommendation from the Rabaul Volcano Observatory,
the East New Britain Disaster Committee declared on 5 January that
the level of alert for Rabaul was being reduced from Stage IV to
Stage I. This was based on the cessation of volcanic activity at
both Tavurvur and Vulcan and the continuing decline in ground
deformation and seismicity, indicating that no resumption of
eruptive activity was likely. Although no longer a hazard to what
remains of the town of Rabaul, the two cones are still off-limits
to the public.

"Rabaul has remained under a State of Emergency, with access to the
town controlled, because mudflows and flooding are still perceived
to be a serious hazard. To date the rainy season has been unusually
dry without any persistent periods of heavy rainfall. Mudflows and
flash floods have already caused some damage and the roads into
Rabaul are washed out after even slight rain, but there is great
potential for more damage. Large amounts of ash still remain on the
high ground surrounding Rabaul and large areas of potentially
unstable land have been exposed by the destruction of vegetation.
The rehabilitation of areas affected by the eruption continues in
a haphazard fashion. With the end of explosive activity, ashfall
stopped being a nuisance and this has accelerated the
rehabilitation of the Nonga area on the N coast. In Rabaul itself,
most of the activity consists of clearing-up operations. However,
a number of businesses have been re-established, the port is open
and taking large ships, and one of the hotels has re-opened."

Information Contacts: same as for Manam.