Smithsonian Institution
Global Volcanism Network Bulletin v. 19, no. 10, October 1994
Rabaul (Papua New Guinea)  Tavurvur activity decreasing; its lava
      flow stops; minor subsidence
New Britain, Papua New Guinea (4.27S, 152.20E)
All times are local (= GMT + 10 hours)
"The eruption that started in Rabaul Caldera on 19 September
continued throughout October (see Bulletin v. 19, nos. 8-9).
However, only one of the two centres initially active, Tavurvur, on
the NE part of the caldera, remained in eruption. It displayed
moderate Vulcanian-type activity, accompanied by the production of
a lava flow. Eruptive activity at the other intra-caldera cone,
Vulcan, on the W side of the bay, ended on 2 October. Thereafter,
its activity was reduced to weak fumaroles and bubbling pools of
water at the bottom of its new NE crater.
"Overall, the level of activity at Tavurvur progressively
decreased, in spite of variations in the strength, frequency, ash
content, and height of its Vulcanian explosions. Only one crater
was active on the E side of the cone; up to 4 were active in the
first few weeks of the eruption. During the first few days of
October, explosive phases occurred at intervals of 30-120 seconds.
They produced billowing columns rising dynamically, with large
ballistic fragments, up to 400-800 m above the crater. In between,
ash emission was usually continuous though less forceful.
Occasionally, the vent remained free of emissions for a few
minutes. A second vent on the W side of the same crater
occasionally produced a darker but weaker emission, with apparently
unrelated frequency. Depending on wind strength, the emission plume
levelled off between 1 and 2 km height, and spread W over the town
of Rabaul, the pale yellow to brown mass remaining visible for 20
"Through October, the interval between explosive phases increased,
though irregularly, to 1-4 minutes. Explosions were irregular in
strength but rose less and less frequently to >600 m, and the ash
content of the plume decreased. The visible extension of the plume
also decreased to ~15 km. Longer periods of weak activity were
commonly followed by larger (and louder) explosions that ejected
ballistic material as far as 1.5 km from Tavurvur's summit, onto
the lower slopes of the cone or into Greet Harbour. During periods
of lesser ash content in the emission, these projections caused
incandescent night displays (22-27 October). At times of dense ash
emission, lightning occurred under and around the plume. Sound
effects of the eruption were variable. Rumbling sounds were the
most common and apparently louder during periods of lesser ash
content in the emission. At other times, Tavurvur could be silent
for a couple of hours, or even days, without noticeable change in
activity. The largest explosions (like at 0640 on 14 October or
2125 on the 16th) were heard as impressive, sharp detonations up to
20 km away and their air-waves were felt up to 10 km away.
"Backfall of material around the vent progressively built a cone
~30 m high with a radius of ~80 m. Light ashfall on the town of
Rabaul and beyond it on the N coast continued throughout October.
The first torrential rainfalls of the pending rainy season
contributed to the major destruction within the town area. Most
buildings in the S and central parts of Rabaul township collapsed
under the weight of 0.3-1.2 m of ash/mud. Subsequent rainfalls also
caused large flash-floods of mud that temporarily cut off access
roads and flooded several buildings and villages. Earthmoving
equipment was used to construct drains and barriers in an attempt
to alleviate destruction in the remaining parts of town from
expected mudflows at the start of the rainy season in December.
"A viscous lava flow, aa to blocky in texture, began on 30
September from a source SW of the main active vent of Tavurvur. Its
flow rate was extremely low and its progression slow. On 5 October,
as this lobe was still moving within the lower W part of the
crater, a new lobe formed and started to override it. On the 8th,
an outbreak of apparently more fluid, darker lava started on the W
side of the original lobe source. The two initial lobes merged
together on 12 October as they started to spill over the lower side
of the crater rim onto the W flank of Tavurvur cone. On the 14th,
a new lobe started to form from an outbreak through the flow, near
the initial source. This became the main feeder to the combined
flow system, although it progressed slower and slower until 25-27
October when the flow-front stopped ~100 m below the rim of the
cone, 2/3 of the way to the coast.
"The extensive pumice raft, formed as a result of the early Plinian
phases and pyroclastic surges, kept drifting across the bay in
response to wind shifts. At times of strong SE winds it occupied
the N half of the bay, packing to thicknesses of up to 1.7 m (G.
Halls, Hydrographic Surveys, Pty Ltd, pers. communication). A few
hours of lull or a reversal in the trade wind, and it decompressed
and spread over the SE part of the bay, only to drift back a few
hours later.
"Ten of the 14 stations of the RVO seismic network were
progressively disabled by volcanic products, lightning,
interruption of power supply, or vandalism, within the first week
of the eruption. By early October, however, in a prompt response to
an RVO and PNG Government invitation, a team from the USGS Volcano
Disaster Assistance Program was on-site deploying a network of 10
digitized stations with P-picker, Tom Murray's real-time seismic
amplitude measurement program (RSAM), and Willie Lee's data
management systems on personal computers.
"Following the end of eruptive activity on the Vulcan side,
seismicity was scattered under the whole caldera, including outside
the usual annular seismic zone. A high concentration of events at
Tavurvur corresponded to explosion earthquakes. The level of
seismicity indicated by RSAM and the number of detected events
showed a general decline, with some fluctuations, throughout the
month (figure 2). Most detected events consisted of low-frequency
and explosion earthquakes with delayed air-phases distinctive
throughout the network.
"All real-time ground deformation monitoring (electronic tilts and
tide gauges) had progressively been lost over the last few years
prior to the eruption by lack of appropriate funding. From the
onset of the eruption, ash density in the bay prevented EDM
monitoring. For the first week thereafter the only accessible
ground deformation data were from two water-tube tiltmeters on the
outer caldera rim. They indicated radial deflation of the caldera,
which started with the triggering earthquakes (ML 5.1) on 18
September and amounted to 30 and 37 frad, respectively, by the end
of September. By late September a few other stations had been
recovered, including a dry-tilt array near the centre of the
caldera at the S end of Matupit Island. In early October two
electronic tiltmeters were deployed by the USGS team. Sea shore
surveying around the bay resumed on 27 September, and geodetic
levelling to Matupit Island on 4 October.
"All collected data revealed a caldera-wide subsidence amounting to
~1 m near the centre and 20-30 cm near the edges. The resulting
bowl-shaped subsidence is, however, perturbed by the residuals of
a pre-eruption uplift on the night of 18-19 September around the
two pending eruptive centres, which amounted to 5-6 m on the E
shore of Vulcan and 1-2 m at Tavurvur and Matupit Island. Minor
caldera subsidence continued through October, although mainly
affecting the central area within 3 km of Tavurvur. The maximum
measured subsidence amounted to 20 cm at the Tavurvur tide gauge,
near the long-recognized apex of ground deformation, with
progressively decreasing rates from ~1.5 to 0.4 cm/day.
Simultaneously, the Matupit Island tiltmeter recorded a deflation
of >110 frad, radial to the same centre of deformation, at a slowly
decreasing rate (figure 3).
Information Contacts: Chris McKee and Patrice de Saint-Ours, with
additional contributions from RVO Staff, Rabaul Volcano Observatory
(see Manam); T.L. Murray, A.B. Lockhart, and E. Endo, USGS Cascade
Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661 USA;
R.W. Johnson, AGSO; H.L. Davies, University of Papua New Guinea,
P.O. Box 414, University, Papua New Guinea.
Figure 2. Fluctuations in the level of seismicity recorded at
Rabaul, October 1994. Courtesy of RVO.
Figure 3. Changes recorded by the Matupit Island tiltmeter, October
1994. Although an upward trend is seen on the plot, the change
reflects a steady deflation of the central part of the caldera.
Courtesy of RVO.