ARCHITECTURE AND FAILURE MECHANISM OF THE OFFSHORE
SLUMP RESPONSIBLE FOR THE 1998 PAPUA NEW GUINEA TSUNAMI
D.R. TAPPIN
British Geological Survey, Keyworth, Nottingham, NG12 5GG United Kingdom.
P. WATTS
Applied Fluids Engineering, Private Mail Box 237, 5710 E. 7th Street Long Beach CA
90803 United States of America
T. MATSUMOTO
Japan Marine Science and Technology Center, 2-15 Natsushima-Cho, Yokosuka,
Kanagawa, 237-0061, Japan
Abstract
After considerable controversy over the origin of the July 1998 PNG tsunami, there is
now a large body of evidence that supports a sediment slump offshore of the devastated
area. Between 1999 and 2000, four surveys were carried out offshore of the affected
area, acquiring bathymetry, sediment cores, 3.5kHz seismic, multi-channel seismic and
seabed imagery. In 2001, the same area was surveyed using single channel seismic that
has been used to interpret the northern margin of PNG and the internal architecture of
the slump. The susceptibility to slumping of the area offshore of northern PNG can be
more definitively assessed.
Keywords: Submarine slump, Papua New Guinea, mass movement, Tsunami,
convergent margin,
1. Introduction
The tsunami that struck the north coast of Papua New Guinea (PNG) in the evening
of July 17th 1998 has been, from the outset, controversial. Argument as to whether
the tsunami was the result of an earthquake or an offshore sediment slump
continues. The scale of the devastation is well known, with 2000 people dead and
numerous villages destroyed. As a result, there has been a comprehensive
investigation into the event and now we have a compendium of survivors’ accounts,
onland study, offshore seabed imaging, geological interpretation, seismological
analyses, and computer simulations (Davies, 1998; Kawata et. al., 1999; Tappin et
al., 1999, 2001, 2002; McSaveny, 2000; Synolakis et al., 2002). Further, the event
identified the danger to coastal communities worldwide of tsunamis that are
generated from local sources and with no adequate warning. This message has had a
major import on tsunami science that has been taken up internationally.
We present here further results from the surveys offshore of the north coast of PNG
conducted during 1999 to 2001 carried out from the JAMSTEC vessels Natsushima
and Kairei (Figure 1). The interpretation is based on 1600km of single channel
seismic data acquired in February 2001 from the area previously
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Figure 1. Bathymetry and main morphologic elements offshore of northern Papua New Guinea. Filled
triangles identify the area devastated by the 17th July 1998 tsunami. Box is the area of the amphitheatre
where a close-spaced grid of seismic was acquired. NNE-SSW oriented lines mark the regional seismic
lines. Large star is location of the Earthquake of September 9th 2002. Inset map shows tectonic
configuration of the area and the box marks the area shown in the main figure.
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surveyed by multibeam in 1999. Most particularly there was acquired a closely
spaced grid of data from within the area of the amphitheatre, the location of the
slump that is thought to have generated the 1998 tsunami (Figure 2).
Figure 2. Amphitheatre area off of Sissano Lagoon (Located in Figure 1) with features identified in the
text. The solid white line is the slump area identified from the multibeam data. The dashed line is the
slump identified from the single channel seismic. Fine black lines are seismic acquired during the survey
of February 2001. Numbered thick black lines 23 and 43A are seismic lines illustrated in Figures 3 and 4.
Contours at 100m intervals.
2. Offshore survey – data acquisition
The data was acquired using a Single Channel Seismic profiling system comprising a GI
gun and Streamer cable. The GI gun had a volume of 150cu.in and was used in GI mode
with the Primary pulse controlled by the Injector pulse. Shooting rate was every 16
seconds and the ship speed varied between 6 and 8 knots. Data was recorded using an
analogue streamer comprising a 65m active section with 48 hydrophones and a 200m
lead cable. 780km of data were acquired on regional grid of 16 lines oriented NNE to
ESE with a line spacing of between 13 and 17km (Figure 1). Over the amphitheatre area
800km of data were acquired on 25 north to south trending lines with a spacing of
mainly 900 m and seven ENE-WSW trending tie lines (Figure 2).
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3. Results
3.1 REGIONAL GRID OF DATA
Using the regional grid of data the area has been subdivided into an inner trench slope
and a subducted (Pacific) Plate (Figure 1). We focus our attention here on the Inner
Trench slope because of its tsunamagenic potential.
The inner trench slope has been subdivided along its length into two sections, the
boundary of which lies in the area of the Yalingi Canyon and amphitheatre. In the east,
the inner trench slope is narrow and steep with a thin sedimentary sequence on its
uppermost part. On the steepest gradients a sedimentary section is not distinguished but
on less steep slopes a trenchward-dipping sedimentary sequence is imaged. In the west
the inner trench slope is wider with the increase in width mainly accounted for by the
presence of a series of lower inner slope sedimentary basins. There are numerous
backward tilted fault blocks on the lower sections of the slope. Within the lower slope
basins, there are at least two sedimentary units present, with the lower unit backtilted
towards land (south). To landward of the basins, inner trench slope gradients are again
steep and the sedimentary sequence poorly imaged.
In the west, and also in the area of the Yalingi Canyon and amphitheatre, there is a thick
sedimentary sequence on the upper inner trench slope. In both eastern and western areas
there are numerous incised canyons that dissect the upper part of the slope. The two
largest submarine canyons (the Pual and Yalingi) continue downward to (respectively)
the inner slope basins in the west and the New Guinea Trench in the central area. In the
area around the Pual Canyon there is a major sedimentary accumulation. Within this
region, at the foot of the steep inner trench slope, there is a sediment failure.
3.2 AMPHITHEATRE
In the region adjacent to the amphitheatre and the nearby Yalingi Canyon there is a
sedimentary depocentre. There are numerous slumped areas here, located on the steep
slopes of the inner trench wall. Their presence signifies this area to be a region of
common sediment failure.
In the eastern part of the amphitheatre, the slump proposed as the probable source of the
1998 tsunami (Tappin et al., 1999), and mapped previously with multibeam and seabed
observation (Tappin et al., 1999, 2001 and 2002) and multichannel seismic (Sweet and
Silver, in press) was located and mapped. The slump lies between the Bliri Delta to the
southwest and the upraised block in the north (Figure 2). On the seismic data it displays
a complex morphology (Figures 3 and 4). There are three seismic units, the lowest of
which is only clearly imaged on east-west Section 43A (Figure 4) where its base lies at
1500ms (TWT) below seabed. The internal seismic reflections at the bases of the two
upper units are very disturbed.
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The seabed expression of the topmost seismic unit agrees generally with that of the
slump area identified from the multibeam data (Tappin et al., 2001). The extent of the
second seismic unit has a greater extent particularly to the west, in which direction it is
overlain by slumps derived from the steep headscarp that is present here (Figure 4).
These later slumps have a characteristic morphology with numerous benches formed by
discrete sediment failures down the headscarp. In the east the slump is more
homogeneous. At its thickest, the base of the second seismic unit lies at a maximum of
700ms two-way time (TWT) below seabed that, using a velocity of 2km/sec, gives a
thickness of 700 metres.
Upraised
block
Dipping
sequence
14km Fault
Figure 3. Seismic line 23 (north-south), showing the two slump seismic units 1 and 2.
At its base, the slump over-rides a sedimentary sequence that dips to the northwest and
is downfaulted at the 14km fault, against the upraised block (Figures 2 and 3). The
dipping sequence thins to the southwest, whereas to the northeast it is overlain by a
horizontally stratified seismic sequence (Figure 3). Above the slump headscarp there is
a morphologic high that, to the south, descends into the Yalingi Canyon (Figure 2).
West of the area of the amphitheatre and the slump, the morphology changes. The
upraised block is absent and the inner trench slope descends without interruption to the
trench. The sediment thickness on the slope seaward of the Bliri delta front is thinner.
The delta front sediments prograde northward, with the margin of this sediment wedge
deeply incised and, at many locations, truncated by slumping. There are slumps
identified on the slope and at its foot. A large slump is traversed by line 12 (Figure 2).
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Figure 4. Seismic line 43A (west-east) showing seismic units 1, 2 (slump) and 3.
4. Discussion and Conclusions
The inner trench slope outside of the areas of fluvial input is sediment starved. Slumps
are rare except where associated with localised sources of sediment supplied from major
rivers. In the vicinity of the amphitheatre, sediment from the Bliri River has constructed
a delta. As the delta front progrades there is collapse along the delta front leading to
slumping. These slumps are both cohesive rotational failures and translational slides.
The seismic data has been use to map out in detail the sediment failure located in the
eastern part of the amphitheatre, identified on previous surveys. The upper two seismic
units are interpreted as the slump proposed previously as the source of the 1998
tsunami. This agrees with the multichannel interpretation of Sweet and Silver (in press).
The architecture of the slump indicates a rotational failure mechanism (Tappin et al.,
1999, 2001 and 2002; Sweet and Silver, in press). However, the main slump is 7-8km
wide rather than the 5km mapped previously by multibeam and multichannel seismic.
Its mapped maximum length of 4 km agrees with previous results. The maximum slump
thickness is of the order of 700m and confirms that measured previously (Sweet and
Silver, in press). The slump is parabolic in morphology with well-defined margins that
taper outward to the sides.
The slump failure is complex, and involves two sedimentary units. The disrupted bases
of the two upper units suggest internal sliding on sub-horizontal planes in addition to the
‘domino’ type faulting identified on the multichannel seismic (Sweet and Silver, in
press). In the west it is overlain by small slumps derived from the headscarp that may
have been part of the same major slumping event of 1998. In the south, located at the
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foot of the headscarp, there is a major fault. At the base of the slump, there is a sharp
discontinuity with the underlying northwest dipping seismic sequence. In the northwest,
this underlying sequence is faulted against the upraised block along the 14km fault.
The slump mapped in the eastern part of the amphitheatre has been modelled on the
basis of previous interpretations of its architecture and location as a probable source of
the 1998 tsunami. The new data indicates that there may have been movement along
internal sub-horizontal discontinuities during failure, also that the failure may have
taken place episodically over a short time scale. Based on the new slump size and the
architecture presented above, new modelling should be attempted.
On a regional scale, the main areas of slumping are associated with sediment
depocentres at the mouths of major rivers in the area. Whereas slumping in these areas
is common, majortsunami events are probably due to slump size and failure mechanism
as well as to the local morphology such as that represented by the amphitheatre. A
major (Magnitude 7) earthquake in September 2002, located to the west of the
amphitheatre (Figure 3) created a tsunami wave of less than 2 metres.
5. References
Davies, H.L., 1998. The Sissano Tsunami 1998. University of Papua, New Guinea Printery, Port Moresby.
Kawata, Y., B.C. Benson, J.L. Borrero, H.L. Davies, W.P. de Lange, F. Imamura, H. Letz,J. Nott, &
Synolakis C., 1999. Tsunami in Papua New Guinea was as intense as first thought. Eos, Transactions
American Geophysical Union. 80: 101,104-105
McSaveny, M.J., Goff, J.R., Darby, D.J., Goldsmith, P., Barnett, A., Elliot, S. and Nongkas, M., 2000. The 17
July 1998 tsunami, Papua New Guinea: evidence and initial interpretation. Marine Geology. 170: 8192.
Sweet, S. and Silver, E.A., in press., Seismic reflection images of the source region of the 1998 Papua New
Guinea tsunami. In: Watts, P., Synolakis, C. E. and Bardet, J-P. Prediction of Underwater Slide and
Slump Hazards. Balkema, Rotterdam, Netherlands.
Synolakis, C.E., J-P. Bardet, J.P. Borrero, H.L. Davies, E.A. Okal, E.A. Silver, S. Sweet, D.R. Tappin.
The Slump Origin of the 1998 Papua New Guinea Tsunami. Proceedings Royal Society, London
A. 458: 763-789.
Tappin, D.R., Matsumoto, T. & shipboard scientists. 1999. Offshore Surveys Identify Sediment Slump as
Likely Cause of Devastating Papua New Guinea Tsunami 1998. Transactions American Geophysical
Union. 80: 329, 334, 340.
Tappin, D.R., Watts, P., McMurtry, G.M., Lafoy, Y. and Matsumoto, T., 2001. The Sissano, Papua New
Guinea tsunami of July 1998– offshore evidence on the source mechanism. Marine Geology.
Tappin, D.R., Watts, P., McMurtry, G.M., Lafoy, Y. and Matsumoto, T., 2002.Prediction of slump generated
tsunamis: the July 17th PNG event. Science of Tsunami Hazards. 20: 228-238