THE 1996 FINNEIDFJORD SLIDE; SEAFLOOR FAILURE AND SLIDE
DYNAMICS
O. LONGVA1, N. JANBU2, L. H. BLIKRA1, R. BØE1
1
NGU, N-7491 Trondheim, Norway
2
Norwegian University of Science and Technology, 7491 Trondheim, Norway
Abstract
The 1996 Finneidfjord slide was a combined submarine/subaerial, retrogressive flow/
quick clay slide that mobilized 1 mill. m3 of sediments and killed four people.
Interpretation of the morphology of the submarine slide scar and the slide deposits
combined with eyewitness observations suggest that the initial failure occurred on the
steepest slope of the foreshore platform. Detachment along a weak layer within silty
clay caused unloading, oversteepening and erosion and triggered retrogressive sliding.
Outrunner blocks from the slide occur up to 1.6 km outside the main debris lobe. Excess
hydrostatic pressure related to meteorological and anthropogenic influence is thought to
have triggered the slide.
Keywords: Submarine slide, weak layer, pore pressure, swath bathymetry, high
resolution seismic.
1. Introduction
The understanding of failure and slide processes is essential for evaluating the different
aspects of slide hazard. Swath bathymetry combined with high-resolution seismics and
sidescan-sonar images can contribute significantly to the understanding of failure
mechanisms and slide dynamics. The slope failure along the shoreline of Finneidfjord,
North Norway in 1996 (Fig. 1), has been used to test these methods (Longva et al.
1998). These data are further compared and discussed with more classical methods
including geotechnical investigations and stability modelling. The morphological
features formed by the Finneidfjord slide are comparable to what is found in mega
slides on the continental slope (Locat & Homa 2002) and provides a unique opportunity
to study slide processes independent of scale in shallow and sheltered water.
2. The 1996 Finneidfjord slide
Around midnight of June 20, 1996, a shoreline slope failure occurred in Finneidfjord,
Northern Norway (Fig. 1). Janbu (1996) reconstructed the sequence of sliding from all
available information, including eye witness accounts and sea bottom video
investigations. The initial slides started below sea level, before midnight, at the steepest
part of the slope some 50 to 70 m from highway E6, which ran along the shore (Fig. 2).
Eye witnesses saw waves, bubbles and whirls moving away from the shore some time.
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Figure 1. Index map, 1998 seismic grid and location for figures.
before midnight. The slide developed retrogressively towards land. About 25 minutes
after midnight a driver felt that his car and the road were shaking violently and stopped
The beach below the road was gone. Minutes later he witnessed 250 m of the road
breaking in three parts and slumping into the sea. A car with one person also
disappeared. Shortly afterwards, the nearest house started to move, then sank into the
mud and disappeared into the sea. Three people inside did not manage to escape. All
this happened within 5 minutes or less. Several minor mass movements occurred along
the edges of the slide, but after 1 hour, no more slide activity was observed.
Ground investigations prior to the slide, in connection with the re-alignment of the E6
highway, showed that the beach sediments comprised soft sensitive clay with layers of
quick clay and silt, overlain by up to 5 m of sand. Rockhead sloped towards the fjord,
with the clay layer therefore thickening downslope towards the shore. At the shoreline,
bedrock was encountered at a level of about –15 m (Fig. 2B). Geotechnical properties of
the sediments are summarized in Best et al. (2003, this volume).
3. Methods
In 1997 the Norwegian Hydrographic Service, on behalf of the Geological Survey of
Norway (NGU), performed a swath bathymetry survey (Fig. 1) deploying a Simrad
EM100 multibeam echosounder. NGU collected a few lines of high resolution seismic
in 1997 and a denser grid in 1998. Survey instruments were parasource (TOPAS) and
boomer (Geopulse). Niemistoe cores and vibro-cores were collected in 1998 and 2001,
but except for parts of one core these data will be published elsewhere.
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Figure 2. A: Slide development defined by Janbu (1996) into 5 phases. B: Profile based on survey in 1984,
illustrating slide mechanisms. The failed section involved a cap of Holocene silty clay over late glacial clay
with the initial detachment within the Holocene succession. The beach was relatively flat before the failure,
with the steepest slope about 18° 5-25 m below sea level.
4. Failure and slide mechanisms
The swath bathymetry gives an excellent image of the geomorphic features of the slide
(Figs. 1 & 3) and it has been possible to differentiate the event into three main stages
(not to be confused with Janbu´s five phases, Fig. 2).
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4.1 STAGE 1 – INITIAL SLIDE
There are several indications of instability and sliding prior to the main slide event. On
the northern slope, outside the main slide scar, sediments have travelled down-slope,
leaving an exhumed surface behind and a wedge of compressed sediments at the foot of
the slope (Fig. 3). A seismic section down this slope, across the main slide (behind "H",
Figure 3. Shaded relief image showing the slide scar and the slide deposits. Sediments mobilized during
various slide stages are indicated. Tracks of seismic profiles illustrated in Figs. 4 and 5 are indicated.
Fig. 3) and up the southern slope (Fig. 4) shows that the detachment occurred along a
well-defined bed, expressed as a band of high-amplitude seismic reflectors. This bed
can be identified over the entire area at a depth varying from 1 m to 9 m. The weak
layer has not been sampled. However, a study of gassy sediments in the fjord (Best et al.
2003, this volume) indicates that the high-amplitude reflectors may result from free gas
trapped in porous sand layers between layers of silty clay. This may have contributed to
excess pore pressure and is further discussed below.
On the north flank of the main debris lobe of slide stage 2 (Fig. 3) a wedge of sheared
sediments with concentric folds is found. On high-resolution seismic data (Fig. 5), the
displaced sediments are compressed to a ridge at least 2 m high, are bordered by distinct
shear zones on either side and are thrust along the weak layer, which in the section lies
at c. 5 m depth. The ridge is in the south partly capped by a thin sheet of debris-flow
deposits from the main stage 2 event. It supports the main debris flow lobe
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Figure 4. Sliding along a weak layer expressed as a high-amplitude seismic reflector, probably reflecting
alternating beds of sand and silt/silty clay. Failure on the north side of the main lobe occurred during the 1996
Finneidfjord Slide. On the south, failure probably occurred some years earlier.
Figure 5. Seismic profile (location Fig. 1) across the outer part of the main debris flow lobe as seen on Fig. 3.
Notice debris flow, undisturbed seafloor and thrust along weak layer.
and is therefore older. The entire succession above the detachment (Fig. 5) is deformed
during the shear, which demonstrates that this wedge is young. The wedge lies in the
lowest depression of the pre-slide seafloor in direct continuation of the proposed initial
slide of Janbu (1996), but can not be traced upslope because of later slide activity. The
"fresh" morphology, young deformation, and similar expression (Fig. 3) of this wedge
and the failure along the northern slope, suggest that both are part of the 1996 event.
Hence, detachment along a weak layer is thought to represent the initial slide
mechanism. This probably "punctured" the quick-clay chamber either by unloading,
oversteepening or erosion and triggered the retrogressive slide with development of a
typical "bottle neck" slide. There is morphological evidence on the swath of deeply
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rooted decollement, slumps and hydrostatic "blow outs" (Fig. 3) that might have
triggered larger slides given the right soil conditions. Whether such processes were
involved during the initial detachment is not known.
The slope outside the southern part of the main slide gate is also exhumed along the
same bed and the failed sediments lie folded at the base of the slope in a length of more
than eight hundred meters. The gentler morphology and seismic indications of a thin
sediment drape lead us to conclude that this section failed some years ago. However, it
shows that the weak detachment layer controls the stability in the area. Side scan sonar
images of the slopes revealed solifluction lobes and slow creep processes may have
been important for a destabilisation of the fjord slopes prior to the slide event.
4.2 STAGE 2 – THE MAIN SLIDE
Deposits from the main slide – stage 2 – cover large portions of the sea floor, and are
characterized by a hummocky surface with shear ridges, lobes and large outrunner
blocks (Fig. 3). Video inspection of the slide has revealed structures interpreted to be
remnants of the road, c. 100 meters from the old shoreline (Janbu 1996). All debris from
houses and constructions were found landward of these remains. Outside this, the
Figure 6. Outrunner block that has travelled 1.6 km outside the main debris flow lobe by hydroplaning. A
seismic section across the block indicates the depth to the gliding plane. The gliding plane (transition zone)
can be observed in an x-ray image of a core through the block and into the underlying sediments.
terrain is very rough, with densely stacked ridges, many meters high. These occur up to
300 m offshore. The remains of the road are not easily recognized in the swath
bathymetry, but the hummocky terrain is well expressed ("H" – Fig. 3). This part of the
main slide has ploughed into older debris, forming low, lateral ridges for a distance of
200–300 m and folded sediments 60 to 70 m in front, before it has come to a halt. The
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video surveyors described the sediments in the inner part of the slide as very soft. The
"plug" must obviously consist of more competent material. As it is located southwest of
the road, and no debris originating from the landward side of the road has crossed it, the
"plug" most likely represents dry crust or sandy sediments from the beach area. This
constrains the debris lobe outside the "plug" and the rafted blocks on the surface (Fig. 3)
to the submarine part of the slide. The larger blocks are typically elongated, 40–70 m
long, 10–20 m wide, 1 m to 2 m thick and oriented transverse to flow direction. This
accounts for blocks both on the debris lobe and outrunners in front of the lobe, possibly
with one exception ("x" – Fig. 3). The largest block (100 x 50 x 2 m, 15 000 tonnes) has
travelled 1.6 km downslope (past the outer margin of the debris lobe) on a slope of only
1º before it has stopped against the flank of a moraine ridge (Fig. 6). In the swath
bathymetry, a shallow, linear depression, less than 10 cm deep, marks the slide path. Xray investigation of a core through this block shows layered glaciomarine sediments
with ice-dropped gravel, in the lower part of the block, and bioturbated undisturbed
sediments below, separated by a sheared interval (Fig. 6). Hydroplaning has been
suggested as a mechanism for long-distance transport of blocks like this, across flat or
gently sloping surfaces (Elverhøy et al. 2002). Flow tank experiments show that
accelerating hydroplaning fronts of debris lobes can detach from the lobe and fly as
outrunners (Harbitz et al. 2002). In front of the debris lobe (Fig. 3), there is one lobeshaped outrunner block (x) oriented with its long axis in the flow direction. This block
might consist of remolded sediments, but that is not confirmed.
4.3 STAGE 3 – MINOR SLIDES ALONG THE SLIDE SCARP
Smaller debris-flow lobes, situated above the stage 2 deposits in the eastern part of the
slide scar, reflect the latest instability phase of the Finneidfjord slide (see arrows - Fig.
3). One example is the lobe from south east where the video inspection identified debris
from a construction area, which from eyewitness accounts slid out at a late stage.
After the interpretation of seafloor morphology and seismic data, Janbu´s (1996)
reconstruction (Fig. 2A) still stands except for evidence that the initial slides – phase 1 –
involved massive detachment of surface sediments over a larger area of the steepest
slope. This demonstrates the value of combining modern technology like swath
bathymetry, high-resolution seismics and sidescan-sonar images with more classical
methods including geotechnical investigations and stability modelling.
5. Trigger mechanisms
A critical evaluation of all available data suggests that excess pore pressure was the
fundamental cause for triggering the initial slide. The elevated pore pressure most likely
resulted from a combination of climatic and anthropogenic factors (Janbu 1996). There
were long periods of heavy rain and a high groundwater table prior to the slide.
Incipient water was reported to percolate into open cracks on the surface and perhaps
through slits between the steep rock surface and the overlying sediments. There may
also have been leakage from the main water pipeline. Ground tremors from heavy traffic
on the rough, bumpy road, and detonations from nearby tunnel construction works could
have affected the location of the hydrostatic front. These factors together may have led
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to decreased stability and sliding along the weak layer, which was the most obvious
failure surface.
Stability analyses showed that the beach slope had a priori low margin of safety,
perhaps less than 10 % (i.e. F 1) (Janbu 1996). Thus, any man-made activity, or
naturally induced cause, amounting to 10% reduction of safety, could trigger the initial
slides. This agrees well with almost all registered cases of quick clay slides and
submarine slope failures. The initial slide usually starts without a priori warning, due to
minor destabilisation, and develops gradually until the "main event" happens rapidly
and often very dramatically.
6. Conclusions
Initial sliding during the 1996 Finneidfjord slide event occurred as a detachment along a
weak layer due to the introduction of excess pore pressure from a combined influence
from weather conditions and human impact. Prior to the slide, the slope had relatively
low stability, and the initial slide changed the slope enough to trigger a retrogressive
quick clay slide that involved 1 mill. m3 from sub-sea and land areas. Outrunner blocks
from the sub-sea debris lobe travelled 1.6 km outside the main debris lobe, probably by
hydroplaning.
7. Acknowledgement
Colleagues at NGU and NTNU that have participated in field, laboratory or during
preparation of the manuscript are thanked. A. Lyså and M. Stoker are acknowledged for
constructive reviews of the manuscript. This work forms part of the EC COSTA project
(EVK3-CT-1999-00006).
8. References
Best, A. I., Clayton, C. R. I., Longva, O. & Szuman, M. 2003. The role of free gas in the activation of
submarine slides in Finneidfjord. First International Symposium on Submarine Mass Movements
and their Consequences, EGS-AGU-EUG Joint Meeting. Kluwer, Nice, France. This volume.
Janbu, N., 1996. Raset I Finneidfjord – 20. juni 1996. Unpublished expert´s report prepared for the County
Sheriff of Nordland. Report Number 1, Revision 1.
Elverhøi, A., De Blasio, F, Butt, F.A, Issler, D., Harbitz, C., Engvik, L., Solheim, A. And Marr, J.,2002.
Submarine mass-wasting on glacially-influenced continental slopes: processes and dynamics. in:
Dowdeswell, J.A. and O'Cofaigh, C., (Editors), Glacier-Influenced Sedimentation on HighLatitude Continental Margins, Geological Society, London, Special Publication, 203: 73-87.
Harbitz, C.B,. Elverhøi, A., Parker, G. and Mohrig, D. 2002: Theoretical description and analytical
calculations of hydroplaning debris flows on high latitude glacigenic deep sea fans. Journal of
Geophysical Research.
Locat. J. & Homa, J. L. 2002. Submarine landslides: advances and challenges. Canadian Geotechnical
Journal, 39: 193-212.
Longva, O., Blikra, L. H., Mauring, E., Thorsnes, T. & Reither, E. 1998. Testprosjekt Finneidfjord; integrert
skredfarekartlegging – metodevurdering. NGU Rapport 1999.051: 62 s.