Environmental Engineering Reference
In-Depth Information
9.2.12
Seafloor Instability
General
Various forms of slope failures have been recognized offshore, including deep rotational
slides
(Figure 9.57)
and shallow slumps, flows, and collapsed depressions
(Figure 9.58).
Major causes are earthquakes, storm waves inducing bottom pressures, depositional loads
accumulating rapidly and differentially over weak sediments, and biochemical degrada-
tion of organic materials forming large quantities of gases
in situ
which weaken the
seafloor soils.
Offshore failures can occur suddenly and unpredictably, destroying oil production plat-
forms, undersea cables, and pipelines. Large flows, termed “turbidity currents,” can move
tremendous distances.
Recognition
Occurrence is most common in areas subjected to earthquakes of significant magnitude
and on gently sloping seafloors with loose or weak sediments, especially in rapidly accret-
ing deltaic zones.
After failure the seafloor is distorted and scarred with cracks, scarps, and flow lobes
similar to those features which appear on land as illustrated in Figure 9.58. Active areas
are explored with side-scan sonar
(Figure 9.59)
and high-resolution geophysical surveys
(see Figure 9.57).
Examples of Major Failures
Gulf of Alaska
The major slide illustrated on the high-resolution seismic profile given in Figure 9.57
apparently occurred during an earthquake, and covers an area about 15 km in length.
Movement occurred on a 1° slope and is considered to be extremely young (Molnia et al.,
1977). As shown in the figure, the slide has a well-defined head scarp, disrupted bedding,
and a hummocky surface.
Gulf of Mexico
Movements are continually occurring offshore of the Mississippi River delta. During hur-
ricane Camille in August 1969, wave-induced bottom pressures caused massive seafloor
movements that destroyed two offshore platforms and caused a third to be displaced over
a meter on a bottom slope that was very flat, less than 0.5% (Focht and Kraft, 1977).
South
North
0
100
200
300
0
0.25
0.5
0
5 km
FIGURE 9.57
High-resolution seismic reflection profile showing a portion of the Kayak Trough slump slide in the Gulf of
Alaska. (From Molnia, B.F. et al., in
Reviews in Engineering Geology
, Vol. VIII,
Landslides
, Coates, D.R., Ed.,
Geologic Society of America, 1977, pp. 137-148. With permission.)
