Geoscience Reference
In-Depth Information
(Shepard
1933
). These changes are indicative of submarine
landsliding. In Central America, the coincidence of earth-
quakes and tropical cyclones has a higher probability of
occurrence than the joint probability of both events. Finally,
failures on the submerged front of the Mississippi Delta have
occurred primarily during tropical cyclones.
The internal pressure within submerged sediment can
also be increased by the formation of natural gas through
anaerobic decay of organic matter that has accumulated
through the deposition of terrestrially derived material
(Masson et al.
1996
). This process is known as under-con-
solidation and occurs when fluid pressure within sediments
exceeds the hydrostatic pressure. Methane is also locked
into sediment on lower slopes, but because of the near-
freezing water temperature and the extreme pressure, it is
preserved as a solid gas hydrate. If this hydrate decomposes
back to methane, then either the increased pressure due to
the release of the gas into the sediment or the formation of
voids can cause failure.
Slopes of ongoing sediment accumulation can become
overloaded and oversteepened, leading to failure (Keating
et al.
1987
; Holcomb and Searle
1991
; Whelan and Kelletat
2003
). This was a major process on continental slopes
during glacials when sea level was lower. The increase in
grade and exposure of continental shelves to subaerial
weathering permitted increased volumes of sediment to be
emplaced on the shelf slope. The process dominated the
terminal end of major river deltas. Slides in these envi-
ronments consist of rotational slumps. In solid rock, cohe-
sion may be low because of shear planes that exist along the
bedding planes between sedimentary layers or by joints
running through the rock. Stratigraphically favorable con-
ditions for landslides include massive beds overlying
weaker ones, alternating permeable and impermeable beds,
or clay layers. Structurally favorable conditions include
steeply or moderately dipping foliations and cleavage; joint,
fault, or bedding planes. Rock that is strongly fractured or
jointed; or contains slickenside because of crushing, fold-
ing, faulting, earthquake shock, columnar cooling, or des-
iccation; is also likely to fail. Many rock units and sediment
deposits on shelves thus have the potential for collapse.
Mid-ocean volcanic islands are particularly prone to
large landslides (Keating et al.
1987
; Masson et al.
2006
).
On average, four such failures have occurred on these types
of islands each century over the past 500 years. Shield
volcanoes over hot spots accrete through successive lava
flows that solidify quickly when they contact seawater. The
islands consequently can rise 4,000 m or more above the
seabed as steep pedestals emplaced on poorly consolidated,
deep ocean clays. If weathering occurs between flow events,
then subsequent flows are deposited higher in the edifice
over weaker strata that may become zones for failure. If a
landslide occurs early in the building phase of a volcano,
then its seaward-dipping surface and its cover of blocky
debris can become an unstable, low-friction foundation for
ensuing lava flows. Eruptive events may inflate the volcano,
steepening bedding planes, and fracturing the dome-like
structure. Fracture lines may become zones for dyke
intrusion that can over-pressurize rock and increase its
volume by more than 10 %. For example, on the east side of
Oahu, Hawaii, dykes make up 57 % of the horizontal width
of the shield volcano. In addition, the weight of edifices
standing over 4,000 m high can cause crustal subsidence
that fractures the volcano further. Many volcanoes contain
rift zones extending throughout the edifice along these
zones of weakness. The flanks of these volcanoes are being
pushed sideways, over the top of the sediment layer at their
base under the force of gravity or by magma injection.
Rates of spreading range between 1 and 10 cm per year. For
example, the southern flank of Kilauea is presently moving
at this higher rate away from a rift zone known as the Giant
Crack. Giant landslides occur on the unbuttressed flanks of
such volcanoes giving the volcano a tristar shape etched by
steep headwall scars. This process is common on young
volcanoes usually less than a million years in age, and has
been a notable feature of the Hawaiian and Canary Islands.
While, the above causes appear logical, the database of
dated slides available to prove these factors is limited. The
best data set contains 68 submarine slides that have occurred
over the past 175,000 years (Urlaub et al.
2013
). It encom-
passes both low and high sea-level stages and periods of
sediment accretion off major rivers. Landslides are distrib-
uted randomly through time showing no significant trends,
peaks or clusters. While river-fed systems appear to have
more slides during low and rising sea levels, the relationship
is not statistically significant. Finally it is not possible to
ascribe submarine landslides to earthquakes. While earth-
quakes trigger slides, a significant number of slides have
occurred on passive continental margins with generally low
levels of seismicity. At present, submarine landslides and any
tsunami that they may generate appear to be random events.
7.3
How Submarine Landslides Generate
Tsunami
The characteristics of tsunami generated by landslides are
different from those simply generated by the displacement of
the seabed by earthquakes. One of the more important dif-
ferences is the fact that the direction of propagation of tsu-
nami generated by landslides is more focused (Watts
1998
).
The slide moves in a downslope direction, and the wave
propagates both upslope and downslope with the slide. As a
result, the tsunami wave has a shape that becomes exagger-
ated close to the source and is best characterized by an N-
