Great Landslides - Tsunami - page 150

deposits, the largest of which covers an area of 32,000 km 2 .
Two debris flows have been mapped in 1,000-3,000 m
depth of water off the Mississippi delta. The larger of the
two is 100 km wide and 300 km long. There are also the
two

other, slides can develop in subduction zones. The Ranger
slide off the coast of California is one of the biggest of this
type identified to date (Moore 1978 ). It covers an area of
125 km 2 and incorporates 12 km 3 of material. Even mid-
ocean ridges can generate slides. Along the Mid-Atlantic
Ridge, one such slide, comprising 19 km 3 of material, was
caused by the failure of a 4 km 9 5 km block on the flank
of a mountain bordering the rift (Tucholke 1992 )
More worrisome are the coasts where no mapping has
been carried out. Most of the coastline surrounding the
Indian Ocean has not been mapped in enough detail to
identify individual slides. Even in a developed country such
as Australia, parts of the east coast have only been mapped
since 1990 using side-scan sonar. This coastline is passive
and assumed to have low seismicity. However, numerous
slides off the continental shelf have been identified (Jenkins
and Keene 1992 ; Clarke et al. 2012 ). One of the largest
measures 10 km 9 20 km, and is located 50 km south of
Sydney. The age of the latter slide is unknown, but there is
now substantial evidence for the presence of recent large
tsunami along the adjacent coastline (Young et al. 1995 ). As
described in Chap. 4 , the signatures of tsunami are common
elsewhere around Australia, but unfortunately any link to
submarine

mega-turbidite deposits in the Mediterranean Sea

300-600 km 3 mentioned in Chap. 1 with volumes of

(Rothwell et al. 2000 ).
Passive continental margins lie along tectonically inac-
tive edges of crustal plates (Moore 1978 ). Many of the
slides emanating from these margins are derived from
sedimentary units that are only 10-100 m thick. Sediment
has accumulated over time along these margins through
subaerial erosion. Failure occurs on slopes parallel to bed-
ding planes. While the size of these slides is small com-
pared to the Hawaiian ones, the widespread nature of the
evidence is worrisome. For example, the eastern seaboard
of the United States has at least four large submarine can-
yons cutting through the shelf edge, leading to distributary
fans on the abyssal plain. Levees on the fans indicate that
large debris or gravity flows have occurred often. Slides are
numerous off the west coast of Africa, where hummocky
slides, block fields, debris flows, and turbidity deposits have
all been mapped. Few submarine slides have yet been
detected off the west coast of North America, mainly
because bathymetry has not been mapped in detail. How-
ever, a 6.8 km 2 slide with a volume of 1 km 3 occurred on
April 27, 1975 off the fjord delta at Kitimat, British
Columbia (Lipman et al. 1988 ). The resulting tsunami had a
run-up height of 8.2 m. A 75 km long slide also has been
detected off the Monterey Fan in California (Lipman et al.
1988 ). One area that has been mapped well is the conti-
nental slope on the north side of the Aleutian Islands facing
the Bering Sea (Carlson et al. 1991 ). The area is relatively
quiet seismically but is underlain by gas hydrates or a zone
of sedimentary weakness. Here, mass failures up to 55 km
long and containing blocks 1-2 km across have been
identified emanating from some of the largest canyons in
the world on low slopes of 0.5-1.8. The volume of the
landslides ranges between 20 and 195 km 3 .
The sides of deep ocean trenches are only susceptible to
submarine slides if ocean sediment has accumulated here as
part of a tectonic process. Thick sediment layers can pile up
on oversteepened slopes as the result of tectonic off scrap-
ing (Moore 1978 ). Slides from trenches have been reported
in the Sunda, Peru-Chile, Puerto Rican, and eastern and
western Aleutian trenches. One of the largest slides occurs
in the Sunda Trench off the Bassein River in Burma (Moore
1978 ). The slide covers an area of 3,940 km 2 and has a
volume of 960 km 3 . The slide was triggered either by an
earthquake or by overloading of sediments brought down
the Irrawaddi River at lower sea levels. Along other tren-
ches, localized slides of 10 km length appear to be a
common feature. Where crustal plates are sliding past each

slides remains speculative without detailed

mapping.

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