Submarine Landslidepnkaj

8/18/2019 Submarine Landslidepnkaj

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Submarine landslideSubmarine landslides are marine landslides that transport sediment across

thecontinental shelf and into the deep ocean. A submarine landslide is initiated when the

downwards driving stress (gravity and other factors) exceeds the resisting stressof the

seafloor slope material causing movements along one or more concave to planarrupture surfaces. Submarine landslides take place in a variety of different settings

including planes as low as 1° and can cause significant damage to both life and

property. ecent advances have been made in understanding the nature and processes

of submarine landslides through the use of sidescan sonar and other seafloor mapping

technology.!1"!#"!$"

%auses

Submarine &andslides have different causes which relate to both

the geological attributes of the landslide material and transient environmental factors

affecting the submarine environment. %ommon causes of landslides include' i)

presence of weak geological layers ii) overpressure due to rapid accumulation

of sedimentary deposits iii) earthuakes iv) storm wave loading and hurricanes v) gas

hydrate dissociation vi) groundwater seepage and high pore water pressure

vii) glacialloading viii) volcanic island growth and ix) oversteepening.!1"!#"!$"

Weak geological layers

*he presence of weak geological layers is a factor which contributes to submarine

landslides at all scales. *his has been confirmed by seafloor imaging such as swath

bathymetric mapping and $+ seismic reflection data. +espite their ubiuity very little is

known about the nature and characteristics of the weak geological layers as they have

rarely been sampled and very little geotechnical work has been conducted on them. An

example of a slide which was caused by weak geological layers is the Storegga slide

near ,orway which had a total volume of $$-- km. !$"!/"

Overpressuring

0verpressure due to rapid deposition of sediment is closely related to weak geological

layers. An example of landslides caused by overpressure due to rapid deposition

occurred in 12 on the 3ississippi delta after 4urricane %amile struck the region.!#"

https://en.wikipedia.org/wiki/Marine_(ocean)https://en.wikipedia.org/wiki/Landslidehttps://en.wikipedia.org/wiki/Sediment_transporthttps://en.wikipedia.org/wiki/Continental_shelfhttps://en.wikipedia.org/wiki/Deep_seahttps://en.wikipedia.org/w/index.php?title=Driving_stress&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Resisting_stress&action=edit&redlink=1https://en.wikipedia.org/wiki/Sidescan_sonarhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Geologyhttps://en.wikipedia.org/wiki/Sedimentary_depositshttps://en.wikipedia.org/wiki/Earthquakeshttps://en.wikipedia.org/wiki/Hurricaneshttps://en.wikipedia.org/wiki/Hurricaneshttps://en.wikipedia.org/wiki/Gas_hydratehttps://en.wikipedia.org/wiki/Gas_hydratehttps://en.wikipedia.org/wiki/Groundwaterhttps://en.wikipedia.org/wiki/Glacierhttps://en.wikipedia.org/wiki/Volcanic_islandhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Multibeam_echosounderhttps://en.wikipedia.org/wiki/Multibeam_echosounderhttps://en.wikipedia.org/wiki/Seismic_reflectionhttps://en.wikipedia.org/wiki/Geotechnicalhttps://en.wikipedia.org/wiki/Storegga_slidehttps://en.wikipedia.org/wiki/Norwayhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-4https://en.wikipedia.org/wiki/Deposition_(geology)https://en.wikipedia.org/wiki/Sedimenthttps://en.wikipedia.org/wiki/Mississippi_deltahttps://en.wikipedia.org/wiki/Hurricane_Camilehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Landslidehttps://en.wikipedia.org/wiki/Sediment_transporthttps://en.wikipedia.org/wiki/Continental_shelfhttps://en.wikipedia.org/wiki/Deep_seahttps://en.wikipedia.org/w/index.php?title=Driving_stress&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Resisting_stress&action=edit&redlink=1https://en.wikipedia.org/wiki/Sidescan_sonarhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Geologyhttps://en.wikipedia.org/wiki/Sedimentary_depositshttps://en.wikipedia.org/wiki/Earthquakeshttps://en.wikipedia.org/wiki/Hurricaneshttps://en.wikipedia.org/wiki/Gas_hydratehttps://en.wikipedia.org/wiki/Gas_hydratehttps://en.wikipedia.org/wiki/Groundwaterhttps://en.wikipedia.org/wiki/Glacierhttps://en.wikipedia.org/wiki/Volcanic_islandhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Multibeam_echosounderhttps://en.wikipedia.org/wiki/Multibeam_echosounderhttps://en.wikipedia.org/wiki/Seismic_reflectionhttps://en.wikipedia.org/wiki/Geotechnicalhttps://en.wikipedia.org/wiki/Storegga_slidehttps://en.wikipedia.org/wiki/Norwayhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-4https://en.wikipedia.org/wiki/Deposition_(geology)https://en.wikipedia.org/wiki/Sedimenthttps://en.wikipedia.org/wiki/Mississippi_deltahttps://en.wikipedia.org/wiki/Hurricane_Camilehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Marine_(ocean)


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Earthquakes

5arthuakes are a key factor which trigger most ma6or submarine landslides.

5arthuakes provide significant environmental stresses and can promote elevated pore

water pressure which leads to failure. 5arthuakes triggered the 7rand 8ankslandslideof 1# where a #- km$ submarine landslide was initiated after an earthuake. !$"!9"

Stormwave loading

Stormwave loading and hurricanes can lead to submarine landslides in shallow regions

and were recognised as one of the factors which contributed to the slides which

occurred on the 3ississippi delta in 12 following 4urricane %amille.!#"

Gas hydrates

A number of studies have indicated that gas hydrates lie beneath many submarine

slopes and can contribute to the triggering of a landslide. 7as hydrates are ice like

substances consisting of water and natural gas which are stable at the temperature and

pressure conditions normally found on the seabed. :hen the temperature rises or the

pressure drops the gas hydrate becomes unstable allowing some of the hydrate to

dissociate and discharge bubble phase natural gas. ;f pore water flow is impeded then

this gas charging leads to excess pore water pressure and decreased slope stability.

7as hydrate dissociation is thought to have contributed to slides at water depths of

1--- to 1$-- m off the east coast of the


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kilometres. =actors which are significant in glacial loading induced landslides are the

flexing of crust due to the loading and unloading of a fluctuating ice front variation in

drainage and groundwater seepage uick deposition of low plasticity silts rapid

formation of moraines and till above hemipelagic interstaidal sediments. An example

where glacial loading lead to submarine landsliding is the ,yk slide of northern ,orway.!#"!>"!?"

Volcanic island growth

Slope failures due to volcanic island growth are among the largest on earth involving

volumes of several cubic kilometres. *he failure occurs as large bodies of lava form

above weak marine sediments which are prone to failure. =ailure is particularly common

on edifices which are over #9-- m but rare on edifices which are less than #9-- m.

@ariation in the behaviour of the slides is significant with some slides barely keeping upwith the growth on the upper part of the volcano while others may surge forward great

distances attaining landslide lengths greater than #-- km. @olcanic island submarine

landslides occur in places such as the 4awaiian ;slands!1"!"!1-" and the %ape @erde

;slands.!11"

Oversteepening

0versteepening is caused by scouring due to oceanic currents and can result in the

triggering of submarine landslides.!#"

;n some cases the relationship between the cause and the resulting landslide can be

uite clear (e.g. the failure of an oversteepened slope) while in other cases the

relationships may not be so obvious. ;n most cases more than one factor may contribute

towards the initiation of a landslide event. *his is clearly seen on the ,orwegian

continental slope where the location of landslides such as Storegga and *raenad6upet is

related to weak geological layers. 4owever the position of these weak layers is

determined by regional variation in sedimentation style which itself is controlled by

large scale environmental factors such as climate change

between glacial and interglacial conditions. 5ven when considering all the above listed

factors in the end it was calculated that the landslide needed an earthuake for it to

ultimately be initiated.!1"!$"

*he environments in which submarine landslides are commonly found in are f6ords

active river deltas on the continental margin submarine canyon fan systems

open continental slopes and oceanic volcanic islands and ridges. !1"

https://en.wikipedia.org/wiki/Plasticity_(physics)https://en.wikipedia.org/wiki/Silthttps://en.wikipedia.org/wiki/Silthttps://en.wikipedia.org/wiki/Morainehttps://en.wikipedia.org/wiki/Morainehttps://en.wikipedia.org/wiki/Tillhttps://en.wikipedia.org/wiki/Norwayhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-7https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-8https://en.wikipedia.org/wiki/Volcanic_islandhttps://en.wikipedia.org/wiki/Lavahttps://en.wikipedia.org/wiki/Hawaiian_Islandshttps://en.wikipedia.org/wiki/Hawaiian_Islandshttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-9https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-10https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Le_Bas-11https://en.wikipedia.org/wiki/Oceanic_currenthttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Storeggahttps://en.wikipedia.org/wiki/Glacialhttps://en.wikipedia.org/wiki/Interglacialhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Fjordshttps://en.wikipedia.org/wiki/River_deltahttps://en.wikipedia.org/wiki/Continental_marginhttps://en.wikipedia.org/wiki/Continental_slopehttps://en.wikipedia.org/wiki/Continental_slopehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Plasticity_(physics)https://en.wikipedia.org/wiki/Silthttps://en.wikipedia.org/wiki/Morainehttps://en.wikipedia.org/wiki/Tillhttps://en.wikipedia.org/wiki/Norwayhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-7https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-8https://en.wikipedia.org/wiki/Volcanic_islandhttps://en.wikipedia.org/wiki/Lavahttps://en.wikipedia.org/wiki/Hawaiian_Islandshttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-9https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-10https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Le_Bas-11https://en.wikipedia.org/wiki/Oceanic_currenthttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Storeggahttps://en.wikipedia.org/wiki/Glacialhttps://en.wikipedia.org/wiki/Interglacialhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Fjordshttps://en.wikipedia.org/wiki/River_deltahttps://en.wikipedia.org/wiki/Continental_marginhttps://en.wikipedia.org/wiki/Continental_slopehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1


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Submarine landslide processes

*here are a variety of different types of submarine mass movements. All of the

movements are mutually exclusive for example a slide cannot be a fall. Some types of

mass movements such as slides can be distinguished by the disrupted step likemorphology which shows that there was only minor movement of the failed mass. *he

displaced material on a slide moves on a thin region of high strain. ;n flows the slide

one will be left bare and the displaced mass may be deposited hundreds of kilometres

away from the origin of the slide. *he displaced sediment of fall will predominantly travel

through the water falling bouncing and rolling. +espite the variety of different landslides

present in submarine environment only slides debris flow and turbidity currents provide

a substantial contribution to gravity driven sediment transport. !#"!$"

ecent advances in $B+ seismic mapping have revealed spectacular images ofsubmarine landslides off Angola and 8runei showing in detail the sie of blocks

transported and how they moved along the sea floor.!1#"!1$"

;t was initially thought that submarine landslides in cohesive sediments systematically

and seuentially developed downslope from slide to debris flow to turbidity current

through slowly increasing disintegration and entrainment of water. 4owever it is now

thought that this model is likely to be an oversimplification as some landslides travel

many hundreds of kilometres without any noticeable change into turbidity currents as

shown in figure $ while others completely change into turbidity currents near to the

source. *his variation in the development of different submarine landslides is associated

with the development of velocity vectors in the displaced mass. *he inBplace stress

sediment properties (particularly density) and morphology of the failed mass will

determine whether the slide stops a short distance along the rupture surface or will

transform into a flow which travels great distances.!1"!#"

*he initial density of the sediment plays a key role in the mobiliation into flows and the

distances that the slide will travel. ;f the sediment is a soft fluid material then the slide is

likely to travel great distances and a flow is more likely to occur. 4owever if the

sediment is stiffer then the slide will only travel a short distance and a flow is less likely

to occur. =urthermore the ability to flow may also be dependent upon the amount of

energy transferred to the falling sediment throughout the failure event. 0ften large

landslides on the continental margin are complicated and components of slide debris

flow and turbidity current may all be apparent when examining the remains of a

submarine landslide.!1"!#"!2"!1$"

https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/w/index.php?title=Seismic_mapping&action=edit&redlink=1https://en.wikipedia.org/wiki/Angolahttps://en.wikipedia.org/wiki/Bruneihttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-12https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Gee2007-13https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Densityhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Huhnerbach-6https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Gee2007-13https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/w/index.php?title=Seismic_mapping&action=edit&redlink=1https://en.wikipedia.org/wiki/Angolahttps://en.wikipedia.org/wiki/Bruneihttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-12https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Gee2007-13https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Densityhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Huhnerbach-6https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Gee2007-13


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4aards

*he primary haards associated with submarine landslides are the direct destruction of

infrastructure and tsunami.

&andslides can have significant economic impacts on infrastructure such as the rupture

of fibre optic submarine cables and pipelines and damage to offshore drilling

platforms and can continue onwards on slope angles as low as 1°. An example of

submarine cable damage was discovered in the 7rand 8anks slide of 1# where the

landslide and resulting turbidity current broke a series of submarine cables up to nearly

2-- km away from the beginning of the slide. !1"!$"!9" =urther destruction of infrastructure

occurred when 4urricane %amille hit the 3ississippi delta in 12 causing a landslide

which damaged several offshore drilling platforms. !#"

Submarine landslides can pose a significant haard when they cause a tsunami.

Although a variety of different types of landslides can cause tsunami all the resulting

tsunami have similar features such as large runBups close to the tsunami but uicker

attenuation compared to tsunami caused by earthuakes. An example of this was the

Culy 1> 1? Dapua ,ew 7uinean landslide tsunami where waves up to 19 m high

impacted a #- km section of the coast killing ##-- people yet at greater distances the

tsunami was not a ma6or haard. *his is due to the comparatively small source area of

most landslide tsunami (relative to the area affected by large earthuakes) which

causes the generation of shorter wavelength waves. *hese waves are greatly affected

by coastal amplification (which amplifies the local effect) and radial damping (which

reduces the distal effect).!$"!1/"

ecent findings show that the nature of a tsunami is dependent upon volume velocity

initial acceleration length and thickness of the contributing landslide. @olume and initial

acceleration are the key factors which determine whether a landslide will form a

tsunami. A sudden deceleration of the landslide may also result in larger waves. *he

length of the slide influences both the wavelength and the maximum wave height. *ravel

time or run out distance of slide will also influence the resulting tsunami wavelength. ;n

most cases the submarine landslides are noticeably subcritical that is the =rounde

number (the ratio of slide speed to wave propagation) is significantly less than one. *his

suggests that the tsunami will move away from the wave generating slide preventing the

buildup of the wave. =ailures in shallow waters tend to produce larger tsunamis

because the wave is more critical as the speed of propagation is less here.

=urthermore shallower waters are generally closer to the coast meaning that there is

https://en.wikipedia.org/wiki/Tsunamihttps://en.wikipedia.org/wiki/Submarine_cable_(disambiguation)https://en.wikipedia.org/wiki/Drilling_platformhttps://en.wikipedia.org/wiki/Drilling_platformhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Nisbet-5https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Papua_New_Guineahttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-McAdoo-14https://en.wikipedia.org/w/index.php?title=Frounde_number&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Frounde_number&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Frounde_number&action=edit&redlink=1https://en.wikipedia.org/wiki/Tsunamihttps://en.wikipedia.org/wiki/Submarine_cable_(disambiguation)https://en.wikipedia.org/wiki/Drilling_platformhttps://en.wikipedia.org/wiki/Drilling_platformhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Hampton-1https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Nisbet-5https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Locat2002-2https://en.wikipedia.org/wiki/Papua_New_Guineahttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-McAdoo-14https://en.wikipedia.org/w/index.php?title=Frounde_number&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Frounde_number&action=edit&redlink=1


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less radial damping by the time the tsunami reaches the shore. %onversely tsunamis

triggered by earthuakes are more critical when the seabed displacement occurs in the

deep ocean as the first wave (which is less affected by depth) has a shorter wavelength

and is enlarged when travelling from deeper to shallower waters.!$"!1/"

*he effects of a submarine landslide on infrastructure can be costly and landslide

generated tsunami can be both destructive and deadly.

Drehistoric submarine landslides

• *he Storegga Slide ,orway ca. $9-- km$ (?/- cu mi) ca. ?--- years ago a

catastrophic impact on the contemporary coastal 3esolithic population

• *he Agulhas slide ca. #---- km$ (/?-- cu mi) off South Africa postB

Dliocene in age the largest so far described !19"

• *he uatoria +ebris Avalanche off ,orth ;sland ,ew Eealand ca. $--- km in

volume 1>---- years ago.!12"

• %atastrophic debris avalanches have been common on the submerged flanks of

ocean island volcanos such as the 4awaiian ;slands and the %ape @erde ;slands. !11"

https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-McAdoo-14https://en.wikipedia.org/wiki/Storegga_Slidehttps://en.wikipedia.org/w/index.php?title=Agulhas_slide&action=edit&redlink=1https://en.wikipedia.org/wiki/Pliocenehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-15https://en.wikipedia.org/w/index.php?title=Ruatoria_Debris_Avalanche&action=edit&redlink=1https://en.wikipedia.org/wiki/North_Islandhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-16https://en.wikipedia.org/wiki/Debris_avalanchehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Le_Bas-11https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Mason-3https://en.wikipedia.org/wiki/Submarine_landslide#cite_note-McAdoo-14https://en.wikipedia.org/wiki/Storegga_Slidehttps://en.wikipedia.org/w/index.php?title=Agulhas_slide&action=edit&redlink=1https://en.wikipedia.org/wiki/Pliocenehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-15https://en.wikipedia.org/w/index.php?title=Ruatoria_Debris_Avalanche&action=edit&redlink=1https://en.wikipedia.org/wiki/North_Islandhttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-16https://en.wikipedia.org/wiki/Debris_avalanchehttps://en.wikipedia.org/wiki/Submarine_landslide#cite_note-Le_Bas-11


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S


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&andslide terminology is beyond the remit of this paper but as noted by several authors

(!ampton et al . 1""#F $ulder % &le'ander ())1F*anals et al . ())+) the reader should be aware

of the complexity of landslide nomenclature and the freuent imprecise use of landslide terminology

especially in the submarine environment where information on landslide processes is often limited.

;n this paper GlandslideH is used as a generic term encompassing all forms of slope failure

irrespective of process. 0ther terms used in this paper including GslideH Gdebris flowH Gdebris

avalancheH and Gturbidity currentH each imply a particular process as defined below'

Occurrence, distribution and scale o- submarine landslides

&andslides are widespread on submarine slopes particularly in areas where fineBgrained sediments

predominate (-igure 1). ;n the ,orth Atlantic this corresponds mainly to areas at high and low

latitudes (Weaver et al . ()))) and appears to correlate with the weathering style of rocks on land. ;n

general glacial action at high latitudes and chemical weathering processes at low latitudes produce

fineBgrained sediments that form thick accumulations on the continental slope and appear to favour

landslide formation. 8etween these ones at midBlatitudes fluvial weathering and sedimenttransport produces greater uantities of coarser grained sediment. *his sediment is often

transported directly to the deep ocean basins by turbidity currents which pass through submarine

canyons and bypass the slope. Although small landslides freuently occur on canyon margins

ad6acent slopes are relatively starved of sediment and appear less likely to be affected by landslides.

*he submarine deltas and fans of large rivers are also sub6ect to widespread landsliding related to

the rapid accumulation of fineBgrained sediments on the continental slope (e.g. 3ississippi

=anF rior % *oleman 1"/(). &andslides in f6ords (0orstad 1"#/) and on the flanks of oceanic

islands pose particular haards to humans. ;n f6ord environments landslides are freuently

associated with Guick clayH a particularly unstable sediment created when marine clays are

uplifted above sea level (usually by glacial rebound) and leached by fresh water (ocat et al .

())2). =ailure of submarine deltas formed where rivers discharge sediment onto the steep

submarine walls of f6ords is also common (rior et al . 1"/(F 3ulikov et al . 1""#). &andslides on

oceanBisland flanks have been cited as posing a tsunami threat on a transBoceanic scale (Ward

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% 4ay ())1) potentially comparable in magnitude to or even larger than the #--/ ;ndian 0cean

tsunami (ay et al . ())5). :hile this extreme scenario can be uestioned ($ader ())1FWynn %

$asson ())2) there is little doubt that the threat posed by oceanBisland flank landslides is both

real and significant.

*he largest submarine landslides can involve many thousand km$ of material two to three

orders of magnitude larger than any terrestrial landslide (table 1F !ampton et al . 1""#). =or

example the Storegga slide involved some $---km$ of sediment affected 9---km # of the

,orwegian slope and basin and had a runBout distance of around ?--km ( !a-lidasonet al .

())+). *o put this into perspective the area is about #-I bigger than Scotland and the runBout is

close to the length of mainland 8ritain.

*he largest landslides occur mainly in two settings on open continental margins and on oceanic

island flanks. *his appears to be a function of specific aspects of the geology and morphology of

these areas. %ontinental margin slopes that are sub6ect to largeBscale failures are typically of low

gradient (from less than 1° to 9°) with gentle topographyF however the GdropH from shelf edge to

basin floor can be up to 9km over distances of a few hundred kilometres. DarallelBbedded sediment

seuences with little variability over large areas characterie their subBsurface structure with the

result that should the conditions for slope failure occur they can simultaneously affect large areas.

:hen measured from the top of the highest volcano to the bottom of the ad6acent ocean basin

ocean islands such as 4awaii and the %anary islands have the greatest relief of any topographic

feature on 5arth. *he island slopes can be very steep (e.g. on *enerife in the %anary islands the

average slope from the top of *eide volcano to the coast is 19°) and volcanic processes tend to

build load and steepen these slopes with time. +espite this not all these slopes are unstable with

landslide occurrence apparently closely controlled by geology particularly the trends of dyke

intrusion (rift ones) on the islands ($oore et al . 1"/"F *arracedo 1""#F

4owever as discussed in the following section a landslideJs sie is not necessarily proportional

to the haard it poses. ;n particular continental margin landslides that occur on very low slopes

far from land may form relatively slowly by retrogressive processes similar to uick clay flows

on land (6entley % Smalley 1"/+). 3any such landslides appear to have limited tsunamigenic

potential although they may still pose a threat to cables or other seabed installations.

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Submarine landslides and ha7ards

*he haard posed by submarine landslides will vary according to landslide scale location type and

process. 5ven small submarine landslides can be dangerous when they occur in coastal areas. *he

12 =inneidf6ord slide in northern ,orway mobilied only -.--1km$ of sediment and little of this

material travelled more than a few hundred metres from source but four people were killed when a

house and car were carried away (ongva et al . ())2). *he 1> ,ice airport slide also cut back

onto land killing several men working on the airport extension (&ssier897adkiewic7 et al . ()))).

4owever the effects of this landslide were felt at least 1--km offshore where a turbidity current

generated by the landslide broke submarine telephone cables. A local tsunami which resulted in the

death of one person was also observed. *he 1# 7rand 8anks earthuake resulted in submarine

landslides a turbidity current and a tsunami that caused significant casualties (!ee7en % Ewing

1"5(F iper et al . 1"""F:ine et al . ())5). *his is one of the bestBknown submarine landslides

because the resultant turbidity current broke several submarine cables

seuentially downslope allowing the speed of such a current (up to $-ms K1) to be measured for the

first time. ;t also illustrates one of the main difficulties in submarine geohaard studyLwhen a

coupled earthuakeMlandslide generates a tsunami which of the two haards produces the tsunami

or could their effects even be combined (:ryer et al . ())+F :ine et al . ())5)N *he latter has been

suggested for the 1? tsunami that struck Dapua ,ew 7uinea (D,7) killing over #--- people

(Satake % ;anioka ())2). 4owever there is now considerable evidence that many GunusualH

tsunamis particularly those with high nearBfield runBups that decay rapidly away from source are

directly caused by landslides (6ardet et al . ())2F Okal % Synolakis ())+). otational slides (often

referred to as slumps) where a thick slide block with a steep headwall can move rapidly downward

may be particularly effective in generating tsunamis even when the lateral distance moved is small

and little effect is seen on the seafloor downslope of the immediate landslide site. *he D,7 tsunami

is most likely to have been generated in this way ($atsumoto % ;appin ())2F Sweet % Silver

())2).

=inally it should not be forgotten that an increasing proportion of the worldJs oil and gas is now

recovered from deepBwater areas offshore where slope instability can be a ma6or geohaard. *he

6uxtaposition of the 0rmen &ange gas field which is set to supply some #-I of


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+. *auses o- landslides

3any factors have been suggested as probable or possible contributors to the initiation of submarine

landslides (table (). *hese vary from sudden impacts operating on timescales of minutes (e.g.

shaking due to earthuakes) to geological processes operating on timescales of tens to hundreds of

thousands of years (e.g. climate changeF Weaver % 3ui


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(ongva et al . 1""#) and the 1> ,ice Airport landslide where loading of the upper slope during

construction work at the airport probably contributed (&ssier897adkiewic7 et al . ()))). ;n other

areas where submarine landslides have directly affected humanBmade structures such as where

buildings on deltas have been submerged or

offshore oil installations destroyed by landslides it is often impossible to determine (and sometimes

unlikely) that human activities triggered the landslides. 4owever even if human activity is not to

blame for triggering such landslides building in these areas has undoubtedly contributed to the

landslide conseuences.

*he concept that Gweak layersH oriented parallel to sedimentary bedding might control the location of

many continental slope landslides is not new (see O=eary >1""1? and references therein). 4owever

it reuired the advent of modern seafloor survey technology such as swath bathymetric mapping

and $+ seismic systems to demonstrate that this concept was correct. ;ndeed it seems that

submarine landslides at all scales are often controlled in this way (astras et al . ())+F Wilson et al .

())+F 6ryn et al . ())5). 4owever we know very little about the nature and characteristics of these

weak layers since they have rarely been sampled and very little geotechnical work has been done

on sediments recovered from them. *he weak layers that underlie parts of the Storegga slide are a

notable exception (6ryn et al . ())5F 3valstad et al . ())5). 4ere it has been shown that the weak

layers are composed of contourites (sediments deposited by ocean currents flowing along the

continental slope) that are clayBrich and have higher water content and greater plasticity than the

overlying lessBwell sorted glacial and glaciomarine sediments. As a result the contourites are more

sensitive and brittle (i.e. they lose strength rapidly when their strain bearing capacity is exceeded).

apid loading of the

waterBrich contourites by glacial sediments appears to have raised pore pressures within the

contourites and is the main factor contributing to landsliding.

3any sedimented slopes prone to submarine landslides show a history of landsliding that extends

back through geological time. *his observation can often be applied at uite local scales with areas

showing stacked landslide deposits sharply demarcated from those showing longBterm stability

(Solheim et al . ())5). *he same is true of some volcanic island slopes for example in the %anary

islands where part of the north flank of *enerife has experienced at least five landslides in the last

#Q$3yr while ad6acent regions have seen none ($asson et al . ())(). ;n a regional sense repeated

landsliding will be a natural conseuence if the sediment deposition processes that generate the

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preconditions for landslides persist over long periods of time. %yclic conditions such as

glacialMinterglacial transitions may also contribute to repeated landsliding on timescales similar to the

cyclicity (6ryn et al . ())5). 0ne of the key reasons for repeated landslides at a site specific scale is

that the scars created by landslides often act as traps for subseuent sedimentation leading to

enhanced sedimentation rates and increasing the risk of further landslides. *hus contourites are

preferentially deposited in landslide scars on the ,orwegian margin enhancing weak layer

development within these scars (6ryn et al . ())5F Solheim et al . ())5). Similarly landslide scars in

the %anary islands are freuently the loci of subseuent volcanism probably

because the landslide removes some of the overburden and creates an easier path for magma to

reach to the surface (*antagrel et al . 1"""). ;t is also possible that the debris left by a landslide

when loaded by subseuent volcanic products can act as a weak layer for future landslides.

5. andslide processes

Submarine landslides can be subBdivided into a bewildering variety of types (-igure 2). 4owever in

terms of volume of gravityBdriven sediment transport in the ocean only slides debris flows and

turbidity currents make a significant contribution (see R1 for definition of flow types). +ebris

avalanches are less significant in terms of total transport but they pose a particular threat to human

populations.

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*here is a generally accepted GparadigmH that landslides in cohesive sediments evolve downslope

from slide to debris flow to turbidity current through gradually increasing disintegration and

entrainment of water (e.g.$ulder % *ochonat 1""#F lstad et al . ())+F 6ryn et al . ())5). 4owever

this is probably an oversimplification in that some landslides travel many hundreds of km without

appreciable transformation into turbidity currents while others transform entirely into turbidity

currents very close to source. ;n truth the formation of large turbidity currents in which a few

hundred km$ of (usually cohesive) sediment are rapidly mixed with much larger volumes of seawater

is a very poorly understood process (;alling et al . ())().

&arge landslides in continental margin sedimentary seuences are often complex events and

elements of slide debris flow and turbidity current may all be evident in the aftermath of a single

landslide. 0ften the slide scar will contain displaced but coherent slide blocks made up of sediments

that have travelled only a short distance from source (-igure +). =urther downslope the landslide

deposit may show flow structures characteristic of debris flow processes (-igure 5). A turbidity

current initiated by the landslide may travel hundreds of km beyond the obvious limit of the debris

deposit with no obvious connecting pathway or deposit. ;n this situation

the correlation between turbidite and landslide can often only be established on the basis of precise

dating sedimentology andMor geochemical analysis (earce % 0arvis 1""(F Wynn et al . ())().

*heoretical and experimental studies have shown that landslides on slopes as low as -.9° are only

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possible where excess pore pressures at the level of the detachment surface support a large fraction

of the weight of the landslide mass thus decreasing the effective stress and the friction with the

underlying substrate (verson 1""@F 3valstad et al . ())5). 4igh pore pressures can be created

through rapid sediment deposition (especially in fineBgrained sediments with low permeability)

collapse of the sediment structure (soBcalled Gsensitive claysH) due to earthuake shaking or possibly

due to melting of gas hydrates contained within the sediment.

+ebris avalanches occur when a mass of cohesionless material usually fragmented rock moves

downslope. ;n the subaerial realm debris avalanches typically occur on slopes that range from #9°

to near verticalF in this situation they can attain speeds as high as 1--ms K1 ($2-kmh K1FVoight %

ariseau 1"@/). *he most widely known debris avalanches in the submarine realm are those that

occur on the flanks of volcanic islands ($oore et al . 1"/"F Watts % $asson 1""5F Ollier et al .

1""/F $asson et al . ())() although they also occur in consolidated sedimentary rocks on active

continental margins due to failure of steep slopes generated by tectonic processes (!Ahnerbach et

al . ())5). 0n the submarine flanks of

volcanic islands such as the %anary islands debris avalanche failure planes dip oceanward at 1-° or

less suggesting that these avalanches are less energetic than their subaerial counterparts. *he

blocky character of submarine debris avalanche deposits probably reflects the friable nature of the

volcanic material and the distance of transport (typically an order of magnitude greater than the

biggest subaerial avalanches) rather than the speed of the emplacement process. *his is further

discussed in R?.

#. ;he Storegga slide

As noted in a previous section the Storegga slide which occurred on the continental slope west of

,orway around ?#-- calendar years ago is one of the largest and bestBstudied landslides on earth

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(6ugge et al . 1"/@1"//F !a-lidason et al . ())2F !a-lidason et al . ())+F 6ryn et al .

())5F3valstad et al . ())5).

*he Storegga slide illustrates many key aspects of landslides on low angle continental slopes

(-igure +). *hese include'

1. Several (at least /) distinct gently sloping (-.9Q#°) failure planes parallel to the sedimentary

bedding.

#. Steep (1-Q$9°) headwall scarps separating the different glide plane levels.

$. &andslide debris showing clear evidence of brittle deformation preserved in the landslide

scar.

/. etrogressive behaviour.

An elegant geotechnical model for the Storegga slide was constructed by3valstad et al . >())5? who

demonstrated that the best explanation for the slide reuired a combination of one or more weak

layers identified as marine clays (contourites) deposited during interglacial periods and excess pore

pressures developed as a result of the rapid sedimentation that loaded the ,orwegian slope during

intervening glacial periods. 5vidence that high pore pressures existed in the area comes from

measurements ad6acent to the slide where remnant high pressures can still be found. 3odelling ofthe pore pressures found that excess pore pressure ratios of the order of -. were reuired to cause

failure. ;n simple terms this means that the strength of the sediment was reduced to about 1-I of its

normal level. *here is clear evidence that instability only developed at certain levels in the sediment

and that conseuently most of the sediment pile was inherently stable. *his is best seen on highB

resolution seismic data that show faulted and fractured slide debris preserved within the slide scar

(3valstad et al . ())5). ;t is also shown by the long term stability of the steep slide headwall scarps

which have stood with angles of up to $9° for over ?--- years since the excess pore pressure in the

weak layers was released by the landslide.

*he area of the Storegga slide shows a longBterm history of sedimentation and landsliding that

reflects glacialMinterglacial cyclicity (Solheim et al . ())5). *his leads to the conclusion that the state

of stability that has characteried the Storegga slide area since the last landslide occurred is unlikely

to change until the next interglacialMglacial cycle has been completed.

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@. *anary island landslides

&argeBscale landslides are a common feature of volcanic ocean islands such as 4awaii the %anary

islands and eunion island ($oore et al . 1"/" 1""+F Watts % $asson 1""5F $asson

1""#F Ollier et al . 1""/F$asson et al . ())(). *he ,uuanu landslide off 0ahu in the 4awaiian

islands with an estimated volume of 9---km$ may be the largest single landslide on earth

($oore et al . 1"/"). &andslides on volcanic islands typically take two formsLdebris avalanches and

slumps (in the terminology used in this paper a slump is a type of slide). As defined by$oore et al .

>1"/"? a debris avalanche is a relatively thin (-./Q#km thick) landslide with a clear evacuated

headwall and a distal train of blocky debris. 5ach debris avalanche appears to be a single event at

least in terms of geological time and some show evidence for rapid and energetic emplacement. ;n

contrast a slump involves gradual intermittent downslope movement of a thick (up to 1-km)

coherent block of the island flank.

*he history of landslides in the %anary islands over the past one million years is now well

understood (see $asson et al . >())(? and references within). *he bulk of landslide activity is

associated with the youngest and most volcanically active islands of *enerife &a Dalma and 5l

4ierro (-igure #). 0n average one landslide has occurred somewhere in the %anary islands every

1----- years although this figure masks an irregular distribution through time ($asson et al .

())(). *he youngest landslide occurred on the island of 5l 4ierro some 19--- years ago. 3ost of

the landslides are debris avalanches with slumps only recognied on the youngest island 5l 4ierro

perhaps suggesting that this landslide style is a feature of early island development. A typical %anary

island debris avalanche is marked by a nearBvertical amphitheatreBshaped headwall on the island

an erosive chute on the upper part of the submarine island slope and a pile of avalanche debris at

the foot of the steepest island slope usually at $---Q/---m waterdepth ( -igure @). *his typical

avalanche has a volume of 9-Q#--km$ covers an area of a few thousand km# and has a runBout of

9-Q1--km. 7lide planes at the base of the landslide are typically up to 1-° on the upper slope

decreasing to less than 9° on the lower slope (Watts % $asson 1""5F Gee et al . ())1FWatts %

$asson ())1). &arge accumulations of debris such as seen north of *enerife or west of &a Dalma

are clearly the cumulative result of several landslides rather than single larger events that have

occurred in the past. 5ven some deposits thought to be the result of a GsingleH landslide event (in

geological time) show signs of more than one phase of emplacement (Watts % $asson ())1). ;t is

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notable that %anary island debris avalanches are an order of magnitude smaller than those on

4awaii possibly reflecting the larger sie and higher magma production rates of the 4awaiian

islands or the more rapid development of instability on the relatively steeper %anary island slopes.

%anary island landslides identified as debris avalanches according to the definition of $oore et al .

>1"/"? show a variety of structures that suggest elements of both debris avalanche and debris flow

emplacement mechanisms (-igure /F $asson et al . ())(). ;n addition these landslides can also

initiate turbidity currents that are capable of flowing considerable distances downslope. Sediment

cores recovered from the deep Agadir 8asin about $--km to the north of the islands contain

turbidite deposits which based upon their mineralogy geochemistry and age are interpreted to be

linked to %anary island landslides (Wynn et al . ())(FWynn % $asson ())2). Specifically a turbidite

deposited at ∼19kyr is linked to the 5l 7olfo landslide on 5l 4ierro while an older turbidite dated

at ca 1>-kyr is linked to the ;cod landslide on *enerife. 3ost turbidite deposits in the Agadir 8asin

are actually derived from the 3oroccan continental margin to the east (Wynn et al . ())() and show

the typical smooth upwardBfining grainBsie profile that is typical of graded turbidite deposits (-igure

"). 4owever the two turbidites derived from %anary islands landslides show a stepped grainBsie

profile that appears to represent deposition from a series of GminiBturbiditesH (-igure "). *his pattern is

interpreted to be the result of a multiBstage source landslide as other potential causes e.g. flow

reflection multiple pathways or pulses can be ruled out (Wynn % $asson ())2). *his hypothesis is

also supported by the fact that similar turbidites linked to 4awaiian landslides show the same pattern

of stacked miniBturbidites (Garcia 1""#). +etailed sedimentological analysis of the Agadir 8asin

turbidites has revealed that their source landslides probably occurred in several retrogressive stages

over a period of hours or days rather than weeks or months (Wynn % $asson ())2). Assessing the

sedimentary record of deposits derived from these landslides is therefore critical when assessing

their tsunamigenic potential since it is clear that a series of smaller landslides spread over several

hours will have a much smaller tsunamiBbuilding potential than a single large instantaneous

landslide (see R?).

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. andslide8generated tsunamis

*he generation and propagation of tsunamis resulting from earthuakes have been studied for the

last 9- years and are now relatively well understood (6ardet et al . ())2). ;n contrast the importance

of tsunamis generated by landslides has only become widely recognied during the last fifteen years

or so when it became apparent that a landslide source could explain the unusual runBup

distributions and propagation characteristics of certain particularly deadly tsunami such as the 1?

D,7 event (Ward ())1F 6ardet et al . ())2 and references thereinF Okal % Synolakis ())+).

4owever the complexity and variability of submarine landslides means that we are still some way

from a comprehensive understanding of the range of tsunamis that landslides are capable of

producing. 3odelling of landslide tsunamis has shown both that extreme wave heights of hundreds

of metres might be possible (Ward % 4ay ())1F $c$urty et al . ())+) but that models are sensitive

to the geological input parameters and the hydrodynamic assumptions adopted in the model (Ward

())1F!augen et al . ())5F ovholt et al . ())5) with the result that poorly constrained model

predictions will have large uncertainty.

+espite the variability of submarine landslides that might cause tsunamis many such tsunamis show

similar general characteristics. ;n particular these tsunamis often have very large runBups close to

the landslide site but appear to propagate much less efficiently than earthuake tsunami so have

limited farBfield effects (Okal % Synolakis ())+). *his was exemplified by the 1? D,7 tsunami

where waves up to 19m high affected a #-km segment of coast killing ##-- people ( $cSaveney et

al . ()))) even though farther a field the tsunami was not a significant event (Okal % Synolakis

())+). *his is a conseuence of the relatively small source areas of most landslide tsunami

(compared to the areas affected by large earthuakes) that leads to the generation of shorter

wavelength waves. *hese are more prone to coastal amplification (increasing the local effect) and to

radial damping (decreasing the distal effect). *his contrasts with the lack of radial damping seen in

earthuake tsunamis that are generated by elongate twoBdimension sourcesF these tsunamis

propagate perpendicular to the source fault with little radial spreading.

*he tsunami generated some ?#-- years ago by the Storegga slide off ,orway is one of the bestB

understood landslide tsunamis. A wellBpreserved record of tsunami deposits on land in ,orway

Scotland and the =aroe islands (4awson et al . 1"//F 6ondevik et al . 1""@ ())5) coupled with an

unprecedented knowledge of the landslide processes (6ryn et al . ())2 ())5F !a-lidason et al .

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())2 ())+F 3valstad et al . ())5) provides the basis for rigorous testing of tsunami models and

allows the key landslide parameters to be identified (6ondevik et al . ())5F !augenet al .

())5F ovholt et al . ())5). *he key findings are that landslide volume velocity initial acceleration

length and thickness all contribute to the determination of tsunami character. *he best indicator of

tsunamigenic potential is the product of volume and initial acceleration (ovholt et al . ())5). An

abrupt deceleration might also contribute to larger surface elevations. *he slide length affects both

the wavelength and the maximum surface elevation (!augen et al . ())5) while the wavelength is

also determined by the travel time or runBout distance of the slide. Submarine slides are normally

clearly subcritical i.e. the =roude number (the ratio of slide speed to the speed of wave propagation)

is much less than one. *his implies that the tsunami will run away from the waveBgenerating slide

limiting the buildBup of the wave. Slides in shallow waters are more critical since the speed of wave

propagation is lower here. 3oreover shallower water normally means less distance to the coast anda shorter distance available for radial damping. ;n contrast tsunamis generated by earthuakes are

more critical when the seabed displacement occurs in deeper waters as the initial wave (which in

this case depends much less on the water depth) will become shorter and more dangerously

amplified when propagating from deeper to shallower waters.

*he Storegga slide is best modelled as a retrogressive slide with a peak velocity of #9Q$-m

sK1 (6ondevik et al . ())5). *he retrogressive slide of total length L is modelled as a train of N fixed

block slides released at different times t but moving with identical velocity distributions (!augen et

al . ())5). =or simplicity the blocks have the same thickness h and the same length LMN . 3oreover

the time lag Δt between release of two ad6acent blocks is assumed to be eual. =or waves

propagating in the same direction as the slide increasing Δt increases the distance between the

surface elevations caused by the individual block modules. *his decreases the overlap and results in

a smaller amplitude and longer wave (-igure 1)). =or small time lags the wave remains smooth but

as Δt increases the distances between the individual block modules become large and the discrete

nature of the retrogressive slide starts to show. 5ventually when Δt is sufficiently large the waves

generated by the block modules are completely separated.

a larger amplitude but shorter wave. :hen the time lag euals the time it takes for the wave to

traverse a block module i.e. Δt LMN (gH )K1M# where H is the water depth the individual surface

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