Great Landslides - Tsunami - page 126

7

Great Landslides

7.1 Introduction

generated historically by earthquakes (Moore 1978 ; Masson
et al. 1996 ). The most notable event to occur in the twen-
tieth century was the Grand Banks Tsunami of November
18, 1929. This event, to be discussed in detail subsequently,
resulted from a slide that had a volume of 185 km 3 (Piper
et al. 1999 ). Geologically, larger slides with volumes over
5,000 km 3 are known (Masson et al. 1996 , 2006 ). For
example, debris flows around the Hawaiian Islands have
involved these volumes, while the Agulhas slide off the
South

Chapter 6 on tsunamigenic earthquakes continually alluded
to submarine landslides as being a contributing factor in the
generation of anomalous tsunami. For example, the tsunami
that swept Prince William Sound following the Great
Alaskan Earthquake of 1964 and the one that swept the
Pacific Ocean following the 1946 Alaskan earthquake are
now recognized as being the products of submarine land-
slides (Sokolowski 1999 ). The latter event was large enough
to affect Hawaii. Unfortunately, the evidence of sliding is
difficult to obtain, especially in regions where detailed
side-scan sonar surveys have not been performed before-
hand (Fig. 7.1 ).
About 70 % of the Earth's surface consists of oceans
containing tectonically and volcanically active areas near
subduction zones. The oceans also consist of very steep
topography along continental shelf margins, on the sides of
ocean trenches, and on the myriad of oceanic volcanoes,
seamounts, atolls, and guyots that blanket the ocean floor
(Moore 1978 ; Lockridge 1990 ; Canals et al. 2004 ). Sedi-
ment moves under gravity down these slopes through a
variety of processes that include slumps, slides, debris
flows, grain flows, and turbidity currents. Debris flows can
move without incorporating water; however, where material
mixes with water, a dense turbid slurry of sediment can
move as a current along the seabed under the effects of
gravity. The latter are known as turbidity currents and form
distinct deposits that were described in Chap. 3 . Substantial
volumes of material are transported long distances, on
slopes as low as 0.1, across the deep ocean seabed by
slides, flows, and turbidity currents. Slides consist of basal
failure of topography that moves downslope in coherent
blocks. As the slide progresses downslope it may disinte-
grate, producing a debris flow that mainly consists of di-
saggregated sediments. Where large volumes of material are
involved, these processes can generate tsunami ranging
from small events concentrated landward of the failure to
mega-tsunami an order of magnitude larger than those

20,000 km 3 Africa coast contains a total of of

material (Table 7.1 , Fig. 7.2 ).
Landslides can take the form of slumping of rock and
unconsolidated sediment, or rotation of material along
planes of weakness in the rock (Moore 1978 ). The latter
often leaves a distinct scar or headwall eroded into the
continental shelf slope or exposed on land as an amphi-
theater formed in cliff lines. Rotational slides may also
generate transverse cracks across the body of the slipped
mass and tensional cracks above the head scarp, which
gives rise to subsequent failure. As shown in Chap. 5 ,
earthquakes are the most likely triggering mechanism for
landslides, especially submarine ones that have commonly
been associated with tsunami.

7.2 Causes of Submarine Landslides

On dry land, slope failure can be modeled using the Mohr-
Coulomb equation (Bryant 2005 ) as follows

s s ¼ c þ r n ð Þ tan u ð 7 : 1 Þ

where
s s = the shear strength of the soil (kPa)
c = soil cohesion (kPa)
r = the normal stress at right angles to the slope (kPa)
n = pore water pressure (kPa)
u = the angle of internal friction or shearing resistance
(degrees)