Geoscience Reference
In-Depth Information
the ocean bottom with high velocities, transferring part of the accumulated potential
energy to tsunami waves. Precipitations, annually accumulated in some canyons,
amounts to 10
6
-10
9
m
3
, while the bottom slopes often exceed 0.1. Precipitations on
slopes of the ocean bottom often exhibit thixotropic properties, i.e. they are capable
of becoming fluid in the case of sharp enhancement of the threshold pressure due to
blows, shaking and vibrations. The unstable friable sedimentary material, possess-
ing a high content of subcolloidal fractions, may, when losing stability, form dense
suspension (muddy, turbidite) flows. Moving down a bottom slope with a velocity
exceeding 10 m/s, such a flow leads to waves of the tsunami type being generated at
the water surface, and it also severs underwater cables. The strong earthquake that
destroyed the city of Messina on December 28, 1908, gave rise to a landslide or
muddy flow, that severed seven underwater cables connecting continental Italy and
Sicily.
It must be noted that well-known large underwater canyons were located inside
the source areas of some strong tsunamis: the Lisbon canyon (tsunami of 1755),
the Messina canyon (tsunamis of 1783 and 1908), the Kamchatka canyon (tsunamis
of 1791, 1923, 1937) and others. N. L. Leonidova was, evidently, the first to note
that most aftershocks of strong tsunamigenic earthquakes, even when approxi-
mately equal in force to the main shock, do not cause noticeable tsunami waves.
Thus, the well-known Kanto earthquake, that destroyed Tokyo in 1923, gave rise in
the Sagami bay to a tsunami wave 12 m high, while its aftershock, that originated
in about the same place and with practically the same energy, was accompanied by
waves less than 0.3 m high. Measurements showed that the volume of the landslide,
provocated by the first earthquake, amounted to about 7
10
10
m
3
, the average width
of the flow was 2 km, its length 350 km, it power (thickness) 100 m, the flow velocity
in the canyon was estimated to be 25 m/s. The potential energy of the landslide that
covered a path from a depth of 1,500 m (the average position of the landslide body
at the beginning of its movement) down to 7,000 m (the bottom of the deep-water
depression) can be estimated to have been 10
18
J. The energy of the tsunami waves
generated was of the order of 10
16
J.
Note that after the earthquake of December 26, 2004 (
M
w
= 9
.
3) that gave rise
to a catastrophic tsunami with run-ups as high as 35 m, another strong earthquake
took place in March 2005 (
M
w
= 8
.
7) approximately in the same region, but caused
quite a weak tsunami with heights up to 2 m.
Much of the information on ground or underwater landslides, avalanches and
cliff collapses indicate that the models, in which the movement of a landslide is
considered just forward displacement of a solid body, not subject to deformation,
are too simplistic and do not describe the character of these processes adequately.
The idea of a landslide representing a flow of a heavy viscous fluid is much closer to
the true nature of landslide dynamics. In the region of river estuaries the sedimentary
silt masses usually consist of diluted fractions, which after the breakdown of an
unstable sedimentary mass form a dense dirt (mud) flow, behaving like a viscous
fluid.
In problems concerning landslide tsunami generation the notion of a landslide
in the form of a flow of a heavy viscous fluid has started to be applied only quite
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