Great Landslides - Tsunami - page 130

Fig. 7.3 Schematic
representation of a coherent
submarine slide

Tsunami wave propagation
onshore

Tsunami wave
propagation offshore

Slide

β

Thickness

slope angle

Debris
flow

Turbidity current

wave followed by a trough up to three times greater in
amplitude. The lead wave may travel faster than the slide. The
second wave in the wave train has the same amplitude as the
trough, but over time, it decays into three or four waves with
decreasing wave periods. The initial inequality between the
crest of the first wave and the succeeding trough enables
landslide-generated tsunami to obtain greater run-up heights
than those induced by earthquakes.
Wave generation by landslides depends primarily upon
the volume of material moved, the depth of submergence,
and the speed of the landslide (Watts 1998 ). The volume of
material can be determined knowing the height of the slide,
its horizontal length, and the initial slope (Fig. 7.3 ). How-
ever, the characteristics of a slide critical to modeling tsu-
nami are inevitably determined well after the event. The
easiest slide to model is one where material fails as a
coherent block. However, most slides disintegrate into a
debris avalanche and eventually a turbidity current. Turbidity
currents are irrelevant in the generation of tsunami, because
by the time sediment has become mixed with water and
begun to stratify in the water column, the tsunami has been
generated and is moving away from its source area. Turbidity
currents are only important in laying down distinct deposits
that are more than likely a signature of paleo-tsunami.
Most slides slowly accelerate reaching a terminal
velocity that depends upon the slide's mass and density, and
the angle of the slope (Watts 1998 ). Hence, a submarine
landslide takes time to develop and generate any tsunami. If
an earthquake triggers a tsunamigenic submarine landslide,
the time between the earthquake and the arrival of the
tsunami at shore is longer than expected. The wavelengths
and periods of landslide-generated tsunami range between
1-10 km and 1-5 min respectively. These values are much
shorter than those produced by earthquakes. The wave
period of a landslide-generated tsunami increases as the size
of the slide increases and the slope decreases. It is inde-
pendent of water depth, the depth of submergence, and the

mass of the block. As a first approximation, the simplest
slide to model is one that occurs on a slope of 45. In this
case, the maximum height of a tsunami wave above still
water can be approximated by the following formula:

H max c 1 : 75 d 0 : 75

ð 7 : 2 Þ

where
c = the thickness of the slide (m)
d = the initial depth of submergence in the ocean (m)
H max = the maximum height of a tsunami wave above still
water
Submarine landslides rarely exceed 50 m s -1 , while
their associated tsunami travel at 100-200 m s -1 . Recent
modeling indicates that these latter velocities may approx-
imate 1,500 km hr -1 . Hence, tsunami generated by sub-
marine landslides outpace the slide that forms them and
produces several simple long waves in a wave train (Peli-
novsky and Poplavsky 1996 ). In this case, that maximum
tsunami wave height becomes independent of bottom slope
and the following equation may apply:

H max ¼ 0 : 25pc 2 d 1 ð 7 : 3 Þ

These heights can be used to calculate other characteristics
of the tsunami when it reaches shore using the equations
presented in Chap. 2 .

7.4 Historical Tsunami Attributable

to Landslides

Both terrestrial and submarine landslides can produce tsu-
nami. While historically rare, both are impressive. For
example, the largest tsunami run-up yet identified occurred at
Lituya Bay, Alaska, on July 9, 1958 following a rockfall
triggered by an earthquake (Miller 1960 ). Water swept 524 m
above sea level opposite the rockfall, and a 30-50 m tsunami