Geoscience Reference
In-Depth Information
30 Hz dies out completely in sand at a distance of 2 km from the source, and in
consolidated sedimentary rock at a distance of 10 km. In water such a signal is reli-
ably registered at distances of up to 1,000 km.
Hence follows the important conclusion that hydroacoustic monitoring of the pre-
paration process of an oceanic earthquake may lead to success in resolving the dif-
ficult problem of revealing in good time a nearby earthquake at its preparation stage
and of issuing a timely and effective warning of the possible rise of a local tsunami.
During strong underwater earthquakes the ocean bottom in the epicentral area is
deformed, and the deformation not only has a horizontal component, but a vertical
one as well. If the motion is directed vertically upward, then a wave of compression
forms in the water and propagates towards the surface; if the motion is downward,
then a decompression wave forms. When reflected from a free surface of water, an
elastic wave changes its polarity, therefore independently of the sign of the defor-
mation, there may always be realized a wave of decompression, which tends to 'tear
apart' the liquid. The amplitude of pressure variations related to elastic waves can
be calculated by the formula
p
d
=
ρ
cU
,
ρ
where
is the water density,
c
is the velocity of sound in the water and
U
is the ve-
locity of motion of the ocean bottom. If the bottom moves with a velocity of 1 m/s,
the pressure amplitude will amount to
p
d
= 1
.
5MPa.
Besides variations of pressure due to elastic waves, in the water layer there exists
a hydrostatic pressure, the increase of which with the depth is approximately linear,
p
st
=
p
atm
+
ρ
g
z
.
The total pressure
p
d
+
p
st
at large depths is always positive, but in the layer
near the surface (for
U
= 1 m/s down to
z
140 m) a situation may arise, when
the dynamic pressure exceeds the hydrostatic pressure in absolute value, so the total
pressure turns out to be negative.
The limit strength of water under tension is known to be about 0.25 MPa.
Therefore, in the subsurface layer, where the total pressure
p
d
+
p
st
exceeds in ab-
solute value the limit strength of water, violation of the water continuity is possible,
and it is called cavitation. The influence of the described mechanism taking place
above the epicentre of an underwater earthquake results in the formation of a zone of
cavitating (partly foaming) water. This zone has a reflection coefficient (albedo) dif-
fering from the reflection coefficient of all the remaining surface of the aquatorium.
In this case the perturbated zone of the water surface can be registered by remote
methods (from satellites, airplanes, etc.). Note that cavitation effects in the subsur-
face layer are observed in the case of underwater explosions.
≈
1.8 Killer Waves in the Ocean
The killer-wave phenomenon is in no way related to the seismic activity of the
sea-floor; however, the authors considered it expedient to present this rare and
catastrophic oceanic event in the topic. It is a striking example of the origination
