Geoscience Reference
In-Depth Information
understanding of the processes of wave excitation and propagation. Compared to
coastal measurements, deep-water tsunami registration displays a whole number
of important advantages [Titov et al. (2005)]. First, owing to the tsunami veloc-
ity depending on the ocean depth, the deep-water sensor registers a wave faster
than a coastal mareograph, located at the same distance from the source. Second,
a tsunami wave approaching the coast is strongly distorted (for instance, owing to
resonances in bays) and it 'forgets' the properties of the source that generated it.
Therefore, coastal mareographs are not sensitive to the true frequency spectrum
of a tsunami. At the same time, a tsunami signal in the open ocean is not dis-
torted or filtered and contains all the components of the original spectrum. Third,
the frequency response function of bottom pressure sensors is totally flat within
the range of tsunami waves, while the response function, peculiar to many coastal
mareographs, is complex and not constant. Most mareographs are, generally speak-
ing, not intended for tsunami measurements, since they were created for observing
relatively low-frequency tidal-level oscillations. Fourth, the amplitude of a tsunami
in the open ocean is small compared to the ocean depth, therefore, wave propagation
is described with a very good accuracy by simple linear models. For this reason,
the results of deep-water measurements can be applied effectively in resolving
inverse problems (reconstruction of perturbation forms at sources etc.).
Level registration in the open ocean (at large depths) is a difficult technical task
that has been accomplished only in recent decades. Of the various numerous sys-
tems, quartz sensors provide the best measurement precision and stability.
Variations of pressure at the ocean bottom exhibit a broad frequency spectrum
and are due to a whole complex of processes in the atmosphere, ocean and litho-
sphere. Surface, internal and elastic waves in the water column, as well as seismic
surface waves and changes in the atmospheric pressure all contribute to variation of
the bottom pressure.
Pressure variations, related to surface waves, are known to be felt at the bot-
tom only in the case of long waves (
H
), so for sensors established at large
depths high-frequency sea-level oscillations (wind waves) do not influence the bot-
tom pressure. But in the case of tsunamis or tidal waves changes in the bottom pres-
sure
λ
∆
p
bot
are mostly determined by displacements of the ocean-free surface,
∆ξ
,
with account of changes in the atmospheric pressure
∆
p
bot
=
ρ
g
∆ξ
+
∆
p
atm
, where
ρ
is the density of water and g is the acceleration of gravity. Estimation [Wunsch
(1972)] shows that the influence of baroclinity on the ocean bottom pressure can be
neglected. Elastic oscillations of the water column, caused by seismic movements
of the bottom (for instance, due to surface seismic waves), may provide a signifi-
cant, and even definitive, contribution to the bottom pressure
cU
∗
, where
c
is
the velocity of sound in water,
U
is the vertical velocity of movement of the bottom.
But, owing to the difference in propagation velocity between tsunamis and seismic
waves, the arrival times of 'seismic noise' and of the tsunami signal may differ.
In those cases, when the pressure sensor is located near the epicentre, the tsunami
signal can be singled out by filtration of the high-frequency seismic component.
∆
p
=
ρ
∗
On condition that
T
>
4
H
/
c
(incompressible ocean):
Ha
,where
a
is the vertical acceler-
ation of movement of the bottom,
T
is the period of movement of the bottom.
∆
p
=
ρ
