Geoscience Reference
In-Depth Information
3.1.6 Elastic Oscillations of the Water Column at the Source
of the Tokachi-Oki Tsunami, 2003
Till recently all the information on tsunami sources was obtained exclusively by
remote measurements using mareographs (coastal or deep-water), hydroacoustic
systems or seismographs. The absence of direct measurements at the tsunami
sources, in part, explains why processes at the epicentral zones of underwater earth-
quakes have been studied relatively weakly.
The possibility, in principle, of investigating the formation of a tsunami at its
source arose in 1996, when a set of registering devices, comprising sensors of ocean
bottom pressure (JAMSTEC, Japan Agency for Marine-Earth Science and Tech-
nology) was established on the continental slope close to the islands of Japan. The
sensors are connected to the registration site on the coast by cable lines. Variations
of the pressure at the ocean bottom are recorded continually with a sampling fre-
quency of 1 Hz.
The Tokachi-Oki earthquake of 2003 was the first strong seismic event, the epi-
centre of which was located in the immediate vicinity of the JAMSTEC sensors.
According to the seismic catalogue NEIC, this event took place on September 25
at 19:50:06 UTC; the coordinates of its epicentre were 41.78 N, 143.86 E; its
hypocentre depth was 27 km, its magnitude was 8.3
M
w
(HRV). The earthquake
gave rise to a tsunami wave, the height of which amounted to 4 m along the south-
east coast of Hokkaido island.
In this section data are analysed concerning variations of the bottom pressure
registered by JAMSTEC sensors at the source of the Tokachi-Oki tsunami of 2003
[Nosov et al. (2005), (2007); Nosov, Kolesov (2007)]. We consider the water col-
umn at the source to behave as a compressible medium—this is essential, here.
A theoretical analysis of the role, played by water compressibility in the tsunami
problem, carried out in Sect. 3.1.1 permits to assert that elasticity effects turn out to
be essential only at the stage of tsunami generation by an earthquake, while wave
propagation or the runup of a wave onto the coast can be described as the motion
of an incompressible liquid. Simple estimation reveals that the energy of elastic os-
cillations of a water column in the tsunami source area may exceed the energy of
the gravitational tsunami wave by more than an order of magnitude.
If the case of a horizontal absolutely rigid ocean bottom is considered, then
the main difference in the behaviour of a compressible ocean as compared to an
incompressible model medium consists in the formation of elastic oscillations of
the water column, which are characterized by a discrete set of normal frequencies
ν
k
=
c
(1 + 2
k
)
/
4
H
, where
k
= 0
,
1
,
2
...
,
H
is the ocean depth and
c
is the velocity
of sound in water. For typical conditions of a tsunami source the minimal normal
frequency
0
.
1 Hz is excited most effectively.
Real tsunami sources are, naturally, located not on a horizontal ocean bottom, but
in a region of complex bathymetry. But the slope of the oceanic bottom usually does
not exceed the value of 0.1. Therefore, the surface of the bottom can arbitrarily be
represented as a set of quasihorizontal segments, each of which is characterized by
ν
0
=
c
/
4
H
∼
