Geoscience Reference
In-Depth Information
Fig. 3.13 Relative arrangement of the earthquake epicentre (full black circle) and sensors of ocean
bottom pressure (triangles). The dotted line shows the region of ocean bottom deformation (esti-
mation). The isobaths are drawn with an interval of 1 km. The numbers in the figure are charac-
teristics of the sedimentary column (from top to bottom: thickness in km, velocity of longitudinal
waves in km/s, density g/cm
3
)
of pressures may be several times larger than the value of
ρ
cU
. Therefore, the value
of
U
0
.
1 m/s is a good estimate for the velocity of vertical motion of the ocean
bottom. We recall that seismologists consider the destruction of buildings and civil
structures to start when the ground moves with a velocity on the order of 0.1 m/s.
In Figures 3.14b and 3.15b, the variations of near-bottom pressure are presented
in a magnified scale, permitting to see the formation of residual displacements of
the ocean bottom, caused by the earthquake. To discard the high-frequency compo-
nents (
>
0
.
02 Hz) the intial time series were subjected to numerical filtration. The
result of filtration is shown in the figures by dotted lines. The smooth decrease of
pressure down to the starting point of the earthquake is related to tidal variations of
the ocean level. The behaviour of the dotted curve clearly shows that the earthquake
resulted in the average pressure at sensor PG1 decreasing by
∼
∆
p
PG1
≈
4 kP, and at
sensor PG2
1
.
5 kP, which corresponds to a reduction of
the water level (elevation of the bottom) by
by the quantity
∆
p
PG2
≈
∆
H
PG
1
≈
0
.
4 m and
∆
H
PG
2
≈
0
.
15 m,
∆
∆
p
/
ρ
respectively (
g, where g is the acceleration of gravity). Note that resid-
ual deformations of the ocean bottom were first revealed from the data considered
by the authors of [Watanabe et al. (2004)].
According to the Harvard CMT Catalog, the half duration of the process at
the source amounted to
H
=
τ
EQ
= 33
.
5 s. But from Figs. 3.14b and 3.15b it is seen
that the pressure decreases during an essentially longer period of time (
∼
900 s).
