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B
0
j
H
l
z
h
l
y
V
x
Fig. 10.12
A schematic plot of tsunami waves in an open ocean. The mass velocity
V
and Hall
current density
j
H
are shown with
red and blue arrows
, respectively
at the front of wave and gradually decrease at its end. It is unimportant as far as
the estimate of electromagnetic effect since only the total volume of moving water
will enter this estimate. Assuming that both the unperturbed magnetic field
B
0
and
the mass velocity
V
are in
z
x plane, the Hall current
j
H
D
w
V
B
0
is parallel to
y axis as shown in Fig.
10.12
. Moreover we shall ignore the variations of seawater
conductivity
w
with depth. As the distance r to the tsunami wave is smaller than its
lateral size l, the linear infinite current should be considered as the most appropriate
model for the calculation of GMPs ı
B
. Taking into account that ıB
D
0
I
H
=.2r/
where the total Hall current is equal to I
H
D
j
H
h, we obtain the simple estimate
0
w
VB
0
h
2r
ıB
:
(10.56)
Using the following numerical values of the parameters: V
D
5 cm/s,
w
D
5 S/m, B
0
D
5
10
5
T,
D
100 km, h
D
2 km and the distance r
D
500 km,
we get ıB
D
1 nT. This value is close to other estimates (Tyler
2005
) in which
the magnetic field variations generated by the Indian ocean tsunami could have
reached an amplitude of 4 nT. Manoj et al. (
2010
) have reported that tsunamis could
also induce an observable electric potential difference in deep submarine cables. It
should be emphasized that the electromagnetic effect of tsunamis is essentially due
to the large volume of the moving water although the mass velocity is as small as
several centimeters per second.
The hypothesis that tsunamis can generate detectable magnetic variations has
been recently examined for the strong Chilean EQ (M
D
8:8) on February 27, 2010
(Nair et al.
2010
) followed by a moderate tsunami that crossed the Pacific Ocean
westward. The measurements provided by the magnetic observatory on Easter
Island, 3,500 km west of the epicenter have shown a periodic signal of 1 nT in the
vertical magnetic component, coincident with the arrival time of the tsunami. These
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