Geomagnetic Perturbations (GMPs) - Ultra and Extremely Low Frequency Electromagnetic Fields - page 275

beginning of both the effects can be associated with the moment of seismic wave
arrival at the ground-recording stations.
Nevertheless such interpretation of the phenomena should be made with some
care because there is no information on how the ground shaking due to seismic
vibration affects the sensor operation. In the above experiments (Sweeney 1989 )
the magnetic and electric sensors were embedded in the rock, so that the factor
of the rock vibrations should be taken into account for correct interpretation of
the experimental data. Here we mean the so-called microphone effect, vibration
of transistors, ferrite cores, changes of the effective capacity and inductance of the
electric circuits of the instruments and etc. This problem has been discussed in more
detail by Eleman ( 1965 ).
In the last decades some empirical evidence of the electromagnetic signals
possibly associated with EQs has been reported by Eleman ( 1965 ) and Belov et al.
( 1974 ). These signals lie in the ULF band, i.e., 0.1-1 Hz or lower, which is close
to typical frequencies of the seismic waves radiated by EQs. The co-seismic signals
have been observed practically simultaneously with the moment of the seismic wave
arrival (Nagao et al. 2000 ; Skordas et al. 2000 ; Huang 2002 ) or within several
minutes around this moment (Takeuchi et al. 1997 ; Molchanov and Hayakawa
2008 ). In the early study it was supposed that such low-frequency variations can
be carried out by seismic waves at the distances of thousands kilometers from the
EQ hypocenters (Gogatishvili 1983 , 1984 ; Sorokin and Fedorovich 1982 ). Much
more treatise must be done to either validate or disprove this hypothesis because of
a small value of signal-to-noise ratio (Surkov 2000a ). It seems more plausible that
tsunamis are able to give rise a detectable value of the ULF fields due to the fact that
the magnitude of tsunami and the conductivity of the sea-water are much greater
than the magnitude of the seismic wave and the ground conductivity, respectively
(Pavlov and Sukhorukov 1987 ; Gershenzon and Gokhberg 1992 ; Chave and Luther
1990 ).

7.2.2 Basic Equations

The upper layer Earth's crust is a good conductor despite low conductivity of
dry rocks. Basically, this is because of the presence of groundwater in pores and
channels that occur up to several km depth. The ground conductivity varies with
depth and regions. In the range of seismic frequencies the mean conductivity of
continental crust (the layer of sedimentary rocks) is about 10 2 -10 3 S/m and
the dielectric permittivity of the ground is about " 10.
To model the conducting layers of the Earth's crust, consider first a conductive
elastic space with the specific conductivity immersed in an external constant
geomagnetic field with induction B 0 . Actually the value and inclination angle of the
Earth's magnetic field depend on magnetic latitude. In what follows we consider an
acoustic/seismic wave propagating in this conducting medium. The field of the mass
velocities V D V . r ;t/ is supposed to be a given function of the position vector r