Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 369

to distinguish EQ-induced electromagnetic signatures from the background noise
produced by ionospheric and magnetospheric currents, man-made interference and
so on. The interested reader is referred to the topics and reviews by Johnston ( 1989 ),
Park et al. ( 1993 ), Hayakawa and Fujinawa (Eds., 1994 ), Parrot ( 1995 ), Hayakawa
(Ed., 1999 ), Surkov ( 2000a ), Hayakawa and Molchanov (Eds., 2002 ), Molchanov
and Hayakawa ( 2008 ), Hayakawa (Ed., 2009 ) and Hayakawa (Ed., 2013 )fora
history and recent studies of electromagnetic phenomena which can be associated
with the impending EQs.

10.1.2 Theory of Transient Electromagnetic Field Generated

by Electric Charges on Crack Surfaces

In an early study of the EQ precursors, it was hypothesized by Gokhberg et al.
( 1979 , 1982 ), Sadovsky et al. ( 1979 ), Warwick et al. ( 1982 ), Parrot et al. ( 1985 ) and
Oike and Ogawa ( 1986 ) that the increase in natural radiowave emission occasionally
observed before the EQ occurrence can serve as a possible candidate for the EQ
precursor. The narrow-band receivers at frequencies from f D 81 kHz (Gokhberg
et al. 1979 , 1982 ) to 10 MHz (Warwick et al. 1982 ) have been used to detect
the radiowave emission. It was generally accepted that the possible cause of the
observed emission is oscillations of fluctuating charges on the sides of underground
cracks (e.g., see Gershenzon et al. 1989 ).
First of all it should be noted that the skin-depth in the conducting ground at this
frequency range is about ı . 0 f / 1=2 1-30 m. Actually this means that the

radiowave can come from the depth whose value is no more than 1-30 m because
of strong damping of such a kind of emissions in the conducting ground (Surkov
2000a ; Molchanov and Hayakawa 2008 ). Thus, the radiowaves coming from the
ground surface cannot provide us with any information about tectonic processes in
the EQ focal zone.
Moreover, the above frequency range seems to be typical for emission of
microcracks. The electric charges formed during the microcrack growth can radiate
electromagnetic waves at the frequencies which can be roughly estimated as f
V c =l c , where V c is the velocity of crack growth and l c stands for the crack length.
Taking the mean values: f D 1 MHz and V c D 1 km/s, we obtain the estimate of
the crack size l c D 1 mm. Surkov ( 2000a ) has shown that any reasonable number
density of such microcracks situated in the surface layer of the ground cannot
explain the observed intensity of radiowave emission.
In the theory by Molchanov and Hayakawa ( 1994 , 1995 ) the main emphasis is
on the electric currents generated in the conducting rock due to the formation of
microcracks. The ULF electromagnetic field was assumed to be excited by all the
currents resulted from the development of microcrack ensemble around the focal
zone. Below we do not follow the papers by Molchanov and Hayakawa ( 1994 , 1995 ,
1998 ) and choose the simplest but not rigorous way to reproduce the main results of
these papers.