Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 388

It is generally accepted that there are two candidate mechanisms, which can
be responsible for variations of the rock basement conductivity: (1) The content
variations of the conducting underground fluid (Mazzella and Morrison 1974 ;
Takahashi and Fujinawa 1993 ) and (2) The changes of dry rock conductivity itself
associated with variations of tectonic stresses and strains at higher depths (Varotsos
and Alexopoulos 1986 ; Slifkin 1993 ; Freund 2000 , 2002 ; Freund and Pilorz 2012 ).
The study in the last decades was indicative of the presence of the underground
fluid at higher depths down to tens kilometers (Nikolaevskiy 1996 ). At the moment
there are no reliable data about the physical state, mineral composition, and other
parameters of the underground fluid. However, the underground fluid flow plays a
key role in the preliminary stage of the EQ preparation in many models of the EQ
preparation process such as the dilatancy-diffusion model (Nur 1972 ; Scholz et al.
1973 ) or crack migration model (Surkov et al. 2002b ). On the other hand, the water
conductivity is much greater than that of dry rocks at least at low pressure. In this
picture the EQ preparation can affect the mean rock conductivity due to changes of
the underground fluid content.
As has already been stated in Chap. 9 , the laboratory tests (e.g., see reviews by
Urusovskaya ( 1969 ) and by Mineev and Ivanov ( 1976 )) have shown that the solid
conductivity is enhanced under the applied stress basically due to the increase in
number density of charged dislocations and point defects of atomic lattice. Though
the mobility of interstitial ions, vacancies, and especially dislocations are not so
high because of their large effective masses. Recently Freund ( 2002 ) has suggested
an alternative mechanism of the stress-induced rock conductivity based on the time-
resolved impact experiments. The increase in the rock conductivity is supposed
to be due to the clouds of highly mobile positive hole charge carriers, i.e. defect
electrons/holes. Below we consider this mechanism in a little more detail since it
may be of interest in understanding the rock conductivity at higher depths.
Majority of minerals in the rock basement are silicates which contain O 3 Si-O-
SiO 3 and O 3 Si-O-AlO 3 structural units or bonds, in which all oxygen anions have
the valence 2 . When traces of H 2 O structurally dissolve in the matrix of such
silicates, the following reactions can take place

H 2 O C O 3 Si-O-SiO 3 ! O 3 Si-O-O-SiO 3 C H 2 ; (10.44)

thereby producing the so-called peroxy bond. If the applied shear stresses exceed the
elastic limit that results in the generation of large amount of dislocations, the peroxy
link can be destroyed due to intersection of the mobile dislocation and the peroxy
link. When the broken peroxy link meets O 2 , it acts as an electron receptor, which
can hold the electron for a long time. The O 2 , which has donated the electron, turns
into O . This new anion plays a role of positive hole because it has one electron
less as compared to all other oxygen anions. The lifetime of the holes is assumed to
vary from several seconds to several weeks. In this picture, this process is similar
to the conventional transition from an insulator state to a semiconductor state. It is
conjectured that this p-type conductivity can take place in the middle and partly in
the lower crust in the wide temperature interval from about 400 ı Cto500-550 ı C
(Freund 2000 , 2002 ; Freund and Pilorz 2012 ).