Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 387

It should be emphasized that both theoretical models predict that the ULF
seismo-electromagnetic signals related to the EQ focus become undetectable at the
distance greater than approximately 100 km far from the EQ epicenter. According
to our estimations the magnitude of the electromagnetic noise inside this region can
be of the order of approximately 1V=m Hz 1=2 and 0:1 nT=Hz 1=2 .

10.1.5 Variations of the Rock Basement Conductivity

and of Telluric Voltage Possibly Associated with EQs

Long-term observations in seismo-active regions have shown that the variations of
the rock basement conductivity can be generated synchronously with an enhance-
ment of seismic activity several months before and after the EQ occurrence
(Myachkin et al. 1972 ; Sobolev et al. 1972 ; Sobolev 1975 ; Rikitake and Yamazaki
1978 ; Honkura 1981 ; Rikitake 1987 ; Meyer and Teisseyre 1989 ; Park and Fitterman
1990 ;Park 1991 ; Bragin et al. 1992 ). For example, Rikitake and Yamazaki ( 1978 )
observed the gradual decrease in the ground conductivity down to several percent
several hours before the main shock in 21 cases among 30 ones when the considered
effect was observed. In-situ measurements, both passive and active techniques
have been used for measurements of rock conductivity and telluric voltage. In an
early study, the potential difference between a pair of grounded electrodes was
measured to reveal abnormal behavior of telluric currents prior to and after an
EQ. A net of ground-based stations located 100-200 km apart at Kamchatka shore,
Russia have been operated to detect this effect (Myachkin et al. 1972 ; Sobolev
et al. 1972 ; Sobolev 1975 ). In some case-studies the amplitude of telluric voltage
variations reached the value about 100 mV while the signal-to-noise ratio was 10-
100. Myachkin et al. ( 1972 ) have detected the abnormal variation of the telluric
electric field about 100-300 V/m 3-16 days before a seismic event.
The variations of transient electric current ( 1-100 Hz) flowing between two
grounded electrodes were considered to be indicative of the rock conductivity
variations (Rikitake and Yamazaki 1978 ;Park 1991 ). The active techniques such
as the dipole DC probing, vertical electric probing, and field formation method can
be sensitive to the rock conductivity variations up to the depth 15-20 km (Bragin
et al. 1992 ).
Many researchers have discussed whether the conductivity and telluric current
variations can serve as an EQ precursor (Sobolev 1975 ; Allen et al. 1975 ; Smith
and Johnston 1976 ; Honkura et al. 1976 ; Fedotov et al. 1977 ; Corwin and Morrison
1977 ; Koyama and Honkura 1978 ; Murakami et al. 1984 ; Varotsos and Alexopoulos
1984a , b , 1986 ; Miyakoshi 1986 ; Ralchovsky and Komarov 1988 ; Jackson and
Kagan 1998 ; Uyeda 1998 ; Uyeda et al. 2000 ; Varotsos 2005 ). A few of workers
believed that the correlation between the abovementioned phenomena and EQs is
very weak (e.g., Burton 1985 ; Johnston 1989 ).