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EQ. In this picture the study of seismo-electromagnetic phenomena and search of
electromagnetic precursors of EQs seems to be a promising direction of researches.
10.1
ULF Electromagnetic Variations Possibly Associated
with Earthquakes (EQs)
10.1.1
Observations of ULF Electromagnetic Noise Before
and After EQs
We start our study with observations of ULF electromagnetic noise occasionally
measured before strong crust EQs. There are known at least three authentic seismic
events, which were accompanied by the ULF electromagnetic noise several hours or
days before and after the main shock. First of all, it is worth mentioning practically
simultaneously published observations of the ULF noise before and after the large
Spitak (Spitak, Armenia, December 8, 1988, with magnitude M
s
D
6:9, hypocenter
depth h
D
6 km, Kopytenko et al.
1990
) and the large Loma Prieta (Loma Prieta,
California state, USA, October 17, 1989, M
s
D
7:1, h
D
15 km, Fraser-Smith et al.
1990
) EQs. The electromagnetic activity lasted for several weeks for both events
and they exhibit similar characteristics. For example, the noise intensity began to be
enhanced 3-5 days before the Spitak EQ and 12 days before the Loma Prieta EQ. A
maximum of the noise intensity occurred 3 h before the Spitak event and 4 h before
the Loma Prieta event (Bernardi et al.
1991
; Molchanov et al.
1992
; Kopytenko et al.
1993
).
The power spectrum of electromagnetic noise observed during October 1989 at
Corralatios station, Central California is shown in Fig.
10.1
(Fraser-Smith et al.
1990
). Taking the notice of the frequency of measured channel, one can estimate
the amplitude of electromagnetic noise around the moment of Loma Prieta event as
much as 4-6 nT. The typical noise amplitude associated with Spitak EQ was about
0.05-0.2 nT. To explain the lower amplitude of the signals during Spitak event as
compared to Loma Prieta event, one should take into account the difference in the
epicentral distances; that is, 129 km for Spitak and 7 km for Loma Prieta.
The third authentic seismic event occurred at Guam (50 km south of the island
of Guam, August 8, 1993, M
s
D
8:1, h
D
60 km). An increase in ULF magnetic
activity has been observed one month before the great EQ at Guam (Hayakawa
et al.
1996
;Kawateetal.
1998
). The amplitude of electromagnetic noise around the
moment of main shock reached a peak value about 0:1 nT at the epicentral distance
65 km.
This phenomenon can be considered as a possible candidate for ULF elec-
tromagnetic EQ precursor and thus it has been studied intensively for the last
decade. Wide-band ULF perturbations possibly associated with two EQs happened
at Southern California on April, 17, 1990, M
D
4:6, and at the Northridge
on January 17, 1994, M
D
6:7 were observed before and during these EQs
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