Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 402

to the typical period of acoustic waves. The TEC variations after a major volcanic
explosion at the Soufrière Hills volcano in July 2003 have been measured by
Dautermann et al. ( 2009 ). These measurements exhibit the ionospheric variations
with periods of 17 and 4 min, which is typical for IGWs and acoustic waves. The
influence of volcanic eruption on the ionosphere has been recently analyzed on
the basis of date obtained by DEMETER satellite over active volcanoes during the
period 2004 August-2007 December (Zlotnicki et al. 2010 ).

10.2.2 Electromagnetic Phenomena Associated with Tsunami

The major sources of natural electromagnetic fields in the seas and oceans are the
magnetospheric and ionospheric electric current and perturbation of the Earth's
magnetic field due to the motion of seawater. This latter effect is similar to that
observed during the seismic wave propagation in the continent. However, ocean-
induced electromagnetic fields are poorly understood, primarily due to the paucity
of actual observations and the complexity of the ocean velocity field (e.g., Sanford
1971 ;Chave 1984 ). The conductivity of seawater near the ocean surface can vary
within interval 3-6 S/m depending on the temperature and water saltiness, which in
turn is a function of ion contents in the seawater. The ocean-induced electromagnetic
field is dependent on the type of hydrodynamical source that is surface or internal
waves, sea current, ocean tides, long waves such as tsunami and etc.
The tsunami wave is usually generated by a seaquake followed by sudden
displacements in the ocean floor or by underwater volcanic activity. Though the
tsunami wave arrives at the coast for many hours after a major EQ, it brings about
a significant danger. The largest EQ of the past 40 years (M D 9:1-9:3)inthe
Indian Ocean on December 26, 2004 was followed by a devastating tsunami, which
killed more that 280,000 people in the south-east Asian region (Lay et al. 2005 ;
Manoj et al. 2010 ). Although the spatial scale of tsunami generated waves in the
open ocean can be larger than 100 km, the monitoring of tsunami wave propagation
is difficult because in the deep ocean the tsunami wave is smaller than a few cm
high (Artru et al. 2005 ). The tsunami waves become dangerous near the coastal
line basically because they may increase in height to become a fast moving wall of
turbulent water several meters high.
It has long been hypothesized that tsunamis produce a detectable value of GMPs
(Larsen 1968 ; Gershenzon and Gokhberg 1992 ; Tyler 2005 ). To estimate the GMP
caused by tsunami waves, one should take into account that this effect is sensitive
to the movement of the entire water column. The simplest model of tsunami waves
in a deep ocean is sketched in Fig. 10.12 . The wave has a shape of vertical column
with height h and horizontal sizes and l. The seawater column as a whole is
assumed to move horizontally along x axis parallel to the ocean floor at a constant
speed. The mass velocity V D V O x of the seawater inside the wave is assumed to
be constant as well whereas outside this region the medium is at rest. Certainly
according to hydrodynamical equations, the mass velocity must gradually increase