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Recent Seismic Electric Signals (SES) activities in Greece

Varotsos P.

Acta Geophysica

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2006

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Vol. 54, no. 2

158-164

EN

Quite recently two intense Seismic Electric Signals activities were recorded at Pirgos station in western Greece. They have been followed by two strong earth-quakes with magnitudes 6.1 and 6.9 that occurred in western Greece and southern Greece; the latter is the strongest earthquake that occurred in Greece during the last two decades. The compatibility of these results with some relationship between selectivity and earthquake focal mechanism suggested by Uyeda et al. (1999) is discussed.

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Additional evidence on some relationship between seismic electric signals and earthquake source parameters

Varotsos P., Sarlis N., Skordas E., Tanaka H., Lazaridou M.

Acta Geophysica Polonica

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2005

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Vol. 53, nr 3

293-298

EN

We provide recent experimental evidence which further supports the inter-relation between the earthquake source parameters and the selectivity properties of Seismic Electric Signals. The cases of two major earthquakes with magnitudes larger than 6.0, that occurred in Greece during 2004-2005, are presented.

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Transmission of stress induced electric signals in dielectric media. Part II

Varotsos P., Sarlis N., Lazaridou M.

Acta Geophysica Polonica

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2000

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Vol. 48, nr 2

141-177

EN

In the previous paper the electric field Einside, inside a conductive path (and along its axis), having a conductivity s appreciably larger than that of the host medium, s', was studied in the static approximation, for the case of a current dipole source parallel to the path. Here, the same problem is studied but for a source perpendicular to the path. The following two types of paths we considered: (i) a cylindrical channel of radius R and infinite length, and (ii) a layer of width w and infinite extent. If D denotes the distance of the source from the path, and d the distance of the measuring site from the source, we find that the electric field Einside at remote sites (e.g., d/R or d/w of the order of 10(2)) varies as 1/D for the case of a source neighboring to the conductive cylinder, while it is almost independent of D (and w) for the case of a layer. In the case of the cylinder, the values of the ratio Einside/Ehost at (d/R)crit (see the previous paper); significantly exceed (e.g., by one order of magnitude for usual conductivity ratios between 200/1 and 4000/1, but for appreciably small values of D/R, e.g., D/R=2) the corresponding values when the dipole is parallel to the path. The general case, when the dipole source forms a certain angle with its neighboring highly conductive path terminating inside the host medium, is also investigated. The following four points emerge as far as the electric field Eoutside measured inside the more resistive medium but close to an edge is concerned. First, its direction is regulated from the angle between the emitting dipole and the direction of the (elongated) conductive path (as well as from the distance of the source). Second, its amplitude is usually larger than that of Ehost by a factor of around s/s', but there arc also some cases of over-amplification, i.e., the value of Eoutside/Ehost significantly exceeds the conductivity ratio s/s': such an over-amplification may even occur in cases of conductive paths that are not connected. Third, its amplitude versus the distance from the edge varies only slowly, i.e., [Eoutside] 1/r(t) where t is around (but smaller than) unity. Fourth, for a circularly polarized EM plane wave incident on the surf ace of a conductive half-space (containing a highly conductive path close to the interface), the direction of the electric field variations, measured on the surface but close to the end of the path, is generally different from the direction of Eoutside arising from a dipole source which forms a certain angle with its neighboring conductive path. Finally, the above points are applied to the case of the low frequency electric signals that are detected before earthquakes; this results in a natural explanation of the procedure that is followed to determine the parameters of an impending earthquake from the components of the precursory electric signal.