N. Bogris

Signature of pending earthquake from electromagnetic anomalies

Two electromagnetic (EM) anomalies have been detected in the VLF frequency band before the Athens earthquake (EQ) (Mw=5.9, Sept. 7, 1999) with the following characteristics: (i) The first and second anomaly lasted for 12 and 17 hours respectively with a cessation of 12 hours; (ii) The second anomaly ceased at about 9 hours before the EQ; (iii) The larger anomaly, the second one, contains approximately 80% of the total EM energy received; (iv) No EM disturbance has been recorded in the VHF frequency band unlike with other cases, e.g., the Kozani Grevena and Egion-Eratini earthquakes. The fault modeling of the Athens EQ, based on information obtained by radar interferometry, predicts two faults. The main fault segment is responsible for 80% of the total energy released, while the secondary fault segment for the remaining 20%. Moreover, a recent seismic data analysis supports the hypothesis that a two-event solution for the Athens EQ, is more likely than a single event solution. In addition, the absence of surface rupture explains the absence of EM detection in the VHF frequency band. The present analysis reveals that the properties of the preseismic electromagnetic anomalies might be considered as signatures of a pending earthquake.

A new approach in the determination of characteristic directions of the geoelectric structure using Mohr circles

The use of Mohr circles into magnetotelluric (MT) intepretation was introduced by Lilley (Lilley, 1976). By plotting Zxx-rotated versus Zxy-rotated (real and imaginary parts) important information on the conductivity structure is obtained. In this paper Mohr circles are employed to reveal the directions of polarisation of the electric field. Furthermore, the following procedure for Mohr circle analysis is suggested: plotting Zxx -rotated versus Zyx-rotated (instead of Zxy-rotated) the principal axes system i.e., the strike angle, of the regional 2D structure can be resolved in some cases. The latter analysis is implemented to MT data from Ioannina area, in NW Greece. Results from other intepretation techniques applied to the same dataset, such as decomposition methods, tipper and induction arrows analysis, provide support for the validity of the conclusions reached.

Prediction of the 6.6 Grevena-Kozani earthquake of May 13, 1995

Abstract Seismic Electric Signals (SES) were recorded by VAN-group on April 18–19, 1995, at Ioannina station; they resulted in an official prediction that was sent (two weeks before the earthquake occurrence) to the Greek authorities as well as to various International Institutes. The observation of these electrical variations was confirmed by Gruszow et al. (1996); however, they claim that these signals could be attributed to a (non determined) nearby artificial source with huge intensity (IL≈4 × 10 4 Am, for r ≈ 2 km. or 1.6 × 10 5 Am, for r ≈ 4 km). This claim is not valid, because, such an artificial source (cf. horizontal point current dipole) should have produced: (a) electrical field variations having amplitudes two orders of magnitude, larger than the observed ones; this is theoretically shown and experimentally verified and (b) magnetic field variations mainly on the horizontal field, while, in the present case, they have been recorded mainly on the vertical component. Furthermore, we show that the above SES obey the criteria, suggested by Varotsos and Lazaridou (1991), for discriminating SES from noise.

A review on the statistical significance of VAN predictions

Abstract A Special Issue (Geophys. Res. Lett. 23, 1996) was focused on the question whether the VAN predictions outperform random chance. The majority of the participants of this Debate was selected to be critics against VAN, but Varotsos and co-workers accepted to participate and responded to the critical comments. VAN critics, including Geller (1996), share the same “ requirements ”, which decrease the success rate by 50% and the alarm rate by a factor 4–5. Furthermore, these “ requirements ”, when applied to an ideally perfect earthquake prediction method (which successfully predicts all earthquakes above a certain threshold, and does not issue any false alarm), lead to the following paradoxes: (a) the success rate is below 100%, (b) the alarm rate is 23% only , and (c) the ideal precursors are “postseismic” signals. Recent statistical treatments by Hamada (1996) and by Aceves et al. (1996) coincide to the conclusion that VAN predictions cannot be ascribed to chance.