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Anomalous transient electric signals (ATESs) in the ultra low frequency (ULF) band have been often observed during magnetotelluric (MT) investigations [Nature 319 (1986) 310; Phys. Earth Planet. Int. 114 (1999) 141; Geophys. J. Int. 142 (2000) 948], but their origin was unknown until now. They have the same characteristics as the so-called seismic electric signals (SES) claimed to be earthquake precursors by the VAN group (e.g. [Tectonophysics 110 (1984) 73] and later works by this group). Our analysis suggests that the so-called SES could be of anthropic origin. Following the devastating 7 September 1999 Athens earthquake, the VAN group claimed that a SES had been recorded at LAM station (Lamia, central Greece) some days prior to the main shock and that a second SES, which might correspond to an impending even larger earthquake, had been observed after the main shock. In the 2 years after the Athens main shock, no subsequent large earthquakes have occurred near Athens. We conducted a campaign of measurement in the Lamia region in May and June 2001. The results show that ATESs, which look like SES, have several different sources: pump-stations for ground-water, high power electric lines, and factories located to the SE of Lamia city. The ATESs can be generated by two electrochemical mechanisms of metallic electrode polarization: the “galvanic cell” and the “ac electrolytic cell” which are studied by simulated field experiments and discussed in detail in Appendix A . These two mechanisms can occur over a wide range of length scales in the field. Any isolation failure in buried metallic conductors, such as electrical and telecommunication networks, oil, water and gas pipes, railways, high power electric lines, factories and so on, can produce a galvanic cell or an ac electrolytic cell, or both, which could generate, under some circumstances, an “overvoltage”, the ATES. Finally, the absence of a magnetic signal has been observed during ATES and does not constitute a firm criterion for SES [Acta Geophys. Pol. 44 (1996b) 301]. Thus, great care must be taken when claiming the existence of electric precursors of seismic or volcanic events.

A regional telemetric network of twelve digital broad-band seismic stations has been in full operation since the beginning of 1999, in Greece, operated by the Institute of Geodynamics of the National Observatory of Athens (GI-NOA). On 7 September 1999, a ML = 5.4 main shock occurred just 18 kilometers to the north of the Greek capital Athens, causing severe damage and loss of life. The broad band network recorded the seismic sequence and the main shock and 18 aftershocks were selected in order to determine their seismic source parameters and scaling relations by the spectral analysis method. The results indicate a main shock seismic moment M0 = 5.7×10 24 dyn-cm in general agreement with that reported by other agencies and two different source models were used to determine the respective fault radii and displacements for comparison and evaluation purposes. In addition, by investigating source parameters for the aftershocks, it was found that the seis- mic moment correlates very well with the earthquake magnitude (ML) and corner frequency (FC ) through the following relationships: Log M0 = 1.80ML + 15.19 and Log M0 =− 3.17FC + 22.09, respectively. These results and scaling relations are in general agreement with those obtained by other studies and in view of the fact that digital seismic instrumentation is now expanding in Greece, these first results from spectral analysis of digital broad band data can be considered useful for future relevant investigations.

Based on previous observations of the phenomenon of precursory seismic quiescence before crustal main shocks and recent results that indicate an increase in the occurrence of main shocks in the next years, we focus this study on the detection of the seismic quiescence situation in Greece in the beginning of 1999. We use the declustered seismicity catalogue of the Institute of Geodynamics, National Observatory of Athens (NOA) from 1968–1998, to investigate the significance of seismic quiescence for the region 19°–29°E and 34°–42°N. We searched for seismicity rate changes at every node of the grid by a moving time window and we present the results for the beginning of 1999. The results map four (4) areas having a quiescence which duration ranges from 3.8 to6 years in the beginning of 1999. Three of these areas have been devestated by catastrophic earthquakes 17–21 years ago and significant quiescence also preceded those main shocks. Based on these results, an estimate of the future seismic hazard of these areas is made.

A deep magnetotelluric sounding (MTS) investigation in the western part of the Gulf of Corinth has revealed a complex electrical image of the crustal structure. The geotectonic structure of the Parnassos unit and the Transition zone in the central Hellenides, overthrusting the Pindos zone both towards the west and towards the south, has been clearly identified by its higher resistivity and its intrinsic anisotropy related to the N–S strike of the Hellenides range. Subsequent N–S extension of the Gulf introduced another heterogeneous anisotropy characteristic that corresponds to E–W-trending normal faults on both sides of the Gulf. The 2-D modelling of the MTS results reveals the existence of a relatively conductive layer about 4 km thick at a depth greater than 10 km in the middle crust. It corresponds to a ductile detachment zone suggested by microseismic and seismic studies (King et al. 1985; Rigo et al. 1996; Bernard et al. 1997a). It may be attributed to the phyllite series lying between the allochthonous Hellenic nappes and the autochthonous Plattenkalk basement. Towards the east, under the Pangalos peninsula, approaching the internal Hellenides, the detachment zone could root deeply into the lower crust. Some strong local electrical anomalies are observed, reaching the conductive layer in the middle crust, such as that under the Mamousia fault and under the front of the overthrust of the Transition zone on the Pindos zone. Other anomalies affect only the shallower zones such as that beneath the Helike fault and in the Psaromita peninsula. These shallower anomalies provide complementary information to the study of spatial and temporal variations of the seismic anisotropy in relation to the short- and long-term tectonic activity of the Gulf (Bouin et al. 1996; Gamar et al. 1999).

Claims by the VAN group [Varotsos, P., Alexopoulos, K., 1984. Physical properties of the variations of the electric field of the earth preceding earthquakes, I. Tectonophysics 110, 73–98, and later works] to have developed a short-term earthquake prediction technique in Greece continue to arouse contentious debates [Claims of success in using geoelectrical precursors to predict earthquakes are criticized and defended, 1998. Letters, Phys. Today 51 (6), 15–100; Great debates in seismology: the VAN method of earthquake prediction, 1998. Eos 79, 573–580]. This is partly because of the unknown origin of the so-called `seismic electric signals' (SES) precursors. Their particular characteristics are not those of the usual electromagnetic noise (cultural noise) or of the natural electromagnetic field (magnetotelluric field). In this paper, we show that transient electric signals looking like SES can be generated by digital transmitters of the radio-telecommunication network. Such signals have been observed in different regions of the world, including Greece and Vietnam. Their characteristics have been analyzed in a broad band of frequencies (10−3–103 Hz) in the Ioannina, Greece, site which is considered as the most `sensitive area' of the VAN network. It is concluded that some of the signals recorded at this site and identified as SES are probably of artificial origin, and that the criteria used by the VAN group are not sufficient to guarantee that the so-called SES are not man-made. Without an extended and thorough study of the ambient electromagnetic noise in a broad band of frequencies and better information about the electrical properties of the deep structure beneath the monitoring station, earthquake predictions issued on the basis of signals recorded by the VAN network are of dubious significance.

The study of electromagnetic noise in the Ioannina region (NW Greece) shows the existence of long transient electric signals (duration>10 s) without measurable correlated magnetic signals, as is the case for Seismic Electric Signal (SES) observed in the VAN network of short-term earthquake prediction. Our signals have an artificial origin and are emitted by different kinds of transmitters implanted in the Ioannina region. This origin can be clearly ascertained from simultaneous observation, at stations in direct sight of the transmitters, of four electromagnetic components (two electric and two magnetic) in a broad band of frequencies (10−3–10³ Hz). Relying on these observations, it is proposed that the April 18 and 19, 1995 electric signals, interpreted by the VAN group as SES precursors to the May 13 Kozani earthquake, are due to some transmitter located far to the North of the IOA station of the VAN network. The transmitters of this area would be of digital type, as is increasingly common in radio-telecommunication networks. We believe that careful studies of electromagnetic noise in a broad range of frequencies should precede lengthy discussions of the statistical significance of recorded signals.

A regional dial-up PT telemetric network of eight digital short-period seismic stations has been in full operation since the beginning of 1995 in Greece. During the first year of full operation, three destructive earthquake sequences struck different regions of central and northern Greece. The dial-up network managed to successfully record more than 85% of these seismic events including foreshocks, main shocks and aftershocks, and 45 of these events were selected in order to determine their seismic source parameters by spectral analysis. The results show that seismic moment correlates very well with earthquake magnitude, fault radius and fault displacement. The determined scaling relations are in general agreement with those obtained by other studies for Greek earthquakes that used different methodologies. Since digital seismic instrumentation is now expanding in Greece, these first results from spectral analysis of digital short-period data can be considered useful for future seismic hazard studies.

The Bayesian extreme-value distribution of earthquake occurrences has been adopted to estimate the seismic hazard in some seismogenic zones in Greece and surrounding regions. Seismic moment, slip rate, earthquake recurrence rate and magnitude were considered as basic parameters for computing the prior estimates of the seismicity. These estimates are then updated in terms of Bayes' theorem and historical estimates of seismicity associated with each zone. High probabilities for earthquakes withM≥6.0 have been obtained for the northwestern part of Greece as well as for the southwestern part of the Hellenic arc.

TeleseismicP-waves of some large earthquakes that occurred in the eastern Mediterranean region have been analysed by using an iterative maximum entropy technique in order to obtain the independent spectral parameters, the long-period spectral level Ω0 and the corner frequencyf0 of the far-field displacement spectra.

In the present study, the seismic hazard in the city of Patras, central Greece, is estimated. The computations are based on a slightly modified version of the method proposed by Cornell, allowing the introduction of individual attenuation laws for each seismic source. The obtained results emphasize the dependence of hazard on attenuation and the importance of the use of local attenuation laws in seismic hazard assessment.

The magnetotelluric sounding method at 15 sites was employed to investigate the electrical properties of the crust and upper mantle near the epicentral region of the June 15 1995, Ms = 6.1, destructive earthquake in the Gulf of Corinth, Central Greece. The magnetotelluric results indicate the presence of a conductive zone in the mid-crust at a depth of 9 to 12 km near the seismogenetic region. The existence of this zone with a thickness of around 7 km can be explained by the presence of fluids in a zone of ductile shear. A second electrical discontinuity was also found at a depth of about 28 km and this may well correspond to the Moho below the Gulf of Corinth.

In the present paper the source process of some large earthquakes occurred in Greece is discussed in terms of the distribution of the asperities or barriers on the fault plane. The source process has been revealed by the inversion of teleseismic long period body waves using two different techniques. Based on the obtained source time functions an attempt is made to explain the low stress drop values for the earthquakes occurred in Greece. It seems that the stress drops computed by using Brune’s model are an average estimate and do not reflect to some seismotectonic characteristics of the focal region.

International audience; Shear-wave splitting studies on seismo-grams from local earthquakes at the site of the Mornos delta (Gulf of Corinth, Greece) has revealed a clear seismic anisotropy, with a fast S polarization striking N55 + 25 ø. Magneto-telluric soundings in the frequency range 0.01 to 100 Hz on the delta showed a clear electric anisotropy, with a N55 ø + 10 ø direction for the highest frequency. This anisotropy spans through the whole layer of sediments, about 1 km thick. Comparison of the 1966-1972 triangulations and the 1991-1995 GPS positions of geodetic points in and around the delta showed rapid extension strain (2x10-S/year) in the direction N340ø + 30 ø, perpendicular to the fast S and the highest conductivity directions, suggestive of a causal relationship between these observations. We thus propose that this strain controls the two reported anisotropies, by the formation and maintaining of fluid filled, steeply dipping antithetic faults and fractures, and fluid filled vertical cracks, all striking N55 ø 4-20 ø. The source of strain is likely to be active normal faulting near and under the delta, as independently evidenced by the recent discovery of nearby offshore faults, striking about N60 ø, significantly different from the dominant E-W strike of the major faults of the Gulf.

This paper describes and reviews the progress of a three year (11/97–11/00) European Commission FP4 (Climate and Natural Hazards) funded project entitled GPS Seismic Hazard in Greece (SING), A major international interdisciplinary consortium is investigating and comparing strain derived using both geodetic and seismic methods.