John Makris

Summary of the five principles suggested by Varotsos et al. [1996] and the additional questions raised in this debate

The present paper cannot be considered, either as a rebuttal to any participant, or our overview of the debate. Its publication became necessary due to the fact that various participants raised additional questions, i.e., beyond the points suggested by Varotsos et al. [1996]. We clarify these questions that concern the noise discrimination from our electrical recordings, the recent laboratory experiments which support the emission of electrical precursors, and the question on whether, or not, a retroactive adjustment of the VAN prediction parameters was made, after the period 1987–1989 discussed in this debate. We draw attention to the fact that a continuous 9 year (i.e., 1987–1995) sample of VAN predictions is now available. For the benefit of the reader, the present paper also summarizes the essence of the five Principles suggested by Varotsos et al. [1996] (as a consequence, attention is drawn to a correct definition of the success rate). This essence remains exactly the same as it was initially suggested, because we do not feel, after the debate, that the various contributions cast a sound doubt on the correctness of any of these Principles. The calculations which claim that VAN predictions can be ascribed to chance strongly violate these Principles; the incorrectness of these calculations is beyond any doubt, because they “reject” even an ideal earthquake prediction method. On the other hand, several well founded calculations convince that the VANs success (and alarm) rate is very far beyond chance. The study of this paper is highly recommended to the reader before going through the details of each of our individual Replies.

Use of engineering geophysics to investigate a site for a building foundation

The combination of geophysical data and geotechnical measurements may greatly improve the quality of buildings under construction in civil engineering. A case study is presented here at a vacant building site. Initially, boreholes indicated a complex geology. A dipole–dipole configuration was selected for electrical resistivity tomography (ERT) implementation and the data were processed and interpreted by applying 2D and 3D inversions. An electromagnetic survey was also carried out at a different time periods and successfully used to verify the results of the resistivity measurements. It is demonstrated that engineering geophysics is able to provide solutions for determining subsurface properties and that different prospection techniques are necessary for developing a reasonable model of the subsurface structure.

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.

On the tectonoelectric zonation in the hellenic arc

Geotectonic, seismotectonic and in-situ stress measurements in Greece indicate a zonation parallel to the active Hellenic arc. The raised question is, whether a similar zonation is characteristic of the geoelectrical structure and if the experimental ratio, R, of the time-averaged absolute values of the electric field variations comprises a competent indicator for the distinction of electric zones. In the present work, the magnetotelluric (MT) impedance tensor, measured at different sites in Greece, is further analyzed assuming that results from the galvanic distortion of the regionally induced electric field at a homogeneous or horizontally layered Earth due to near surface local inhomogeneities. From the comparison of the calculated ratio, R, with the respective experimental values, it is concluded that this ratio is strongly depended on local inhomogeneities.Furthermore, the geoelectrical anisotropy at these sites is investigated using the magnetotelluric representation of Mohr circles. Through this analysis, generalized local and regional strike-directions are estimated at each site, assuming a regional 2D-geoelectric structure aggravated by near surface local 3D-bodies of anomalous conductivity. The concepts of local and regional strikes are clarified in cases where the impedance tensor is singular, i.e. strong polarization of the measured electric field exists. It is concluded that the local strike is solely defined from the influence of the local inhomogeneities. On the contrary, the regional strike-direction is compatible, in a generalizing sense, with the direction designated by the major axes of the isoseismal curves drawn for earthquakes with epicenters in the vicinity of the measuring site

Soft-Computing Modelling of Seismicity in the Southern Hellenic Arc

This letter investigates the possible coalition of time intervals and patterns in seismic activity during the preparation process of consecutive sizeable seismic events (i.e., M S ges 5.9). During periods of low-level seismic activity, stress processes in the crust accumulate energy at the seismogenic area, while larger seismic events act as a decongesting mechanism that releases considerable amounts of that energy. Monthly mean seismicity rates have been introduced as a tool to monitor this energy management system and to divert this information into an adaptive neuro-fuzzy inference system. The purpose of the neuro-fuzzy model is to identify and to simulate the possible relationship between mean seismicity rates and time intervals among consecutive sizeable earthquakes. Successful training of the neuro-fuzzy model results in a real-time online processing mechanism that is capable of estimating the time interval between the latest and the next forthcoming sizeable seismic event.

Electric field polarization around Ioannina VAN station, Greece, inferred from a resistivity mapping

Abstract In the summer of 1997, we made a bipole–dipole mapping survey around Ioannina station of VAN (Varotsos, Alexopoulos, and Nomicos), where detection of the pre-seismic electric signal (SES) has been repeatedly reported. Since we had found the characteristic directional properties of the electric field in the previous study, the present study was aimed to examine it by investigating the shallow electric structure around the station. The apparent resistivity tensor was derived from two sets of measured voltages at each receiver position. From a rough sketch of the resistivity tensor distribution, we found that the electric field was enhanced along the direction parallel to the trend of the basin at receivers located in the conductive basin, and perpendicular to it at receivers in the resistive mountainside. Conductance distribution models with thin plates were constructed by using the measured voltages. The results showed that the VAN station is located on the resistive portion near the contact between the conductive and the resistive part. Furthermore, we simulated the apparent resistivity tensor near the VAN station on the inferred conductance distribution model. Although the directional property similar to those of magnetotelluric (MT) and lightning electric field was not reproduced there, we found that the electric field polarization is affected by heterogeneous structure not only around receivers but also around the source.