K. Alexopoulos

Physical properties of the variations of the electric field of the earth preceding earthquakes, I

Varotsos, P. and Alexopoulos, K., 1984. Physical properties of the variations of the electric field of the earth preceding earthquakes, I. Tecronophysics, 110: 73-98. The electric field variations of the earth that occur before earthquakes have been studied in a network of eighteen stations in Greece. These precursor seismic electric signals (SES) occur 6-115 h before the earthquake (EQ) and have a duration of 1 min to 1 i h. The duration and the lead-time in contrast to other precursors, do not depend on EQ-magnitude (M). These signals appear as a transient change of the potential difference measured between two electrodes (up to a few millivolts for electrodes at a distance of about L = 50 m) depending on M, the epicentral distance r and the local inhomogeneities. The components of electric field are measured in two perpendicular directions (E-W and N-S). The totality of experiments showed that the interesting quantity of each SES is the maximum value AFof the potential change. The SES of an impending EQ appears simultaneously at a number of stations without being accompanied by any significant change in the magnetic field. The following rules have been established: (1) Seismic electric signals recorded on a single line (e.g. E-W) of a given station and emitted from various seismic regions have AI’-values that decrease with the epicentral distance according to a l/r-law (for r > 50 km). (2) For a given line of a given station the SES emitted from a given seismic region (r = const.) have AV-values that increase with the magnitude; to a good approximation log AV versus M gives a straight line with a slope between 0.3 and 0.4. If for the same station and line another seismic region is considered, the straight line is parallel to the previous one but shifted by a constant amount that depends purely on the ratio of the epicentral distances. Therefore, if the quantity log(AV.r) for earthquakes emitted from various seismic regions is plotted versus M, a unique linear relation for each station appears with the same slope. (3) The simultaneous AV-values of a given EQ recorded at various stations do not follow a l/r dependence. The value AV/L of the electric field in each direction, divided by a suitable factor-an empirically determined effective resistivity-gives a quantity characteristic of the variation of the component of the current density in the earth which can be designated as the intensity of the signal in this direction. By combining the values of the two directions the total intensity J of the SES results. This quantity is found to attenuate with the distances of the stations according to a l/r-law so that log( J. r) is an unique linear function of M for all stations and seismic regions. * Mailing address: Knossou Str. 36, Ano Glyfada, 16561 Athens, Greece. 0040-1951/84/

Latest aspects of earthquake prediction in Greece based on seismic electric signals, II☆

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PHYSICAL PROPERTIES OF THE VARIATIONS OF THE ELECTRIC FIELD OF THE EARTH PRECEDING EARTHQUAKES. II. DETERMINATION OF EPICENTER AND MAGNITUDE

Abstract Since 1983, continuous monitoring of the electrotelluric field has been carried out using an array of measuring stations located at various sites in Greece. The basic physical properties of the transient changes—seismic electric signals (SES)—in the electrotelluric field that are forerunners of earthquakes were first described six years ago. Since then a large body of data has been collected resulting in new insight into various aspects of the method. The present paper reviews the latest developments in SES-based earthquake prediction and describes the current procedures used to predict the epicenter and magnitude of an impending earthquake. A detailed list of the predictions officially issued in Greece during the past 3 years (January 1, 1987–November 30, 1989) is also given. Public warnings were issued well before the most destructive seismic activity.

On the possibility of the enthalpy of a Schottky defect decreasing with increasing temperature

Abstract As reported in the preceding paper, a transient change of the electric field of the earth (seismic electric signal), hereafter called SES, appears many hours before an earthquake (EQ). By measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction. Measurements in two directions (E-W and N-S) give the relative signal intensity Jrel at the station under consideration. By measuring Jrel at a number of stations and considering that it attenuates according to a 1/r-law, the epicenter can be determined with an accuracy usually around 100 km. Once the epicenter has been determined, the product Jrel · r can be evaluated so that the magnitude M can be estimated by resorting to an empirical log(Jrel · r) versus M plot. The uncertainty of M is around 0.5 units. Following Sobolev (1975) and for the statistics to be beyond any doubt, predictions were officially documented before the EQ-occurrence. For 23 earthquakes with a magnitude equal or greater than Ms = 5.0 two events were missed. The present method is compared to other electrical methods used in China, Japan and Soviet Union. A number of problems concerning the origin of the effect, its directivity and the attenuation with distance remain open for further studies.

Prediction of the compressibility of mixed alkali halides

The upward curvature of the conductivity or diffusion plots of ionic crystals in the frame of a single mechanism, is attributed to an excessive fall of the Gibbs free energy g per defect. It is shown that, contrary to published suggestions, thermodynamics demands a simultaneous increase of the enthalpy h and the entropy s of the defect with temperature. The excessive fall of g is due to an increasing difference between h and TN.

Official earthquake prediction procedure in Greece

Abstract A model has been recently proposed by Varotsos and Alexopoulos which permits the calculation of the compressibility k d of the volume of a defect from elastic properties of the surrounding lattice. This expression for k d in conjunction with the Born model, as modified by Smith and coworkers, permits the determination of the compressibility and its pressure dependence of an alkali halide mixed crystal. The application of the proposed method to the system KCl-KBr shows excellent agreement between the predicted and the experimental values obtained by Slagle and McKinstry and recently by Cain.

Migration entropy for the bound fluorine motion in alkaline earth fluorides

Abstract Since Jan. 19, 1983, the authors have been operating a telemetric network of electrotelluric stations in Greece. The real-time observation of specific pre-seismic variations in the electric field of the earth (seismic electric signals, SES) allows us to predict the time, epicenter and magnitude of an impending earthquake (EQ). Initially the predictions were announced by expediting telegrams prior to the event. After the Ministry of Public Works established the Earthquake Prediction Council (EPC) in September 1985, predictions were dealt with in special sessions of the council prior to the occurrence of an earthquake. It was decided to announce only EQs with an expected magnitude Ms ⩾ 4.8 for epicenters predicted within or near the perimeter surrounding the network of measuring stations, and Ms > 5 for EQs predicted outside the perimeter. This procedure was followed from September 1985 to March 18, 1986 with the following results. 1. (1) Within the network. Only one EQ (with Ms = 4.9) occurred during the period mentioned. The prediction of the magnitude was accurate to within a few tenths of a magnitude-unit, while the epicenter was predetermined to within some tens of kilometers. No predictions were made that were not followed by earthquakes. 2. (2)Outside the network. Three EQs with Ms = 5.4, 5.2, and 5.3 occurred on Nov. 9, Dec. 18, and Dec. 23, 1985, respectively. The first was missed. The magnitudes of the other two were predicted to be Ms = 5.2 and 5.5 respectively. The corresponding epicentral coordinates were forecast with an accuracy of 150 and 160 km respectively. The sessions were discontinued on March 18, 1986. On March 24, 1986, a Ms = 6.1 EQ was predicted and directly announced to the Ministry of Public Works. The event occurred on March 29 with the predicted magnitude and less than 50 km from the predicted epicenter.

Migration parameters for the bound fluorine motion in alkaline earth fluorides—II

Abstract The alkaline earth fluorides doped with trivalent rare earths have shown a number of dielectric relaxations that are ascribed to the motion of bound interstitials. The entropies for the migration processes corresponding to RI, RIV relaxations in CaF2 and to RI, RII relaxations in SrF2 have been extra from the most recent experimental data. These are compared to the results from a method that has successfully calculated the formation entropy respectively, volume of defects and other properties involved in ionic conductivity and self-diffusion. The method leads to a ratio of the entropy to the enthalpy of the relaxation process ; it is in close agreement with the experimental values. Further the fact is explained why this ratio is a bulk property of the matrix material.

Efficiency test of earthquake prediction around Thessaloniki from electrotelluric precursors

Abstract In a former paper the dielectric relaxation experiments at ambient pressure by Fontanella et al. were analysed and gave migration entropies that agreed with values calculated from a macroscopic model developed in the past by the authors. Newer experimental results under increased pressure show that the migration volumes υm scale with the enthalpies in agreement to a prediction of the macroscopic model. This model successfully connects υm with the long wavelength transverse optical Grueneisen constant.

On recent seismic electrical signal activity in northern Greece

Abstract Since the completion of the network in January 1983, the electric field of the earth has been continuously monitored at four sites near Thessaloniki, the capital of northern Greece. From the present study and from previous investigations by similar measurements in Greece, it is evident that transient changes of the electrotelluric field occur prior to earthquakes. The analysis of these electric forerunners leads in many cases to a successful prediction of the epicentral area, the magnitude and the time of the impending event. Predictions prior to regional earthquakes are issued and documented with telegrams. From November 1983 until the end of May 1984 twelve earthquakes ( M L > 3.5 ) occurred in the vicinity of Thessaloniki. Ten of these were predicted and warnings given by telegram, whereas two smaller seismic events were missed. Two additional predictions were unsuccessful. Independent of their magnitudes, predicted events took place within a time window of 6 hrs to 6 days after the observations of the electrotelluric anomalies. The accuracy of the predicted epicenters in eight cases is better than 100 km, which corresponds roughly to the mean distance between the electric stations. Magnitude estimates deviate by less than 0.5 magnitude units from the seismically observed ones. Considering the two largest earthquakes, it is shown that the probability of making each of these predictions by chance is of the order of 10−2.