METHOD FOR PREDICTING EARTHQUAKES

Abstract: The device monitors the electrical field present in the earth, by means of at least two electrrodes buried in the earth. The electrodes are connected through a shielded lead to a differential amplifier (DA). followed by a filter (F), an amplifier with variable gain (AV), a triggering circuit (T) and an alarm organ (B). The circuitry of the device is dimen­ sioned so as to detect preferentially pulses of less than a second duration and an intensity of 5 to 50 mV m between the electrodes, and or pulses with 1 to 30 minutes duration and an intensity of 40 to 400 µV/m between the electrodes. These two kinds of pulses occur generally a few minutes or less, and 6 - 8 hours before an earthquake, respectively, so that the latter can be predicted with sufficient accuracy.

Description: Method and device for predicting earthquakes The need to predict earthquakes both in a medium time-range (several hours) and at short notice (several minutes) is obvious and could hitherto not be satisfied with sufficient reliability and precision. Various methods for the prediction of earthquakes based on changes of resistivity, magnetic field, earth currents, velocity of propagation and frequency of minor earthquakes have been described in the literature. None of them is able to indicate when the earthquake will occur except in a vague way, and no prediction could be made about the magnitude except in general terms. We refer in particular to the following publication: T. Rikitake et al., J. Geomag. Geoelectr. 32, 721 (1980). The invention aims at providing a method and a device which allow predictions to be made with an excellent precision regarding the time of occurrence and also with a useful accuracy regarding the magnitude of the earthquake to be expected. To achieve this aim, the invention is defined as recited in the claims. This allows to issue two warnings, one about 6 to 8 hours before the earthquake, and the other a few minutes before it. In contrast to all other methods of predicting earthquakes the invention allows a prediction with an accuracy of + 02 hour. Furthermore, it can also predict its intensity. A triggering circuit can even be set to give an alarm for an intensity that might be dangerous to a particular building, depending on. its shape, material, foundations etc. The time lag of about 7 hours give ample time for an orderly evacuation of endangered buildings. Moreover, the uncertainty of about l2 hour regarding the time of occurrence can be borne without undue discomfort as it will limit a possibly unnecessary evacuation time to 1 hour. If a network of such devices is spread over a large region and connected to a central computing station, then the epicenter and the magnitude of each earthquake can be calculated by triangulation and the endangered regions warned in time. The use of a network further reduces the risk of a false estimation of the measurement from a single device. The invention will now be explained at hand of embodiments and of the drawing, where Fig. 1 represents a block diagram of a part of a device according to the invention, and Fig. 2 represents a block diagram of another embodiment. The method and device of the invention base on the detection and analysis of so-called earth currents, or more exactly of varying potential differences, which occur in the earth previous to an earthquake, possibly due to pressure-induced polarization- and depolarizationcurrents, such as occur in crystals. The inventors have found out two variations of the electrical field within the earth, which regularly precede an earthquake, it being understood that all numerical values given in the sequel are characteristic of regions geologically similar to that encompassing Greece, southern Italy, Turkey and Iran, and may vary to some degree elsewhere. Two main electrical signals, detectable through buried electrodes connected through shielded leads to an operational amplifier, were found to precede an impending earthquake. One consists of an electric pulse with a duration of a few milliseconds and an intensity of about 10 mV/meter; this will occur between about Y2 and 4 minutes before the earthquake, depending on the circumstances. The other signal lasts about 1 minute, with an intensity around 100 ,uV/meter, and will occur approximately between 6 and 8 hours before the earthquake. Naturally, these values also depend on the magnitude of the earthquake and the distance from the epicenter at which the measurement is made. In order to detect these signals, it is, for instance, advantageous to use two cylindrical, metallic electrodes having a diameter of 5 - 10 cm and a length of 20 - 60 cm, buried about 100 m apart, and connected through a shielded conductor to an operational amplifier of gain unity, say. This amplifier, designated by DA in Fig. 1, has a floating ground, and its output is connected to a filter F, which suppresses the most important parasitic signals, due to the electrical mains in industrialized countries. Depending on the frequency of the mains, either 50 and 100 Hz, or 60 and 120 Hz should be suppressed. This is particularly important for an effective detection of that signal which precedes an earthquake by only a few minutes. The filter F is followed by an amplifier with variable gain AV, the output of which is in turn connected to a triggering circuit T, which itself triggers some sort of alarm device B, called a bell, for short, in what follows. Preferably, the circuitry just described is dimensioned so as to detect most effectively either the long-term or the shortterm variations of the electrical field between the electrode. In the first case, i.e. for detecting preferentially the signals which occur a few hours before the earthquake, a further filter should be provided which attenuates or suppresses all frequencies above 0.5 - 1 Hz. In the second case, i.e. for the signals which occur shortly before an impending earthquake, frequencies below about 2 Hz should preferably be suppressed. The trigger T can be adjusted to some predetermined threshold value according to the magnitude of the earthquake which must be detected and to the local conditions. Above that threshold value, the bell B will be energized. For general reasons of security, and in particular because earthquakes often occur in series, so that a preceding one may have damaged the mains when a new alarm becomes necessary, the circuitry used should comprise an independent energy source, such as a battery. The strength of the electrical field which precedes an impending earthquake is directed essentially radially with respect to the epicenter. If, therefore, the direction in which an expected epicenter will lie is approximately known, then it is sufficient to use only two electrodes, aligned so as to optimally measure a field directed towards the expected epicenter. If the direction towards the most likely seismic region is not sufficiently well known, then it is preferable to use at least two pairs of electrodes with main measurement directions (i.e. essentially the direction in which the electrodes of a pair are aligned) essentially perpendicular to each other. Each pair of electrodes is then con nected to one of the amplifiers DA shown in Fig. 2, where corresponding elements are identified by the same letter as in Fig. 1. In a manner similar to that of Fig. 1, each amplifier DA is followed by a filter F and an amplifier with variable gain AV. The output signals of both amplifiers with variable gain are led to one single triggering circuit T which either considers only the greater of the two or evaluates a function of both and compares the latter to one or several threshold values. The electrodes should have a sufficient surface and be buried deep enough in the earth. Particularly cheap and efficient electrodes are provided by the metallic pillars on which some buildings rest. In order to eliminate the alternative fields due to the electric mains in industrialized regions, it is advantageous to provide electrical filters which filter out the most used frequencies and at least part of their harmonics. The basic frequencies which should thus be eliminated are 50 or 60 Hz - depending on the country - and possibly lower frequencies sometimes used for electrical railways, for instance. Obviously, it is also possible to process the incoming signals more specifically, if necessary by the use of microprocessors which can perform a comparison with predetermined signal shapes wherever the general shape of the expected signal is known in more detail from previous observations. After filtering, amplifying and processing, the signal delivered by the electrode is used to trigger some sort of monitoring or alarm device. Because the strength of the electrical field becomes stronger with all increasing magnitude of the earthquake to be expected, a threshold device can be used to trigger different alarm levels, depending on the strength of the incoming signal. The lowest alarm levels can be used merely to activate a recording organ which will record the signal shape for future reference. Because, even with two pairs of electrodes oriented at right angle to each other, it is impossible to determine with sufficient accuracy the position of the epicenter from measurements taken at one single site, it is useful to combine by triangulation the signals picked up at different geographical sites into a final reading, taking into account that the field variations become weaker as the distance from the epicenter increases.

Claims: Patent Claims 1. Method for predicting earthquakes, characterized by monitoring the electrical field present in the earth, so as to detect electrical pulses of a given duration and field strength. 2. Method according to claim 1, characterized by the detection of pulses having less than a second duration, and an intensity of 5 to 50 mV/m. 3. Method according to claim 2, characterized by the detection of pulses having less than 10 ms duration, and an intensity of 5 to 25 mV/m. 4. Method according to claim 1, characterized by the detection of pulses having 1 to 30 min. duration, and an intensity of 40 - 400 fV/m. 5. Method according to any preceding claim, characterized in that the monitoring is done at several places, and in that the times of occurrence and the intensities of the electrical pulses detected at said places are then compared. 6. Device for predicting earthquakes according to the method of claim 1, characterized by at least two 2 metallic electrodes of more than 100 cm surface each, imbedded in the earth and connected through shielded lines to a difference amplifier. 7. Device according to claim 6, characterized by electrodes which have a surface between 10 and 2 10'000 cm , and are imbedded in the soil at a depth of 20 to 100 cm. 8. Device according to claim 6, characterized in that the electrodes are pillars of a building. 9. Device according to any of claims 6 - 8, characterized in that the amplifier has a floating ground. 10. Device according to any of claims 6 - 9, characterized by at least one electrical filter connected between the electrodes and the amplifier for suppressing a given frequency. 11. Device according to claim 10, characterized in that the filter suppresses signals of a frequency lying between 50 and 100 Hz or between 60 and 120 Hz. 12. Device according to claim 6, for performing the method of claim 4, characterized by a filter which suppresses signals with a frequency above 0.5 Hz. 13. Device according to claim 6, for performing the method of claim 2 or 3, characterized by a filter which suppresses signals with a frequency below 1 Hz. 14. Device according to claim 6, characterized by a switching circuit connected so as to trigger an alarm, when the amplifier detects pulses larger than a selectable predetermined value. 15. Device according to any of claims 6 - 14, characterized by at least two pairs of electrodes, oriented so as to measure the field strength in two mutually perpendicular directions. 16. Device according to any of claims 6 - 15, characterized by a microprocessor circuitry which comprises a memory containing data which defines a predetermined signal shape, where this circuitry is connected at the output of the amplifier, and arranged so as to compare the shape of signals appearing at said output with said predetermined shape, and to generate a signal at its own output when the two shapes are similar.

Inventors: VAROTSOS, PANAGIOTIS, ALEXOPOULOS, CAESAR, NOMIKOS, CONSTANTIN
Applicants: ALEXOPOULOS, CAESAR, NOMIKOS, CONSTANTIN, VAROTSOS, PANAGIOTIS
Application year: 1982
Main classification: G01V3
IPC codes: G01V1, G01V3