ELECTRO-METAMORPHIC HYPOTHESIS OF EARTHQUAKE ORIGIN

L. A. Pukhlyakov

(Presented by the scientific seminar of the departments of general geology, historical geology, and mineral resources)

Several hypotheses have been proposed concerning the problem of earthquake origin. The best known among them is the hypothesis of sudden ruptures occurring in zones of gravitational anomalies as a result of the tendency of the Earth’s crust toward isostatic equilibrium [4, p. 145 et seq.]. In agreement with this hypothesis is the fact that the absolute majority of earthquakes occur in zones where negative gravity anomalies are present, above all in young folded structures and in areas of the deepest depressions adjoining the margins of the Pacific Ocean. Here, however, it should be noted that earthquakes very often occur in regions where no gravity anomalies at all are observed. Such regions include the Mid-Atlantic Ridge [4, pp. 95–99, 143]. Evidently, gravitational anomalies are not required for earthquakes to arise.

A new hypothesis on the problem of earthquake origin belongs to A. A. Vorob’ev [2], who relates these phenomena to electrical discharges inside our planet. The character of seismic shocks at the points of their occurrence does in fact correspond to the character of electrical discharges during the breakdown of dielectrics by high-voltage currents. In addition, it is known that many earthquakes with shallow foci are accompanied by major manifestations of atmospheric electricity (thunderstorms). The Tashkent earthquake of 1967 may be cited as an example.

As for the nature of the electric charges that cause such phenomena, A. A. Vorob’ev assumes that they arise in connection with the activity of the Sun [1]. In this form, however, the hypothesis does not explain the distribution of earthquake foci within the body of our planet. In fact, products of solar activity are distributed over the surface of the Earth primarily as a function of geographic latitude, whereas earthquakes are confined to regions that are clearly not connected with geographic latitude. For example, in North Africa, if one excludes from it the mountainous Atlas country and the regions adjoining the Red Sea, earthquakes practically do not occur [4], whereas in the Mediterranean region situated to the north of it they occur quite often. It follows from this that the activity of the Sun has no relation to the occurrence of earthquakes.

At one time the author of the present article [5] advanced the proposition that the occurrence of earthquakes is connected with metamorphism of the

rocks. This is indicated by the fact that the absolute majority of earthquakes with focal depths of up to 60 km are associated with young folded structures, where, as a result of folding and other geotectonic processes, surface rocks relatively recently (up to two or three tens of millions of years ago) were forced down into the deep zones of the Earth. The situation is analogous with earthquake zones associated with the deepest depressions of the World Ocean, in particular with the Java, Philippine, Japanese, and other trenches. Here sedimentary rocks have been drawn into the deep zones of the Earth (to 700 km and more) as a result of the sinking of accumulations of heavy fragments of a second satellite that had joined the Earth [5, 6]. The metamorphism of these rocks continues at the present time as well.

However, in its original form this hypothesis did not contain a mechanism for the occurrence of shocks, and the entire explanation of this question came down to the following: “There are grounds to expect that from time to time these processes (the processes of metamorphism) must be accompanied by sharp shocks, felt on the surface of our planet as earthquakes” [5, p. 223]. Proceeding from what has been said above, however, the following mechanism for the occurrence of earthquakes may be proposed.

The conductivity of the surface rocks of our planet is connected above all with the presence in them of pores filled with aqueous solutions of salts, that is, with electrolytes [7]. In the process of metamorphism of rocks, the pores within them disappear, and the rocks themselves acquire the properties of typical dielectrics. Such changes, however, do not manifest themselves at once throughout the entire rock, but first encompass some parts of it and then others. As a result, the rock proves to be divided into alternating zones of increased and decreased conductivity.

On the other hand, the processes of metamorphism, like many other chemical reactions, must be accompanied by the appearance of electric charges capable of accumulating in zones of increased conductivity. Depending on the character of the rocks, these charges may have different signs, or at least different potentials.

How long can the process of accumulation of such charges continue? Obviously, until the moment when the potential difference in two neighboring zones of accumulation of electricity becomes sufficient to break through the zone of high resistance separating them. Breakdown, as is known, is accompanied by a mechanical shock (impact) similar to that which occurs in an explosion. The echoes of these shocks are transmitted to the surface of the Earth, where they are felt as earthquakes.

The combination of the charges of any two electrification zones may lead to the possibility of their combination with a third electrification zone. In such a case another breakdown will occur, accompanied by a new mechanical shock (impact). Thus, the hypothesis set forth substantiates the repeatability of shocks in earthquakes.

In the uppermost layers of the Earth’s crust, electrochemical processes also take place. However, the accumulation of electric charges on a large scale cannot occur here, since, owing to the presence of pores filled with aqueous solutions of various salts, they are good conductors. Consequently, earthquake foci can arise here only in the most exceptional cases.

According to the proposed hypothesis of the origin of earthquakes, which may be called electro-metamorphic, in zones where the transformation of sedimentary rocks into metamorphic rocks has ended, earthquake foci cannot arise. In accordance with this is the fact that in

in old mountain structures—such as the Ural Mountains—earthquakes do not occur.

The situation is analogous with the Moon. There is no water there and no sedimentary rocks similar to those on Earth. The weathering of surface rocks is limited to mechanical fragmentation. Furthermore, there are no folded mountain structures there similar to the terrestrial Alps, Himalayas, Cordilleras, etc. Thus, in the deep zones of the Moon there should be no metamorphism of rocks in the form in which it occurs in the deep zones of the Earth. And this, in turn, means that seismic phenomena, in the form in which they occur on Earth, cannot occur on the Moon. This is what theory says; but what is known factually on this question?

Seismographs installed on the Moon show that tremors of its surface occur from time to time; however, as G. A. Burba notes [1], “the energy released by moonquakes in a year is a billion times less than that released by earthquakes.” The nature of the shocks that shake the Moon has also been established. Their sources are, first, impacts of meteorites on its surface; second, tidal changes in its shape; and third, phenomena associated with the heating of its surface by the Sun [3]. In other words, seismic phenomena of the kind that occur on Earth do not in fact occur on the Moon. And if the cause of terrestrial earthquakes were sudden ruptures in zones of gravitational anomalies or electrical discharges caused by the influence of the Sun, then seismic phenomena on the Moon would be the same as on Earth.

LITERATURE

1. Burba G. A. Weak moonquakes. Priroda, No. 11, 1974.

2. Vorob’ev A. A. Physical Processes in the Earth’s Interior. Tomsk State University Press, Tomsk, 1970.

3. Galkin I. Seismogram from the Moon. Newspaper Sovetskaya Rossiya, No. 37 (5980), February 14, 1976.

4. Gutenberg B. and Richter K. Seismicity of the Earth (translated from English). IL, 1948.

5. Pukhlyakov L. A. On the question of the origin of the Pacific Ocean. Izv. TPI, vol. 127, issue 2, 1965.

6. Pukhlyakov L. A. A Review of Geotectonic Hypotheses. Tomsk State University Press, Tomsk, 1970.

7. Sheinman S. M. On the electrical properties of rocks of the upper layers of the Earth’s crust. The system of equations of the electric field. Izvestiya AN SSSR, “Physics of the Earth,” No. 5, 1968.