IUGG XXI
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International Association of Seismology and
Physics of the Earth's Interior

To think about tomorrow is in the human nature. On the eve of 21st century, let us look forward. What will geophysics be like?

Historical examples show that the progress of science is not a sequence of random events. Every discovery, every new geophysical conception, was prepared by some other less visible discoveries and results. A new concept is often rejected by scientific society, especially if it appears "too early". Then after some time a new idea is forgotten, but after all it appears sometimes again in a new form. So, my thesis is that THE NEW IS THE FORGOTTEN OLD.

The ideas and achievements which will play a significant, dominant role in geophysics of the 21st century are already with us today. Some of them are being elaborated and are little known. Some are odd-looking and are rejected by the scientific community. Sometimes it is difficult to recognize these ideas and new results, and to foresee their further development.

A forecast is very much a personal matter. What will be shown here is, too, very much personal. This is only a fraction of the whole potential.

Physical properties intrinsic to in-situ rocks is the basis of geophysics. The primary rock model, which is widely accepted both in geophysical prospecting and Earth's geophysics, suggests the following:

Of course, there is some doubt about every one of these items, but common opinion is that a digression from every principle is negligibly small and they are investigated as some interesting peculiarities, but not as a manifestation of principal properties which govern geological processes and the Earth's evolution.

The new concept is that the real media is:

As a result, on the level of fine structure, all geophysical fields are in complex mutual relations.

These five principles should be accepted as Creeds.

Here are some results of experimental investigations that illustrate these properties in the real Earth.

Seismic nonlinearity is manifested as a nonlinear interference of harmonic waves of small amplitude less than 10-8 (Beresnev et al., 1987, In: Problems of Non-Linear Seismic, Editors: A.V. Nikolaev and I. Galkin, Moscow, Nauka, 251-257, in Russian), as wave-velocity changes due to tidal deformation of the Earth's crust (Aki et al., 1970, Bull. Seismol. Soc. of Amer., v. 60, 1315-1335), and as nonlinear interference of eigen-oscillations of the Earth observed after the Chilean and Alaskan earthquakes (Zadro, 1971, Bull. of Geophys. Teor. ed Appl., V. VIII, 187-195).

Another manifestation of nonlinearity is strong tenso- and vibro-sensitivity of the physical properties of rocks (elastic and nonelastic constants, electric conductivity, etc.) and of geodynamical processes. One illustration of that is the Earth tide influence on a seismic flow and its time variations. The detailed investigation of earthquake catalogs in different seismically active areas shows that the ratio of the number of earthquakes that occurred during the phase of tidal compression to the number that occurred during the time of tidal extension changes both in space and time. Overcompressed areas respond to tidal compression and over extended areas to tidal extension. Periods of strongly expressed tidal triggering effects in many areas are interrupted by periods with no triggering effects, the observed dominant time being 10-20 years. The significant peculiarity is that the triggering effects are connected with some harmonic components of the Earth tide which differ by area (Nikolaev, 1994, In: Induced Seismicity, Editors: A.V. Nikolaev and I. Galkin, Moscow, Nauka, 103-114, in Russian).

Vibro-sensitivity, i.e., sensitivity of geophysical processes to seismic vibrations, was revealed as a large-distance influence of underground nuclear explosions and earthquakes upon seismicity over a wide magnitude range (Nikolaev, 1993, Heralds of the Russian Acad. of Sciences, v. 63, 83-86, English translation; Nikolaev et al., 1994, In: Induced Seismicity, Editors: A.V. Nikolaev and I. Galkin, Moscow, Nauka, 251-257, in Russian). In the course of 5-10 days after strong remote events (M5.5), seismicity increases mostly by 10-30%. This effect is most strongly expressed by low magnitude earthquakes (M=3.0-3.5) at a distance range of more than 1000 km.

Seismic and acoustic emissions are extremely high tenso- and vibro-sensitive processors. The effect of their amplitude modulation by the Earth's tide is especially clearly seen after strong earthquakes when the Earth's crust is excited by the strong shock. Such a phenomenon was observed after the M=7.2 Gazli earthquake in Western Uzbekistan, 400 km from the epicenter (Diakonov et al., 1990, PEPI, v. 63, 151-162). Sometimes the vibro-sensitivity increases up to the level of self-excitation and the media emits not a random but harmonic signals whose frequencies are in the band 10-80 Hz.

Sometimes the self-excitation is not full, like in an oscillator. In this case, the system acts as an active frequency filter. It has a high quality Q and very high magnification to external processes. If the frequency of an external process coincides with the resonance frequency of a such system, it can be amplified very strongly, up to 3-4 orders and perhaps more. Taking this into account, let's turn again to the discovery of Sadech, Ben-Menahem, and Meidav which was rejected by the scientific community (Sadeh and Meidav, 1972, Nature, v. 240, 136-138). They found that the harmonic component in microseisms has a frequency which is exactly equal to the double frequency of the pulsar CP1133. I believe in this result; it could be true. The next century will judge the dispute.

Therefore, even extremely weak internal and external processes may influence distinctly and even significantly the fine structure of geodynamics. An especially important role belongs to the Earth's tide whose influence is permanent and constantly directed during billions of years. The nonlinear seismic and acoustic response on the Earth's tide means that it could cause slow movement towards some direction. Here one should remember the hypothesis of the westward drift of the lithosphere which is practically turned down. Perhaps its time will come again.

Also, nonlinear processes in real media are displayed in a wide range of effects which would be impossible in a linear and passive Earth. The characteristic features of fine structure of real-time tectonics are:

The media behavior is as complex as a living being. The biological principle of emergent evolution is very much applicable in geodynamics. The principle claims that new character and qualities that appear in the process, in the more complex level of organization, can not be predicted solely by studying less complex levels of organization. As a result of that, the behavior of the complex system can not be predicted, even when we know its less complex parts, and many results obtained using comparatively simple models of real media by means of numerical modelling and theoretical speculations will be revised and, perhaps, rejected.

It will take some time to accept widely the ideas and the spirit of nonlinearity. What we have now are some dispersed results and general ideas about the processes in the real Earth. The construction of more or less real physical and mathematical models of the Earth or even its small parts is an extremely difficult problem which will be solved in the course of the 21st and following centuries.

Let's look at the problem of Earth structure. The main role in the study of the Earth's structure belongs to seismic investigations. There are two main types of seismic studies, which use different techniques, seismology and seismic prospecting, that is, passive and active approaches.

S eismic prospecting uses very dense arrays of receivers and sources. The images of media which it obtains are much more detailed than those in seismology. In principle, the seismic prospecting technique might be applied in deep interior studies. The depth interval up to 40-60 km is already available for DSS studies using a seismic vibrator. The expansion of DSS at depth, the detailed study of the whole Earth, its mantle and core, using a powerful seismic vibrator, should be the signatures of the 21st century.

A 200 ton long-frequency seismic vibrator was designed by Novosibirsk geophysicists for deep studies. A continuous 60 min sweep-signal of 6-9 Hz was used. From the point of view of signal-to-noise criterion, it was equivalent to a 4 ton TNT charge. In principle, the design of a seismic vibrator which will be able radiate the whole Earth is today a comparatively simple engineering problem. So the 21st century's radiation of the Earth could be planned today.

Lithosphere monitoring. A new method, which is being developed now using a powerful vibrator, is the monitoring of wave velocity changes in the lithosphere. Systematic observations were conducted on Lake Baikal. Seismic rays radiated by the vibrator established on the eastern shore pass through the Baikal Rift and are recorded on the western shore, at a distance of 130 km. A similar study in a smaller scale was conducted some 10-15 years ago in Central California (Clymer and McEvilly, 1978, Bull. Seismol. Soc. of Amer., v. 71, 1903-1927). The main result of both experiments is that the sensitivity of this technique is high enough to monitor the wave velocity variations which are linked with stress field changes due to geodynamical processes and Earth tides. Such studies of geodynamical processes are expensive. If the following generations are reached than we have, they will monitor the most important volumes of the Earth's interior with high detail and precision.

The method of deep interior seismic monitoring is similar to the geodetical study of surface movement. In this case, the seismic rays are used as light beams in geodesy. Unlike real geodesy, the temporal variations of travel times are connected not with distance changes but with changes in wave velocities connected with temporal variation in the stress field. Such studies will yield information on rock movement in the deep interior. A realistic structure of mantle convection, lithospheric plate movement, elastic energy accumulation in the zones of strong earthquake preparation - these are problems which will be solved in the next century.

Earthquake prediction. Application of this technique to the problem of earthquake prediction looks to be very promising. There is no doubt that solution of the earthquake prediction problem needs comprehensive geophysical information on processes in the seismic source region.

The new understanding of the geophysical catastrophe development is based on the ideas of nonlinear dynamics. In the source area before the shock, the weak seismicity and seismic emissions are getting more and more spatially coherent and sensitive to the inducing processes. Distant earthquakes, explosions and vibro-sources could be used to control the medium condition and to predict strong earthquakes.

The development and verification of new methods, based on the fine spatial-temporal studies of the geophysical processes, need much more dense and sensitive multi-disciplinary networks than those we are able to apply now. This is practically a matter of finance. Being optimistic, let's hope that the earthquake prediction problem will be significantly advanced in the 21st century, the losses will be strongly reduced, not only due to seismic resistance construction, but also due to earthquake prediction.

From the point of view of conventional science, many observed results are looking realistic because they are impossible by definition. The disputes around problems of nonlinearity are the signatures of the end of this century. In the near future, the inertia and resistance will be overcome. A new source of disputes is appearing now and it might be the signature of the next century. This is the problem of anomalous unusual events.

Experimental investigations of consciousness influence on non-stable physical processes, like generation of electrical noise, shows a positive effect with a very high confidence level (Jahn and Dunne, 1988, Harvest/Book 270 pp). If the influence of human consciousness on a delicate physical process is true, why can not the consciousness interfere in and influence geophysical processes? Here we come back to the question, which was raised by ancients, that natural disasters are initiated by the behavior of people, by social events.

Let one propose that this is so. So, the consciousness influences Earth processes in two ways: indirectly through technogenic activity and directly through the impact of consciousness on geodynamical processes. The fine structure of geodynamical processes reflects the metastability of the energy saturated medium evolution. Acoustic and electromagnetic emissions (generation of random noise) have extremely high sensitivity to external influences, so they might be sensitive also to the consciousness.

Vladimir Vernadsky developed teachings on the neosphere, the living sphere of the Earth which has consciousness in the form of humanity. He proposed the anthropogenic impact on the Earth's evolution as the technogenic influence on geological and other natural processes. If the human consciousness affects geodynamical processes directly, it might be accepted as the manifestation of the consciousness of the Earth itself. Since each of us is an atom of the Earth, everyone should understand the deep involvement in and responsibility for the Earth's evolution. An understanding of this mission would be the most significant signature of the next century.

A. V. Nikolaev

IUGG XXI
Comptes Rendus

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