VIOLENT EARTHQUAKES: HOW PREDICTABLE?

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Публикатор:


Viktor MURAVYOV, Dr. Sc. (Geol. & Mineral.),

and Galina KRASNOPEVTSEVA, geophysicist and engineer, RF State Research Center VNIIGEOSYSTEM

Earthquake (seismic) foci and districts of seismic manifestations (above all in the case of anomalous seismicity) - are they really unpredictable as not obeying any definite factor of structural control? The mechanism of seismicity - does it consist in the destruction of lithosphere plate rims at the front of ongoing collisions? And the geodynamics of the process-is it determined by the inhomogeneity of the energy-and-mass transfer field deep in the bowels of the earth? Attempting to answer these and other questions inevitably brings us to a new concept, that of synergetic focal dynamics.

Within the framework of plate tectonics the planetary system of anomalous seismicity belts is visualized as a system of tectonic sutures (geosutures) separating the lithosphere plates. It is assumed that seismicity traces the front of plates interaction: collisions of the plates (abduction and related phenomena) or divergence (rifting). Events occurring in the case of collisions are a consequence of a geomedium's mechanical deformations. In the case of rifting, such events are a consequence of spreading, or wedging out of plates in rifting zones by intruding plumages (droplets of hot mantle fluid). In either case we are dealing with zonal processes. That is to say, there can hardly be any geodynamic effect on the in - depth regions of lithosphere plates.

However, no systematic and purposive studies have been conducted into the dynamic effect of separate plumes ("hot spots"), let alone their cooperative (synergetic) action, on the ambient environment - and that even relative to a high-energy rifting process. Random estimates of their structure-forming role (Khibinytype plumes) give the scope of such action on the earth shell in a radius of up to 200 to 300 km. Multiring systems of concentric divisibility (dislocation), which are spreading from the epicenters of a plumogenic disturbance of the lithosphere, were found to be an indicator of such a plumogenic process.

And so it happens that the plate tectonics theory denies the capability

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of self-organization and self-development to the open, high-dynamics system "Earth". Yet the "uncontrolled" plate kinematics can produce only "chaotic tectonics". This circumstance has become obvious in the light of new geophysical data obtained during seismotomographic studies of petrophysical (thermodynamic?) inhomogeneity of the mantle all through.

In order to develop orderly geo-medium models let us try no delve into the available database (far from exhausted) of seismotectonic and geodynamic materials on the example of the Caspian - Black Sea link of the Mediterranean belt of young orogeny (mountain building) and rifting.

From the standpoint of the plate tectonics theory this link is attributed to the zone of the collision of the East European and Afro-Arabian plates. The complex geological structure and high seismicity of this zone are related to this phenomenon, while the recent disastrous earthquakes (at Spitak, Rachinsk, in northern Iran and Turkey * ) are considered to be random components of the earthquake swarm controlled by this zone.

Viewed from the angle of an orderly geomedium (self-organized by focal geodynamics processes ** ), Caucasian seismicity reveals stable criteria for pinpointing both seismic belts (zones) and separate seismically troublesome districts. The structural plan of the seismicity field is formed with the participation of the linear zones of the Caucasian trend (100-150 km wide) - zones corresponding to the


* See: A. Nikonov, "Catastrophe in Turkey", Science in Russia, No. 1, 2000 - Ed.

** See: V. Muravyov, "Structural Guidelines of Geological Evolution", Science in Russia, No. 1, 1999 .- Ed.

page 19


Mediterranean belt; another formative factor are oblique-angled or transverse lineaments * of the Transcaucasian transverse uplift (rise). Within the zones seismicity is distributed in a banded fashion: the axial part is aseismic over a long stretch. High density of the foci is observed predominantly at their outer edges.

Another type of seismocontrolling morphostructures in the region is represented by systems of arcuate zones located concentrically (a system of concenters): in the middle are the Central and East Caucasian systems, and at the periphery- the East-Black Sea, Ararat, and Stavropol ones. The organizing center for each of them are tectonic nodes formed by crossing lineament zones.

The levels of seismicity manifestation in concenters are discrete and correspond to arc curvature radii of 70 to 500 km at a mean scatter of foci relative to the axial line of the zones not above 15 percent. The highest seismic activity is registered in the East Caucasian system of concentric divisibility (SCD) and, above all, within its circular Circum-Caucasian Belt with a radius of 220 km. This belt is a cluster of eight seismoactive districts known best of all in the Caucasus. Structurally each of them is connected with tectonic nodes formed by intersections of the concenters of neighboring systems.

The authors of the present article singled out this belt and its structural- tectonic confinement to a corresponding concenter of the East Caucasian SCD, and validated their stand right after the Spitak earthquake of December 1989. The subsequent events at Java-Tskhinvali


* Lineaments - linear or arched structural elements of planetary significance related to deep faults. - Ed.

page 20


that occurred in the selfsame zone soon after were a natural proof of the objectivity of the structural criteria used for identification of that belt.

The Java-Tskhinvali district is situated at the juncture of the known seismoactive sections of the belt and is apparently confined to a "reawakened" lacoon within a fault there. It should be noted that the Spitak seismic focus is positioned analogously: it is outside the Akhalkala link of the belt and must be connected with the northern flank of the Aragats aseismic district.

Thus the rule of anomalous field structurization for earthquake foci in Caucasia amounts to the concentration of seismic events in the zones of SCD structural elements.

Dissipation (scattering) of seismic energy is most likely in the nodes of intersection of seismocontrolling concenters. This new result of seismicity studies is proof positive of the importance of SCD as an indicator of the real manifestation of focal geodynamic processes.

At the present stage a geodynamic focus may be compared to an anomalous site of the planetary field of energy-and-mass transfer. The situation in the geological space is determined by depth confinement to the columnar canals of interterrestrial energy discharge conjugated with large tectonic nodes.

Cogent bits of evidence have been obtained on the significant relief formation role of the geodynamic situation in SCD systems. This role, among other things, is reflected in the rigorous orthomorphism of the trend of the Caucasian Mountain Range (in a 500 km stretch from Mount Kazbek to Bazar-Dyuzyu) in the outer concenter of the Ararat system; in the controllability of the frontal slope zones of the Greater and Smaller Caucasus by the con-centers of internal and Stavropol

page 21


SCD; and in the rigorous confinement of the axis of the Smaller Caucasus in a stretch from Alaverdi to Gyamysh.

The geodynamic nature of the East Caucasian SCD is seen in the confinement of its nucleus to the Caucasian arched uplift more than 400 km in diameter. A contrast concentric zonation of its structure is detectable, as it is evident in the para-apical depression (radius, 70 km) being supplanted by a wide zone of orogeny within the limits of the Eastern and Smaller Caucasus. At a distance of about 200 km from the SCD epicenter the mountainous zone changes abruptly into a concentric belt of lowlands (Terek-Caspian, Lower Kura, Arax, etc.).

The in-depth roots of concentric elements of SCD are reflected in the fields of geophysical characteristics. A local rise with a diameter of about 50 km is conspicuous over the surface of the mantle roof in the region of the epicenter of the East Caucasian SCD. The earth shell is less than 40 km thick here, and the rate of transverse seismic waves propagation is down to 8.0-8.1 km/s. In adjacent blocks the respective indicators are equal to 45-50 km and 8.2-8.4 km/s.

According to mean seismic rates the epicenter of the East Caucasian SCD has a characteristic local minimum of 5.6 km/s. The 220 km con-center of this system is traced by

page 22


a contrast gradient zone stretching all the way from Vladikavkaz to Baku and matching the face of the eastern Caucasus. Such anomalies are typical of the epicentral regions of many SCD.

The nucleus of the East Caucasian SCD is crossed by a profile of deep seismic sounding Volgograd-Nakhichevan. The sounding data point to the confinement of the system's epicenter to the tectonically disrupted section of the upper mantle. Detected in the middle is a high-rate intrusive body delineated by numerous dots of diffraction, especially on the southern flank.

On the whole the anomalous characteristics of the SCD medium of the Caucasian region definitely confirm the determinacy of their geodynamic nature by the localized action of the foci of energized excitation of the mantle on the lithosphere. It is such action that causes the crust to contract and, simultaneously, brings down its seismic rate characteristics above the focal region. The latter phenomenon attests to dilatation conditions in the suprafocal region of the lithosphere and to the absence of conditions for accumulation of stresses. This is confirmed by the non-seismicity of the epicentral regions of practically all SCD. The concentrically arranged nuclei of Caucasian SCD, a zone of significant thickening of the crust and the increasing mean rates must rather conform to compression conditions. It is with the outer and inner edges of these zones that the foci of anomalous seismic activity are conjugated.

To check into the reliability of the geodynamic characteristics of the East- Caucasian SCD let us take up also a geological model of the Daghestanian wedge-a sector of the eastern Caucasus between Grozny in the west and Derbent in the east, making up nearly a quarter of the SCD total area. Within its limits we see a clear discrete-wave zoning in the distribution of structural elements. Thus, the zone of anticlinorium (rise) of the eastern Caucasus is delimited in the north by a synclinore (deepened) belt represented by a chain of depressions (troughs): Ansalinskaya, Muradinskaya and Urminskaya dating back to the Upper Cretaceous. Another zone of anticlinorium shows up in conformity with the antecedent synclinore belt along the outer border of Mesozoic deposits (at the level of an erosional section). This anticlinorium gives way to a centrifugal belt of depressions (at Buinak, Karanai, etc.) dating from the Late Cretaceous. The outer front of the segment is closed by an arcuate chain of anticlinaria superposed one upon the other (like that at Dmitrov) and built of Oligocenic and Miocenic molassae * . The main structural element of the outer zone are wedge-shaped "inserts"-plates cleaving the sedimental rock of the Terek-Caspian depression. Here is the focus of most active seismism. Apart from the concentric zonation, the Daghestanian wedge is remarkable for the discrete centrifugal migration of sedimentation centers.


* Molassa-a complex of rocks (conglomerates, sandstones, clays) formed in foredeeps and intermontane troughs with the destruction of rising mountains.- Ed.

page 23


The above geological data testify to the pulsed-focal nature of the geodynamic process and to its directionality from the nucleus of the East Caucasian SCD. We have shown this process to be connected with the intrusion of the mantle mass into the lithosphere, and it may be approximated by the "cleaver" mechanism. Hence the horizontal migrations of lithospheric plates (separate sedimentation strata) and the eventual formation of a discrete-laminated lithosphere.

In the general case the development of a high-amplitude mantle uplift creates conditions for the centrifugal gravitational slumping of shell mantle blocks from its "shoulders". Thereby rock masses will be agglomerating on its front to the extent of formation of sheets and folding that touch off active mountain building (orogeny).

Agglomeration areals are regions of vigorous morphogenesis. Focal morphotectonics, generating deposition of different-scale SCD, has a significant role to play here. Therefore, by using values of specific density of local SCD (20-50 km in diameter) as an indicator of the activity of present-day geodynamic processes, we can make an independent experimental assessment of the relevancy of the suggested geodynamic interpretation of the results considered above.

Analyzing the field of local SCD, we see a correspondence of anomalies within this field to contemporary relief features. Thus, the highest values of specific density are characteristic of the eastern Caucasus. A similar picture likewise characterizes a mountain land next to Lake Sevan in Armenia. Surprisingly, the anomalous zones of high density in SCD prove to be compatible by and large with zones in the earth crust where high-rate and most solid blocks of the lithosphere are being formed.

Focal morphogenesis exhibits diminished activity throughout the Circum- Caucasian seismic belt. The outer edge that controls the belt of concenters is reflected by regional contrast density gradients of SCD. A tangible effect in this respect is exerted by the zones of the Ararat-Vladikavkaz and Aragats- Derbent anti-Caucasian lineaments, which is predetermined by their role of the tectonic constraints of the Daghestanian wedge.

To validate the genetic connection of the morphostructural characteristics of the geomedium with seismicity, we interpreted the distribution fields of earthquake epicenters. Systems of arcuate elements controlling chains of seismic foci were identified. Such foci have a concentric pattern of arrangement. The modal values of the distribution maxima of their radii are fully consistent with what Dr. Muravyov found back in 1982, namely, when he determined some values of the curvature of concentric arcuate morphodislocations (segments of regular circles) represented by benches, mountain ridges, watersheds, edges, axial zones of river valleys, and so on.

This result is direct evidence for a connection of seismotectonic processes with relief formation. Significantly, seismogenic elements of the SCD framework are in practical terms absent in the relief. At the same time the clearly expressed morphotectonic elements are not traced by the known epicenters. There is a distinct correlation of seismoactive SCD with the structural peculiarities of an anomalous magnetic field. Considering these data, the regions of SCD diminished density should be viewed as seismically the most troublesome (the situation of Spitak and Java). Stresses here take the form of compression confined to the frontal zone of a mountain rise.

The geodynamic model of system structurization of the anomalous seismicity field in the Caucasus relies on regional-scale data which may furnish proof only relative to the genesis and structurizing role of provincial-rank SCD. But what about the geodynamics of a network of seismic lineaments and SCD of the planetary rank, and the laws of their spatial development?

A schematic answer is supplied on the example of the Pyrenean-Indonesian segment of the Mediterranean belt. In the Northern Hemisphere it extends from 75W along the arc of the "greater circle" (equator) via 45N and 15E to 105E along the equator. Within Eurasia seismoactive SCD of the Caucasian rank are confined to the nodes of the intersection of the line of the Mediterranean "greater circle" by zones of concenters with a radius of 5,000 and 7,000 km belonging to seismoactive SCD of the planetary type. The epicenters of these SCD are situated in the equator zone and are confined to the nodes of the intersection of its seismoactive belt with geodynamically active meridional belts.

The levels of anomalous seismicity manifestation in the systems of provincial- rank concenters in regions other than Caucasia are likewise discrete according to their distance from the epicenter, and they are concentric as well. By metric indicators they are identical to the SCD of the Caucasus. Which means that on a planetary scale, too, the universality of structural organization of the systems of seismoactive regions obeys the general principles ofgeospace structurality.


Опубликовано 14 сентября 2018 года




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