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Порталус


CENTRAL BASHKORTOSTAN'S SEISMICITY

Дата публикации: 07 сентября 2018
Автор(ы): Yuri KAZANTSEV, Tamara KAZANTSEVA, Murat KAMALETDINOV
Публикатор: Научная библиотека Порталус
Рубрика: ВОПРОСЫ НАУКИ
Номер публикации: №1536330198


Yuri KAZANTSEV, Tamara KAZANTSEVA, Murat KAMALETDINOV, (c)

Studies of seismicity on the territory of the former USSR began decades ago, during the construction of the Turkestan-Siberian Railroad (Turksib, 1927 - 1931). A large number of seismic maps have been drawn up ever since, mostly for areas of heightened seismicity (like the Alpine-Himalayan belt, for instance). But such regions as the Republic of Bashkortostan (Bashkiria), where sensible earthquakes are quite rare, have been bypassed. However, the intensive construction of ecologically hazardous objects (nuclear power stations, chemical mills, major hydrological and oil-mining complexes and the like) makes seismic zoning necessary there too: even moderate quakes can entail tragic consequences. Unfortunately conventional methods, i.e. long-term observations, do not allow to obtain adequate information fast enough. Registration of microquakes (high-frequency seismic noises) would be a good way out. The energy level of such quakes, however, is below the sensitivity threshold of the state-of-the-art instruments used in our country. These tremors can be detected at high frequencies much above 1 Hz.

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By Yuri KAZANTSEV and Tamara KAZANTSEVA, Drs. Sc. (Geol. & Mineral.), laboratory heads, Institute of Geology, Ufa Research Center, Russian Academy of Sciences; Murat KAMALETDINOV, chief researcher of the same Geology Institute

Classical seismology, based as it is on macromeasurements, rejected high-frequency seismic noises as nuisance and interference in the literal sense. But studies carried out in the 1970s by Lev Rykunov and coworkers showed that the endogenic nature of this factor and its geophysical significance allowed to use it for express data acquisition with respect to plutonic activity. We were the first to apply the method of endogenic noise registration in Bashkortostan and to study the parameters of this noise. Accordingly, we drew up a map indicating the areal distribution of the level of noise.

Research into the geological structure of the republic's territory in direct field observations showed dislocations with a break in continuity (ruptures, tear faults) to be the chief elements. Some of them that took body and form in the Southern Urals 350 to 150 mln years ago are prone to activity even today. Outcrops of the zones of these ruptures cause deformation of the ground that in its turn intensifies ravine and karst formation processes; all this is accompanied by numerous landslides. Motorways crossing the dislocation areas present a graphic example as seen in the corrugation, or goffering, of the roadbed immediately after asphalt spreading. Relief deformations along dislocations dating back to the Paleozoic prove the genetic dependence of contemporary geodynamics on the geological structure formed in the past. In this connection the construction strategy relative to large and ecologically hazardous enterprises should be based above all on the knowledge of the in-depth structure of a particular district, and its active and passive sites.

Since the larger part of earth crust ruptures is in the form of overthrusts, the present- day geology of our planet makes up an intricate complex of related elements, most different in age and rank (allochthones), thrusting upon one another in their movement from neighboring folded areas. In the course of this process the frontal parts of allochthones are

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crushed into systems of folds that form bars (ramparts) characterized by a regular arrangement of anticlinal bars* along overthrusts with steeper sides. That's how the "overthrust/ fold" structural pairs come into being. They may be potentially seismoactive in any part of our planet. Such pairs contain information about events of the past as well as a "genetic code" enabling us to forecast the scenario of future events within the earth's crust. There is direct evidence that most of the earthquakes which have ever occurred on earth have been caused by the movement of rock strata along ancient overthrusts that have a long record of development. Thus in Tennessee, USA, the horizontal shift of rock strata for 14 years, between 1965 and 1978, has been found equal to 25 cm. This has caused a 16 cm uplift of the locality. However, the rock movement along the overthrust was not even-paced: from 1966 to 1974 it averaged 3.8 cm a year, but it was at a standstill in 1974 to 1978. Yet 15 km to the east, rock shifts were detected in another rupture: a horizontal displacement of 20 cm, and a vertical displacement equal to 7.5 cm. It's a fact that the growth of anticlines in southern California is always accompanied by earthquakes.

On March 30, 1986, a 5.8 magnitude earthquake shook the central part of the Australian platform at Marryat Creek. That was the tenth quake within the continent's inland areas rated as seismically quiet before. In 1993 geologists of Australia's National University and Seismology Center made a study of this area and detected a zone of Proterozoic (over 600 mln years ago) overthrust deformation. During the earthquake the southwestern block was displaced to underthrust strata in the northeast. As a consequence, part of the locality was uplifted by 45 - 50 cm. The displacement zone was hit by landslides of Holecenic-Pleistocenic pebble and sand carrying granitoid fragments.

Bashkortostan, too, is a land where orogeny (mountain folding, or building) has been found to concur with tectonic processes within the earth's crust. A jump in the rate of present-day movements, equal to 3.9 - 4.4 mm a year, has been registered along the Asha displacement (named after Mount Asha in the Chelyabinsk Region). High- precision leveling data along the Samara-Ufa-Chelyabinsk


* With reference to anticline, a rock fold with its prominence upwards.- Ed.

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railway indicate the mean annual rate of contemporary uplifts at 4.4 - 6.5 mm. All this is characteristic of both the mountain area of the Urals and the surrounding platform zone as well. For example, in 40 years (1904 - 1943) the Tuimazy anticlinal fold (situated near the town of Tuimazy) was dozens of centimeters up, while the adjacent locality to the southeast was just as many centimeters down.

In our tectonoseismic studies carried out with the use of the latest geological techniques, we have collected a database for all the tectonic faults on the territory of our republic. Using PASS instruments, we determine then the activity of each and every fault (PASS is the Russian acronym for field study stations developed in the late 1980s by a team of Sergei Solovyov, Corresponding Member of the USSR Academy of Sciences).

But even the most up-to-date seismic technology does not enable reliable forecasting. Instrumental methods of observations do not suffice. That is why drawing upon historical and archival evidence, and on folk legends and traditions is not at all farfetched. For instance, in Japan and the United States, the countries where seismology is at a very advanced level, scientists are studying seismic events of the dim and distant past, and making accurate catalogs of events dating from periods when no instruments were in use yet. The China catalog lists events that occurred three thousand years ago; and catalogs for some countries of the Near East and Southern Europe take in seismic events as far back as antiquity.

Two Russian scientists, V. Solonenko and V. Khromovskikh, who have studied archeological, historical and geological data, conclude that the earthquake intensity ceiling of 8 points on the macroseismic scale postulated for Caucasia is obviously underrated. Dr. Andrei Nikonov of the Otto Schmidt Institute of Physics of the Earth, after a thorough research into historical, folklore and archeological evidence which is but little known to geologists, has found that seismic events in this region were of great intensity.* And the Armenian epic David of Sasun (8th-9th centuries) has caused him to believe that the intensity of earthquakes could have reached 10 points and more there.** Unfortunately these historical data were not taken into consideration in Armenia's seismic zoning map. The moment of rude awakening came in 1988, with the disastrous earthquake at Spitak.

One catalog published in 1977 listed violent earthquakes on the territory of the USSR from ancient times up until the year 1975. I. Ananyin contributed a list of seismic events on the Russian platform and in the Urals (their intensity not above five points). His map of epicenters indicated two at Ufa, Bashkortostan's capital, dated 1879 and 1904, respectively. And now Rushan Shakurov has compiled a more circumstantial list of such events in Bashkortostan in a period from 1549 to 1990. More events are likely to be added to the list in the future.

Here are some of the most sensible quakes in the Middle Urals: 6-point tremors on July 10, 1892, at Sysert; at Bilimbai on August 17, 1914; 5-point quakes occurred at Perm on May 23, 1798, and at Kushva on April 27, 1847; those of up to 5 points were registered at Nizhni Tagil on June 29, 1956, and at Dobryanka on September 13, 1958. In the 1960s, 1970s and 1980s the Kizel district saw a series of 8 earthquakes. One such quake on August 28, 1973 (its intensity was 6 to 7 points) damaged buildings on the site of the Lenin mine.

The disastrous shock in Spitak, Armenia (1988) and in Tajikistan (1976) showed up the weak points of the earthquake prognostication system. The weakest point is that it ignores the connection of earth tremors with geological processes. It is not enough to predict the exact time when a seismic shock will destroy a particular town, nuclear power station or any other major structure-it is much more important to know the degree of ground mobility within this or that territory and choose safe construction sites.

Earthquakes accompanied by landslips, mud-and-rock torrents, as well as by intensive ravine and karst formation processes pose an immense ecological hazard too. Therefore it is imperative to make a thorough study of earth crust structures and locate active faults in seismically endangered zones, in the regions of major water pools, nuclear power stations, rail- and motorway tunnels and operating mines which may induce "technogenic" quakes.

Dislocations and ruptures that occurred in the Southern Urals and adjacent districts 350 - 150 million years ago are much alive nowadays too. To trace the Paleozoic ruptures we have studied the sections of thousands of deep wells; we have perused numerous reports on geophysical works and interpreted thousands of aerial photos. Thus we have been able to draw up a map indicating the main disruptive dislocations within Bashkortostan. It is now clear to us: the submeridional and northeastern dislocations are overthrusts, and those stretching in the transversal direction are displacements, or shifts.

In the central part of the republic we identified a large number of overthrusts dozens of kilometers long (Sergeyevka - Dema, Tavtiman - Urshak, Beketovo, Kabakovo, Karmaskaly, Zagorsk, among others). Situated along their eastern sides are morphologically monotypic local anticlinal folds with steeper limbs in the west and relatively gentle ones in the east. Uplifts are dozens of meters high, and their size ranges from 1.5x5 to 7x10 km.

A study of geological and geophysical materials and their interpretation in the light of contemporary theoretical findings have enabled us to identify new linear dislocations west of Sterlitamak (120 km south of Ufa) as overthrusts; these are the Zirekla, Talachevka and Kurganovo overthrusts, compatible with the morphologically


* See: A. Nikonov, "Earthquakes in Russia", Science in Russia, No. 3, 2000.-Ed.

** See: A. Nikonov, "European Russia: the Seismic Threat Comes from the South", Science in Russia, No. 2, 2002.- Ed.

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Seismic noise distribution map of Bashkortostan's central part (drawn by Yu. Kazantsev): 1-sites of PASS seismic stations: numerator, numbers; denominator, seismic noise values (in nm); 2-seismic noise isolines (in nm); 3- epicenters of registered earthquakes.

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identical and well-known Sergeyevka - Dema and Tavtiman-Urshak dislocations. The Paleozoic strata along these overthrusts are shifted hundreds of meters to the east. A similar picture is obtained in other, more distant zones of the platform.

In the opinion of Vladimir Trifonov (Geology Institute, RAS), the present geodynamics of the Urals is manifest in the deformation of the surface relief features which is detected by high-precision leveling; in seismicity observed with the help of seismographs; in the enhanced lateral pressure resulting in mountain shocks and destruction of underground proving holes. It is also manifest in the formation of socle fluvial plains, rapids, and coarse boulder rifts; in the change of the granulometric composition of river-bed depositions, and also in the changed alluvial thickness, the changed hydrogeological regime of rivers, lakes and springs, especially in karst regions. Finally, it is manifest in the deformations of buildings and other structures.

The front of the Sergeyevka - Dema overthrust, nearly 3 to 4 km wide, is acted upon by earth tremors. The anticlinal structure of the well-developed arch along it is conspicuous in the uplift of locality in the region of the Sergeyevka fold (25 km west of Ufa). Its vault is fully inscribed in the positive relief features as a ridge stretching from the river Dema near the health resort Yamatovo to the northwest through the villages of Av-don, Sergeyevka, Nachapkino, Dmitriyevka and other communities. One cannot escape the impression that the anticlinal is growing. The intensive formation of ravines as well as the landslips, the numerous deformations of asphalted roads and other indications have been noticed near the communities of Churakly, Bulyakbashevo, Novouptino, Alkino, Kumlekul, between the villages Podymalovo and Mudarisovo and in other population centers.

The Tavtiman-Urshak fault (10 - 15 km east of Ufa) is likewise tectonically mobile. It stretches submerid-

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Bashkortostan's central part: 1 - overthrusts (SD - Sergeyevka-Dema, Ch - Chetyryokhbratsk, Bk - Beketovo, TU - Tavtiman - Urshak, Kb - Kabakovo, Zg - Zagorsk, Krm - Karmaskaly, Zr - Zirekla, Kg - Kuganak); 2-displacements (R - Rayevka, Yb - Yablunovka); 3-anticlinal folds.

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ionally for 200 km. Tectonic movements along it are seen in the contemporary deformations of its frontal part over a stretch of more than 70 km between the villages of Aktyuba (r. Ilenka) and Minzitarovo (r. Lobovka). Landslips have been registered along the river Ilenka, between the villages of Novomusino and Iltuganovo, and along the river Zirekla, between the rural communities of Sikhonkino and Barantsevo. A denuded stratum of green clay has been detected in one of the ravines near the Zirekla; the surface of this stratum is the foot of a water-bearing horizon (just where the gliding of overriding rock takes place).

The front of the Kabakovo dislocation 5 - 10 km east of the Tavtiman - Urshak overthrust, too, is subjected to tectonic movements expressed in the deformations of horizontal strata of pebble, sand and clay, and in the tilted and subvertical ruptures east of the community of Iltuganovo. The amplitude of shift here is 0.2 to 0.5 m. Recent movements at Karmaskaly 40 km southeast of Ufa are also over-thrust impacted. The outcrop of the overthrust front shows how the pebble and loam strata of the Quaternary Age have been ruptured and over-thrust 0.8 m upon one another. Intensive landslip formations are seen in the frontal zone of the Chetyryokhbratsk and Beketovo dislocations passing across Ufa. Landslides have been registered along the river Urshak near the rural communities of Repyevka and Nadezhdino, in the Buzovyazy river valley and elsewhere.

A comparative study of 1:100,000 scale maps for 1942 and 1955 - 1961 within this territory has shown a change in the absolute reference marks of triangulation points of identical class. Thus out of 207 points of the Ufa district compared this way, 87 (or 42 percent) register an increase in the absolute marks; 111 (53.6 percent) show a decrease; and only 9 points (4.4 percent) have remained level. An increase (or decrease) in absolute reference marks averages several centimeters, occasionally, as much as 2 to 3 m. A sharply anomalous value of change (decrease) in the absolute mark up to 15.3 m is at the intersection of the Sergeyevka - Dema and Beketovo overthrusts.

In 1991 to 1993 we conducted microseismic studies in Bashkortostan's central part. Eighteen PASS field stations were installed along Ufa's perimeter in 1991. The seismic registration period totaled 65 days and nights. The earth crust tectonic activity thus detected was expressed in weak tremors (1.9 < M < 2.5; M is the local magnitude). Overall, 120 local shocks, regional and teleseismic quakes were detected. Yet coordinates were determined only for three of them.

The following year, in 1992, PASS stations were set up 75 and 115 km south of Ufa along the Davlekanovo-Tolbazy (at five points) and Rayevka-Sterlitamak (at seven points) profiles. The seismic noise level was found to be rather high-a minimum of 6 nm-at one point along Davleka-novo-Tolbazy, and at two points along Rayevka- Sterlitamak. We could pinpoint the coordinates of the epicenters of four earthquakes that occurred south of Ufa and calculate the depth of their foci. The data on the distribution of weak events depending on their cinematic parameters as well as the four localized epicenters indicate the presence of a seismogenic object at Kurmanayevo at a depth of 10 km.

Next, geophysicists of the Otto Schmidt Institute of Physics of the Earth constructed a plot of seismic events in Bashkortostan on the basis of our data and the historical evidence we collected. Here's the pattern of earthquake recurrence: M=6.5 quakes can recur once in 460 years; M=6.0 - in 180 yr.; M=5.5-in 70 yr.; M=5.0-in 27 yr.; M=4.5-in 11 yr.; M=4.0-in 4.4 yr.; M=3.5 - in 1.7 yr.; and M=3.0 quakes can occur every 0.7 yr.

In 1993 we placed seismographs at eight points along the extension of the Dovlekanovo-Tolbazy profiles, at four points along the Rayevka - Sterlitamak profile, and at twenty points over the latitude profile Kirghiz - Miyaki - Sterlibashevo - Zirgan 20 km south of Rayevsky - Sterlitamak. Higher seismic noises were registered in overthrust frontal zones. As in previous years, zones of the Sergeyevka-Dema and Tavtiman - Urshak overthrusts displayed higher mobility. Confined to them are localities where such exogenic factors as collapses, landslides, and ravine formations have a greater impact. Noteworthy in this respect is the rigorously striated distribution pattern of the earth's endogenic breath with the alternating maxima and minima of seismic noise amplitudes. The striae (bands) are oriented northeast and confined to outcrops of active tectonic dislocations.

Just east of the Tavtiman-Urshak mobile stretch lies the zone of the Karmaskaly active overthrust; this zone is likewise characterized by high (5 - 8 nm) seismic noises. Following next are new stretches with enhanced amplitudes of seismic noise which are connected with the Shikhan-Xblostnovo, Tabyn - Ishtugan, Kisyndin, Zilim - Krasnousolsky and other over-thrust dislocations. However, data interpretation may need some correction here because of the more frequent occurrence of overthrusts and the sparse network of instrumental observation points.

In a nutshell, the territory of the East European platform and of the depression west of the Urals is seismically active now.

 

Опубликовано на Порталусе 07 сентября 2018 года

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