by Andrei NIKONOV, Dr. Sc. (Geol. & Mineral.), Institute of Physics of the Earth (IPE), Russian Academy of Sciences
Articles in this rubric reflect the opinion of the author. - Ed .
For more than a hundred years our seismologists were largely concerned with seismic shocks in the Alpine mobile belt which kind of shores up our country's territory from the South. This is quite natural since most of the disastrous earthquakes occurred in the Carpathians, in the Crimea and the Caucasus, and in the mountains of Central Asia and southern Siberia. But, as it is often the case, there was a reverse side to such preoccupation. Involved overmuch with the high-seismicity regions located in the mountains for the most part, our seismologists would care but little about Russia's lowland expanses that seemed safe and serene.
First, let me stress this point: the network of seismic stations set up in European Russia is still the world's sparsest. A few generations of seismic zoning map- makers in the Soviet Union did not think that the intensity of tremors over this vast territory could exceed 5 points on the macro-seismic scale.
But while our seismologists were looking the other way, the densely populated districts of European Russia kept adding more to their population. New industrial enterprises and communication lines were under construction, including hazardous industries that need stable conditions of operation. All that called for close studies of platforms and their seismicity (even though manifest on a minor scale).
Our scientists had a record of practical experience, of course, where seismicity was concerned. However, they could not make do with the available body of information given so many unknowns in seismotectonics for the specific conditions of lowland relief features. Instrumental records of local seismic shocks were too few. Besides, it was essential to identify nontectonic events, among them of technogenic origin, as well as "noises" (interferences) generated by explosions and other sources.
As we finally came to grapple with these problems and achieved some increase in the network of European Russia's seismic stations, a preliminary map of seismic zoning was drawn up in 1990. But it proved to be no good at all. When another attempt was undertaken in 1991, it became clear one could no longer leave aside platform areas as so many blank spots. It took a large team of researchers several years to do this laborious work which, when completed, was endorsed as "Map of Russia's General Seismic Zoning" (GSZ-97) in 1998.*
This very map was the first one to give probabilistic, not deterministic, estimates of possible shocks of different intensity for different time spans, and it defined limits of their possible exceedances. The map had the European and most of the Asian part of Russia colored in blue, that is it indicated a probability of 5- point quakes - true, for a stretch of 500 years only As to intervals of 1,000 and 5,000 years, the above territories were full of green and yellow zigzag stripes, which meant a possibility of 6- and 7-point quakes.
Thus the problem of seismic zoning, for the southern part of European Russia too, seemed to be solved for good. But our seismologists did not leave the matter at that. Yours truly supplied more accurate estimates of earthquake recurrence and pointed at other zones of probable major quakes in our country's South and within the East-European platform. Thereupon I set myself to determining the possible maximal and submaximal seismic effects expressed in points on the macroseismic scale, and the averaged intervals of their recurrence in various regions of Russia's European south. This approach was predicated on a study of earthquakes over centuries and millennia; the initial data were revised and supplemented by historical evidence. I considered only distant violent events, and examined them both in spatial and in temporal aspects.
KEEPING TABS ON VAST TERRITORIES
Distant earthquakes acting upon the southern regions of the European platform originate in the Carpathian (Vrancea zone), Crimean, Caucasian and West- Turkmenian seismic regions. We know more about tremors from deep-focus shocks occurring in the Carpathian seismic zone since such events were reported as far back as the 11th century (our forefathers would describe in their chronicles sundry natural phenomena, among them "shakings of the earth").** The most violent shocks - of magnitude (M) 6.5 to 7.6 - were felt in the central regions of European Russia as far north as Moscow (years: 1230, 1476 and 1990), and also in Warsaw, Nizhni Novgorod and even much farther north, in St. Petersburg (in 1802, 1940 and 1977).
* See: V. Ulomov, "Seismic Menace in Russia", Science in Russia, No. 6, 2001. - Ed .
** See: A. Nikonov, "Earthquakes in Russia", Science in Russia, No. 3, 2000. -Ed.
Map of seismic excitation on the East-European platform caused by two earthquakes that occurred in the Crimea in 1927. Red asterisks-the epicenters of seismic shocks.
Blue lines - isoseismals of the quake of June, 26;
brown lines - those for the quake of September 12, 1927.
Digits - intensity (points).
In addition to the earlier maps of isoseismals (isoseismal lines delineating regions of identical quakes) for seismic shocks that occurred in the years 1940, 1977 and 1986, I have drawn analogous maps of the Vrancea earthquakes that took place in 1802, 1838 and in later years. This work involved newly collected original records. Until recently we knew but little about those quakes, especially about one that occurred in 1838. Yet looking through documents of the Military- Historical Archives (particularly through reports filed by governor-generals of the Russian Empire on all the various happenings in territories under their rule), our archivists could find many facts related to seismic events in Russia's south. Then I processed all these data according to certain parameters and plotted the pattern of those events in rather circumstantial detail. As it turned out, shocks at Vrancea did not cause destruction beyond the then Bessarabia - as, by the way, was evident from the earlier maps. However, the propagation of seismic waves on the surface happened to be somewhat out of the way For instance, a zone of 5-point quakes, e.g. in the middle reaches of the Dnieper, took in a vast area. But reverberations of the tremors were hardly felt in Moscow.
We have at our disposal rather interesting bits of evidence on the earthquake of May 14, 1230. Judging by reports from Kiev and from Pereya-slavl on the Dnieper (a town on the river Trubezh near Kiev), its source was in the Carpathians. In Kiev there were shocks of 5-6 points, and they were even stronger in Pereyaslavl. The same source reports shakings that occurred farther to the north: in Vladimir the intensity of shocks must have reached 4 or 5 points, and at Novgorod the Great-3 to 4 points. But this is appreciably more than the intensity of tremors that spread from the selfsame Carpathian seismic focus during the earthquake of March 4, 1977, when the shocks in those two towns were not above 3 points.
From the evidence on the earthquake of 1230 we can infer that at Moscow and at Staraya Ladoga (near the future St. Petersburg) impacts of 5 and 3 points, respectively, must have
been felt. Such events, had they occurred in our age, would have caused panic among residents of St. Petersburg and Moscow, and even done slight damage to Moscow's high-rises. Fortunately, should we judge by historical sources, such significant reverberations of Carpathian seismic events in the heart of Russia occurred only twice in a thousand years.
Thus, our study of materials on earth tremors on the East-European platform in nearly a thousand years enables us to conclude: deep-focus shocks in the Carpathian Mountains are felt over vast expanses of our country-as far as Voronezh (longitude) and Moscow (latitude), but quite seldom farther north. In the upper and middle reaches of the Don and in the upper reaches of the Volga the maximum intensity of shocks reaches 4 points, and in Russia's southwest - 4 to 5 points.
We have scant data on how seismic events in the Crimea affect the Russian plain. There is factual evidence only on two shocks in this region in 1927 at magnitude equal to 6 and 6.8, which caused quakes in the region of Rostov-on- Don and on the Taman peninsula not above 4 points. But the intensity of shocks could be higher than that: recently, two seismic zones, the South and the North Azov, were detected; they are capable of generating earthquakes up to M
What concerns tremors caused on the East-European platform by Caucasian earthquakes, here I would confine myself to the impacts of most violent shocks that occurred in the central and northeastern Caucasus in 1668 and 1830 (because they alone had an effect on Russia's territory).
The one that took place in the Great Caucasian Range (M~8) was felt in the Kalmyk steppes, and its intensity there reached 7-8 points; on the lower Volga, at Astrakhan, its intensity was up to 5-6 points on the macroseismic scale. The other one that occurred in Daghestan (M=6.9) had a seismic focus extended toward the northeast, and in the lower reaches of the Terek it had an intensity of 5 points.
But there are no grounds to expect stronger reverberations of Caucasian quakes on the East-European platform than it was in 1668. However, the construction of residential and industrial buildings in northern Daghestan, the Kalmyk Republic and at Astrakhan has for many decades been carried out in keeping with the old building code that did not incorporate the latest recommendations of seismologists.
Now turning to seismic events in western Turkmenia, it might be pertinent to ask: in what way do they concern Russia? But they do, since the impacts were felt on the Russian plain as well. For instance, a very violent earthquake of 9 to 10 points that broke out in July 1895 south of Krasnovodsk (M-7.7). And even though the area of destruction there was not large, with tremors spreading southward a hundred kilometers at the most (the impact was actually not felt in northern Iran), the situation was different at Astrakhan nearly 900 kilometers northward from the epicenter-5 to 6-point quakes were clearly manifest there. Furthermore, according to a map of isoseismals which I drew anew proceeding from original source materials, 4 to 5-point reverberations of the earthquake were felt in the middle reaches of the Volga (tremors were locally as high as 5 and 6 points on the right bank of the Volga, on the Zhiguli upland).
This came as outright surprise at first. But we had a rerun of a similar situation, though in a weaker form, on December 6, 2000. A seismic shock that had its epicenter in the Great Balkhan ridge in the selfsame western Turkmenia (M-7.4), caused 4-5 point tremors in Astrakhan and Saratov, they attained to 4 points in the Zhiguli Mountains, and even went as far as Kazan (3 points). The echo of that shock was also felt on the upper floors of high-rise buildings in Moscow and St. Petersburg. This means that tremors spreading from seismic foci in Central Asia pose no threat to both cities. However, the threat is real for the area of the middle Volga, and it should be taken seriously. Although we have documentary evidence on earthquakes in western Turkmenia for 125 years only (and tentative estimates for 250-300 years), we can hardly expect stronger impacts than those described. Yet we must reckon with the possibility of earthquake recurrence once in a hundred years.
Besides, another seismic zone is detected east of the Caspian-it is on the Mangyshlak Peninsula, much closer to Russia. In the past this region must have been the scene of very violent seismic events with intensity of up to 9 points. The latest one on record (1725) took in Astrakhan where tremors of 5-6 points were felt. Small wonder, for the seismic focus extended toward the west and northwest. Using archeological data and a report sent by A. Grishov (1726- 1760), member of the St. Petersburg Academy of Sciences, which I found in the archives, I have reconstructed that event. True, we have no good reason to think that seismic shocks on Mangyshlak could trigger off quakes above 6 points north of the Caspian. But such intensity can cause damage to underground gas storages in the region, a gas plant and trunk gas pipelines.
Proceeding from revised seismic data, taking in 1,000 years for the Carpathian events and several hundred years for the events connected with the seismic foci in the Crimea, the northern Caucasus and western Turkmenia, I have drawn a general map of most intensive shakings registered within the East-European platform for the period of observations. The possible tremors associated with violent shocks in the North-Azov and North-Mangyshlak seismic zones are not indicated on it yet, but they can-
Map indicating the zones of maximal intensity that spread from distant foci in the South to the East-European platform over 1,000 years. Digits denote the intensity of earthquakes in points on the macroseismic scale.
not be ignored because of the proximity of the above-mentioned enterprises and industries vulnerable to tremors. The seismic zones of Akhtyri, Kerch, Anapa and Sochi likewise need close studies.
As to the seismic impacts of southwestern Siberia on the East-European platform, they give us no cause for worry For instance, the new seismic zoning map GSZ-97 indicates the Omsk and Kurgan Regions as zones of possible 5- point tremors. Well and good, but how about the violent earthquakes of 1904 and 1926 that took place near Tyumen and Omsk? They were obviously underrated by the map-makers, and so were the most violent Tien-Shan shocks registered in the early 20th century Meanwhile, the long-forgotten historical sources tell us: the seismic waves from the Chon-Keminsky earthquake (M=8) of 1911 reached even the Southern Urals.
SHOCKS MAY BE MORE FREQUENT
It is likewise essential to identify the actual and probable intervals between violent earthquakes in the above-considered seismic zones and also, on the basis of historical evidence, specify regions posing the greatest menace in the 21st century This is the second part of the task of determining the long-term seismic hazard coming to European Russia's South. Now in either case it all depends on factual evidence for a particular stretch of time and for a particular region; there is better evidence for western regions (period, 1,000 to 2,000 years), but it is worse for the east (period, 150-500 years). The mean frequency (interval) of most violent earthquakes in each region is not assessed proceeding from reliable statistics but is rather based on expert estimates. In my view, the violent jolts in the Carpathians affect the Russian territory of the East-European platform two or three times in a century; those occurring in the Crimea do it once in 200 years, and those in Caucasia - not oftener than three times in a thousand years. However, the impact of West-Turkmenian events is felt in European Russia every 150 to 300 years. Zones (sites) of enhanced tremors cannot be identified other than by historical evidence.
Now let us collate our estimates of impacts with those put on the map GSZ-97. Here we shall limit ourselves to sites showing significant discrepancies. Thus, for the central and south-
em regions of European Russia (Voronezh - Moscow - lower reaches of the Don) the map forecasts the mean recurrence of shocks of 5 points once in 500 years; but by my data they occur there oftener, two or three times per century Along the middle and lower reaches of the Volga the map allows for jolts not above 5 points every 500 years and up to 6 points - once in 1,000 years. Yet my estimates show something else: on the lower Volga tremors of 6 points can occur 1-2.5 times in a century, and the same rate of recurrence holds for 5-point quakes on the middle Volga. Counting in local weak tremors, we shall see that the frequency of shocks of comparable intensity tends to increase on certain sites (for example, on the Voronezh crystalline massif). If so, we shall have greater discrepancies with the GSZ-97 map as well. I need not say that realistic estimates should also involve specific relief and ground features, hydrogeological conditions and, surely, the ever taller buildings.
Two major seismic zones-the North-Mangyshlak next to the south-eastern borders of European Russia, and the North-Azov in the south-are making a far more serious contribution to estimates of the long-term seismic hazard. These two zones I have delineated are not studied by seismologists as yet. Unfortunately.
But why are there such discrepancies between my estimates and those indicated on the GSZ-97 map? The thing is that until recently we had no valid methods of detecting disastrous earthquakes of the distant past. As to the computational methods used for extrapolating the registered events in regions of current weak seismicity - these methods were unreliable and proved of hardly any practical use.
While assessing the probability of violent earthquakes in the northern Caucasus in the 21st century, I relied on long-term prognostic data that followed from the hypothesis of seismic foci migration. Accordingly, violent shocks (M=6.0 and higher) may occur in the next few decades in the northern foothills of the eastern Caucasus-namely near Grozny and east of Makhachkala. Since seismic foci in this zone are extended meridionally, tremors of 5 to 6 points may propagate along it northward into the platform as far as the Kalmyk steppes. As to the western Caucasus, the probable scenario is as follows: if the migration of the foci of strong earthquakes (M=6.6-7.1) goes farther north, destructive jolts could occur at Mineralny Vodi; and there will be much probability of 5-point shakings spreading up to the Manych depression which separates the region north of the Caucasus from the East-European platform.
Needless to say, the prognostic estimates adduced in the present article are of approximate nature. I have just wanted to call attention to regions where, I think, it is necessary to elucidate the seismic characteristics of the past and present so as to make a more correct judgement about the future course of events.
This work has been done with partial financial support from RFFI, grant 00-05-64274.
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