Reports on earthquakes that periodically take place in the Baikal area contribute to rumors and the most unbelievable hypotheses. The actual situation there is described by K. Levi, an outstanding scientist who for many years has been heading seismological research of Eastern Siberia. He is deputy director of the Institute of the Earth Crust (RAS Siberian Branch), Doctor of Geological and Mineralogical Sciences.

The Baikal depression was formed millions of years ago as a result of action of tectonic forces. It is developing as a structure of extension in the lithosphere like other similar structures in the world rifting system. This process is slow: today we can say that the depression is widening at a speed of about 5 mm/year along the line Irkutsk - Ulan Ude. Besides, in the opinion of geologists, for a long period of time, after the onset of processes reef origin, it was not expressed at all in the relief of the Baikal- Transbaikal region.

Its present site was occupied by a rather wide "slope" of the then relatively high Transbaikal region-in comparison with Central Siberia lowlands. It was only about 2.5 - 3 mln years ago that the lithosphere had split, and there appeared morphologically expressed mountains framed by the lake; its very basin came to be formed. This process took place with the stripping of old and formation of new zones of fractures, whose development is accompanied by seismicity. Events similar to those occurring in the Baikal region make up only 5 percent of the total number of earthquakes taking place on the earth. But taking into consideration the fact that there are many big and ecologically dangerous industries and structures on the territory of the Baikal region, this figure is to be taken seriously.

The seismological science of Eastern Siberia is well known to specialists. At the end of the 19th century Russian researchers I. Mushketov and A. Orlov drew up a catalog of earthquakes in Russia and paid considerable attention to collecting historical chronological data on seismological events occurring on the territory of Asia. It was concluded on the basis of the knowledge thus obtained that it is necessary to develop a network of relevant observation stations. But the first seismological station in Eastern Siberia-"Irkutsk"-remained the only one in the Baikal region till the mid- fifties of the 20th century.

This network moved into a period of rapid growth in the 1970s and 1980s. By the 1990s there were 28 stations in the region that registered seismological situation all the way from Mongolia in the south to Yakutia in the north. Today we have 23 stations, half of them digital, with the Russian-made equipment by and large. All of them work around-the-clock.

In the Baikal groove rift the earthquakes are concentrated in small areas. From 3 to 8 thousand seismic events are registered every year. But strong earthquakes have not occurred just as often there since the 1740s (when data on earthquakes began appearing in the historical chronicles). The earthquake of 1862 in the Tsagan steppe (the Selenga river delta) is considered to be the most violent. It was then that Proval inlet was formed and cracks appeared in the stone church walls in Irkutsk, and bells tolled all by themselves. By its outward effects the quake was around 8 points, but we cannot tell yet whether the origination of Proval inlet was a direct

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result of the event, or it was just a landslide touched off by it. Anyway, all design and construction organizations orient themselves to the probable shocks of 8 points.

When 10 - 12 years ago work began on drawing a map of general seismological zoning of the territory of northeastern Asia, somewhat different methods of assessment of the seismic hazard were applied; this region was considered to be one point more dangerous than before (it must be pointed out that the major structures have been built with regard to this seismicity at least since 1950s). Since the late 1980s and early 1990s an original technological chain of data processing for aseismic construction has been in operation at the Institute of the Earth Crust. Thanks to our work, it has been found possible to add one more floor to nine-story buildings in Irkutsk. In addition, a long-term prognosis, such as the seismic zoning of territories, envisages safety systems. We have also some ideas about a medium-term prognosis that are being updated with regard to new data on seismic regularities within the earth's interior. A short-term prognosis, according to K. Levi, is not feasible at present. It is well known that in order to get a complete picture of seismic processes in the earth's crust, it is essential to have reliable information on its behavior during millennia. Series of seismological observations embrace but a limited interval of time.

On the whole, methods of space geodesy applied today allow to determine the velocity of horizontal displacement of lithosphere blocks. Since Baikal is extending with the speed of 5 mm/year, if the process responsible for rift genesis in the earth's interior does not subside, Baikal may turn into an ocean in hundreds of millions of years. But today such a forecast is premature.

Science in Siberia (Nauka v Sibiri), 2002

Prepared by Vladimir GOLDMAN

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