by Gennady UFIMTSEV, Dr. Sc. (Geol. & Mineral.), Institute of the Earth's Crust, RAS Siberian Branch

One of Lake Baikal's bays, Proval, seems to have nothing special about it: the same low shores, waterlogged in places, against a backdrop of sand dunes and burnished hills... And yet this bay is a standout by virtue of its very birth: on New Year's eve (December 31, 1861 -or January 12,1862, by the Gregorian Calendar) part of the Selenga river delta and adjacent plains suddenly sank in on area 260 sq. km.

The very name, proval, means "sink", or "pit". Its birth was preceded by the most violent earthquake on Baikal's record: its magnitude (M) is estimated to have been 7.5. And about a hundred years later, on August 29, 1959, another disastrous quake hit the Baikal area (M=6.8 and force, 9 points on the macroseismic scale). And farther to the northeast there lies Cape Oblom ("break" in Russian). Both names- proval and oblom-are illustrative enough.

The bay is close to a triangle in shape, with the mainland shore in the east, the edge of the Selenga delta in the southwest, and the subwater bank Sakhalin separating the inlet from the open water area. This very bank (shoal) saved people just after the Tsagan Steppe (plain) suddenly sank below the lake's level. The ice jams and hummocks at low depths prevented the Baikal waters (including the only tidal wave, "tsunami") from rushing into the newly formed sink, and the Buryat population managed to escape, even though thousands head of cattle perished. Russian-populated villages suffered less, situated as they were on a terrace whose ledge turned into the shore of the new bay, from 2 to 6 meters deep now.

Eyewitnesses left their accounts of what happened more than 140 years ago. In 1865 the Gorny (Mining) Journal published perhaps the most substantive eyewitness account by Fitinhof. Unfortunately this article cites no instrumental evidence (since no seismic arrays were in existence then). But be it as it may, the Proval Bay can offer us a key to the seismoge-odynamics of the Baikal area.

For a long time, however, researchers failed to see one important episode attending that grandiose natural calamity. According to eyewitnesses, at first warm water came into the pit of the Tsagan Steppe so that the local people had more time for evacuation. Now, warm water is something quite out of the way for Lake Baikal in summer, let alone winter. The earthquake must have released underground thermal waters which rose to

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Structural-geomorphological pattern of the Proval Bay district: 1-low- mountain rises; 2-hilly elevations; 3-fluvial terraces; 4-delta; 5-avant-delta of the Selenga; 6-subwater slope; 7- "relict" rise of the Svyatoi Nos type; 8-area of Proval sunk during the Tsagan quake; 9-relatively uplifted suture-type block of the Sakhalin Bank; 10-hot water springs; 11-abysmal faults.

the surface during the first phase of inundation. Today there is a string of medicinal hot spas on the mainland shore. As to the group of thermal springs along the foot of the Sakhalin Bank (and facing the bay), these appear to be the route of a recent fault. This goes to prove, among other things, that Proval is more than a giant landslip, as most experts will believe -it is a complex tectonic formation. This is a structure composed of two blocks: one is a broad, sunken one, and the other-a narrow, rather uplifted one, on which the Sakhalin Bank rests. Before the 1861/62 earthquake both were under later-day deposits, with the Tsagan Steppe stretching beyond. The seismic disaster kind of updated these formations-it was not a "shake-up" merely, but a series of shoves affecting several segments of the earth's crust.

Hydrogeological effects likewise accompanied the Mid-Baikal earthquake of 1959, with outbursts of wet ground and formation of smallish mud volcanoes, or cones, often aligned in linear groups.

The nature of Proval and the Selenga delta's relief, structure and recent geotectonics allows us to put forward the following hypothesis: the catastrophic earthquake of 1861/62 precipitated a more complex geological process rather than just rapid shifts of mountain masses in the zones of present-day faults. It involved large blocks: a narrow shore horst of the suture type, and a rear graben (with the central part of the horst being uplifted, and subsidence in the graben's case). Consequently, the energy released thereby had accumulated not only along the fault zones but also in the three- dimensional subterranean space during crustal plate movements.

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Structural pairs "shore horst/rear graben" (A) and rises of the Svyatoi Nos type at Baikal (B): 1-incipient; 2-young; 3-mature; 4-submerging under rift bottom deposits. Arrow shows Proval's location.

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Proval Bay, space photo

Separation of the solid run-off of the Selenga in Lake Baikal.

The implication of volumetric (three-dimensional) structures in the seismic process in Eastern Siberia's south has not received due attention thus far. One opts for traditional explanations about the tie-in of seismic shocks with recent faults. This connection does exist, of course. And yet we should also consider the probable shifts or transformations of mountain masses in the upper parts of the lithosphere. Such processes must have been responsible for a violent earthquake in southern Yakutia on April 20, 1989-its aftershocks kind of indicated a space where seismic energy was being liberated. But this quake occurred beyond the boundaries of the Baikal rift zone, within the confines of the northern limb of the Stanovoi Ridge, with a Jurassic (213 - 144 mln years ago) depression, uplifted subsequently.

Such kind of inverse uplifting is not a singular phenomenon within the limits of the Baikal rift, and it is not characteristic only of the suture-type rise of the Sakhalin Bank and of Proval's rear subsidence. The "shore horst/rear graben" structures are of wide occurrence east of Lake Baikal, too. These pairs find themselves at different tectonic development stages and by this very token group them-

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selves in a genetic series. Young forms combine both narrow shoreline rises and small subsidences in the rear whose pits could be filled with lakes lying above Baikal level. This we can see in the example of the shore horst and Lake Dukhovoye's pit south of the river Barguzin. In more mature forms, the water areas in rear grabens (Lake Kotokel) are actually at the same level as Lake Baikal, while the Sakhalin Bank and Bay Proval close this genetic series. We cannot call them "relict" structures: due to the expansion of the Baikal rift they are part of its floor and covered with loose deposits (with other elements at the periphery). However, their very transition into a buried state is accompanied by brief inverse movements capable of triggering strong "volumetric" tremors.

Lake Baikal has other forms as well that are aligned in genetic series within which the epicenters of seismic shocks are grouped. Like, for instance, the peninsula Svyatoi Nos, which is a solid serrate massif rising right in Baikal's middle. Recent rises of the Svyatoi Nos type are arranged stepwise on mountain slopes descending into the lake. Destroyed by erosion above the surface, these steps get submerged in the water to be buried under the lake's deposits. We can see this in the major rise of Posolskaya Bank on Lake Baikal's bottom southwest of the Selenga delta, where a concentration of seismic foci has been registered. Again, these are but seemingly relict structures within the rift's bottom, a factor responsible for enhanced seismic activity for these forms are subject to particular tectonic inversions against the background of general submergence. On Svyatoi Nos such inversions are represented by the ladder of lacustrine terraces on Cape Nizhneye Izgolovye, as high as 150 meters and even higher.

Now, does Baikal have Proval's analogs? Another bay, Posolsky Sor south of the Selenga delta, is close to it in morphology. A long spit (bar) separates Posolsky Sor from the lake's body. Reports about submerged trees on the bottom are oblique evidence on the fast submergence of this tract of land. Other bays ("sors") in the area are relatively small.

Thus, the outwardly modest Proval Bay suggests unexpected pathways of research into the seismotectonics and recent geodynamics in Eastern Siberia's south. ^* It will enable us to fill in the "provals" (breaks, gaps, blanks) of our knowledge about this wonderwork of Mother Nature, Lake Baikal.

^* More about the geodynamics of the Baikal area, see in an article by G. Ufimtsev published in Science in Russia, No. 3, 2001. - Ed.

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