Phenomena similar to tsunami in Russian internal basins

2. Tsunami of Seismic Origin

[5] Occurrence of tsunamis in lakes and reservoirs, and other internal basins generated by seismotectonic displacements of the bottom is beyond controversy. The data of such observations were accumulated. We already cited a few cases of seismic origin in Lake Kineret [Amiran et al., 1994]. Let us mention the cases of seismic tsunami observations in Russian basins.

[6] First of all let us mention tsunami in Lake Baikal. The most known case occurred on 12 January 1862 (31 December 1861, according to the old calendar) during Tsyganskoye earthquake in the delta of the Selenga River at its southeastern coast. The magnitude of this earthquake is estimated as 7.1 [Golenetsky, 1997; Mushketov and Orlov, 1893]. "The wave collapsed at the shore, destroyed the winter huts of peasants, and spread over more than 2 km inland destroying the forest on its way" [Golenetsky, 1997]. It is likely that during this event a giant seismogenic landslide occurred and a large region of the delta of the Selenga River descended more than by five meters forming Proval Bay whose square is 203 km².

[7] A weak tsunami is mentioned in [Soloviev and Ferchev, 1961] (amplitude of a few tens of cm) recorded by limnographs during Mid-Baikal earthquake on 29 August 1959 (magnitude of the earthquake was M = 6.5). It is noteworthy that more than 300 weak and strong earthquakes are recorded annually only in Mid-Baikal region. The Selenga delta is one of the most active seismic regions here. During the last century, four destructive earthquakes occurred in the delta: the one in 1862 mentioned above (intensity I=8-9, M=7.5), in 1871 (I=8-9, M=6.3), in 1903 (I=8-9, M=6.7), and the one in 1959 mentioned above (I=9, M=6.8) [Dashevsky and Martynov, 2002]. The set of seismotectonic data makes us think that earthquakes with magnitudes 7.5-8 and recurrence of 100-200 years are possible in Lake Baikal, which can lead to the generation of tsunami wave with amplitudes up to 1-2 m. However, tsunami risk for the coast of Lake Baikal has not been calculated yet.

[8] Nikonov, [2004] analyzed the epos of Karelo-Finns. It follows from this analysis that strong earthquakes occurred in the past in Karelia (on the rivers and lakes, as well as on the coast of the Black Sea) with intensity of 7-8, which were likely followed by tsunamis. Although this information is not definite, but taking into account the construction of hydroelectric and nuclear power stations in Karelia, it should be studied.

Figure 2

[9] The major part of lakes and rivers in Russia are in the zones free from seismic activity. However, there were events when weak earthquakes caused tsunamis. For example, on 12 September 1806, near Kozmodemiansk town on the Volga River (in Chuvashia, see Figure 2) Lieut. Balle noticed a "deliberate oscillation in water" caused by an earthquake, which lasted about 5 seconds [Tatevosyan and Mokrushina, 2003]. The analysis of geological and historical data allowed the authors of this paper to estimate the parameters of the earthquake of 1806: its magnitude was 3.7 (depth of the source 5 km), and intensity was equal to 6. We note that after the earthquake in Kozmodemiansk two more earthquakes were recorded in the Middle Volga River region with magnitudes from 3 to 4: in Elabug in 1851 and Soksky Ary in 1914 [Tatevosyan and Mokrushina, 2003], which points to the necessity of studying possible tsunamis in the Volga River. The event in 1806 is analyzed in our papers [Didenkulova, 2006; Didenkulova et al., 2006\linkdide06a].

[10] The most probable mechanism of the appearance of observed oscillations of water surface is the known effect of seaquake over the source of underwater earthquake [Alexandrov et al., 1986; Levin and Nosov, 2005], which is caused by parametric generation of waves at the water surface in the oscillation gravity field. Taking into account that characteristic seismic frequencies of soil oscillations caused by a close earthquake are within the interval from 3 Hz to 5 Hz, the characteristic frequencies of parametrically generated waves at the water surface are 1.5-2.5 Hz, while the wavelengths are 20-70 cm. The maximal amplitude of such wave can be estimated from the known relation for steepness of the limiting Stokes wave, which is equal to 0.143. It follows from this that the amplitude of the limiting wave at the water surface varies in the interval from 3 cm to 10 cm, and such wave can be noted by an observer. It is likely that this was the "deliberate oscillation in water" observed by Lieut. Balle during the earthquake in 1806. Generation of long waves that we usually identify with tsunami is not reported in this case, which can be explained by small amplitude of the earthquake. At the same time, a historical event is known when weak earthquake with the same magnitude 3.7 in 1992 caused a notable tsunami with a wave height of 80 cm on Hainan Island in Tonkin Bay [Lander et al., 2003]. It is not excluded that weak earthquake induced a landslide, which became the source of notable tsunami. These processes are currently studied in detail [Yalciner et al., 2003]. Since one of the coasts of the Volga River is steep, landslides from this coast are very possible (below, we shall present the corresponding data), so that tsunami hazard in the reservoirs located not in a seismic zone should not be underestimated.

Figure 3

[11] Earthquakes of the same strength occur in Lake Ladoga. For example, on 30 November 1921, an earthquake with magnitude 4.2 occurred at the eastern coast of Lake Ladoga (Figure 3). A possibility that even stronger earthquakes occurred in the past is not excluded [Nikonov, 2005]. In the southeastern part of Lake Ladoga, seismic activity manifests itself in the form of subterranean (underwater) rumble sometimes accompanied with slight oscillations of water surface recorded on Valaam and Vysuanny islands [Nikonov, 2005]. After the earthquake in 1921, the depth of the southwestern coast of Valaam Island, where a steep underwater slope is located, increased by 50 m, which most likely was accompanied by tsunami. Observations of "slight water oscillations" allow us to speak about reliable occurrence of tsunami on Lake Ladoga.