Ya. B. Radziminovich

The January 6, 2006, Balei earthquake as a reflection of the present tectonic activity of the east Trans-Baikal Region

The results of detailed studies of the January 6, 2006, Balei earthquake (Kp = 13.1, Mb = 4.7 [7]), which occurred in an almost aseismic area of the east Trans-Baikal Region, are presented. The focal mechanism and dynamical characteristics of the focus are considered, and the structural-tectonic position of the focus is analyzed. It is supposed that the focus of this seismic event coincides with an activated segment of the northwest-striking Balei-Darasun fault. The maximal observed macroseismic intensity was 5–6 points by the MSK-64 scale (in the town of Baleis). The origination of foci for the earthquake with such intensity and magnitude takes place substantially owing to subhorizontal northeastern compression, the varied-rank block structure of the medium, and low rate values of tectonic motions; on aggregate, all these factors promote the accumulation of stresses in the Earth’s crust. The obtained data can be useful for the purpose of assessment in seismic danger of the region.

Application of the ESI-2007 Scale for Estimating the Intensity of the Kultuk Earthquake, August 27, 2008 (South Baikal)

The paper deals with issues related to the testing of the ESI-2007 scale by using as an example the real seismic event that occurred on August 27, 2008 in South Baikal. The main objective of the paper is to carry out a comparative assessment of the earthquake’s intensity based on traditional macroseismic scales and environmental seismic intensity (ESI-2007) scales. The results of the macroseismic survey served as the initial data. Analysis has been made on the ESI-2007 scale in conformity with the requirements for seismic scales. Particular emphasis has been placed on the type (or rank) of the ESI-2007 scale. Such an investigation is one of the first cases of the application of a new scale by the example of a regional seismic event.

The August 27, 2008, MW = 6.3, Kultuk earthquake effects in the Nearepicenter zone: Macroseismic survey results

In this work, the macroseismic effects of the Kultuk earthquake (MW = 6.3), which took place on August 27, 2008 in the southwestern closure of the Baikal Lake, are under consideration. The intensity of shocks in inhabited localities located in the epicentral zone reached 7–8 points on the MSK-64 scale. The earthquake was named after the local settlement of Kultuk, which was the mostly damaged area by the earthquake. The considered seismic event caused significant material damage (about 250 million rubles according to preliminary estimates). In inhabited localities of Southern Pribaikalie thousands of cases of damaged ovens and chimneys were registered. Some buildings were highly damaged and, accordingly, they are unfit for further use. The earthquake was the cause of numerous rockslides, rockfalls, and landslides on steep natural and artificial slopes. A macroseismic survey allowed us to establish the asymmetrical distribution of the intensity of shocks relative to the epicenter.

The historical seismicity of the eastern Transbaikalia

The seismicity of eastern Transbaikalia has not been studied extensively. According to the current estimate, the seismic activity in this area is defined as moderate and low. At the same time, there is some historical evidence to show that major seismic occurrences have repeatedly taken place in various areas of Transbaikalia. The evidence is fragmentary and for this reason has not been taken account of when developing parametric catalogs and for seismic hazard assessment in eastern Siberia. The present paper considers some new data that throw light on the macroseismic effects of some large historical earthquakes in eastern Transbaikalia and permit estimation of their basic parameters. Additional material was discovered when inspecting the regional periodicals that were published in the late 19th and early 20th centuries. This study suggests higher seismic activity for eastern Transbaikalia than thought previously.

On generation conditions of strong earthquakes in southern Baikal

In this paper the features of seismic process in the southern depression of Lake Baikal are considered. By the data on focal mechanisms of the earthquakes of February 25, 1999 (Mw = 6.0), and August 27, 2008 (Mw = 6.3), as well as based on configuration of their aftershock fields, it is determined that foci of strong seismic events in southern Baikal are controlled by the greatest structural elements of sublatitudinal and submeridional strikes. It has been shown that a substantial role in the formation of focal zones is played by low-scale destruction of the Earth’s crust, revealed by geological-geophysical data and proved by clustering of seismic shocks. New data on the August 27, 2008, earthquake have proved the high level of seismic danger of this part of the Baikal Rift Zone and allowed us to determine generation conditions of strong earthquakes more precisely.

The largest recent earthquake in the Upper Amur region on October 14, 2011: First results of multidisciplinary study

916 On October 14, 2011, at 06:10 UT (16:10 Local time), a large earthquake occurred in the Skovorodino district of the Amur region (MLH = 6.2), which was felt in the nearby populated places with an intensity reaching 7 based on the MSK–64 scale. This earth quake was the largest seismic event that had occurred in this region over the entire period of instrumental seismic observations. According to the OSR 97 map [1], the recurrence period of earthquakes with intensity 7 in the territory of the Skovorodino district is 200–500 years. The data of L.S. Oskorbin [2] indicate that during the period from 1863 to 1994, the maximum intensity of ground shaking in the town of Skovorodino (5) was registered during the South Yakutia earthquake in 1989 (MLH = 6.6, h = 27 km), which occurred at a distance of 300 km north to the town. A total of six earthquakes with magnitudes M ~ 5.0–5.5 were recorded over 100 years of instrumental observations in the adjacent territory (Fig. 1). At least three events, the Zeya Pristan earthquake on August 6, 1911 (M = 5.0, h = 10 km) [3], the Gilyuy earthquake on June 13, 1972 (m = 5.6, h = 18 km) [4], and the Zeya earthquake on November 2, 1973 (m = 5.5, h = 20 km), produced ground shaking with intensity up to 7–8 in the populated places near the epicenters. According to the determinations of the Sakhalin Branch of the Geophysical Survey of the Russian Academy of Sciences (SB GS RAS), which bears responsibility for this region, the epicenter of the Sko vorodino earthquake was localized at the point with coordinates 54.05 ± 0.09° N, 123.80 ± 0.06° E. The source of the earthquake was located at a depth of 18 km. The calculated magnitude was MLH = 6.2, which is comparable with the results of determinations by the other seismological agencies. Determination of the earthquake source mecha nism on the basis of the first motion polarities was per formed using the FOCMEC programming code [7] integrated into the SEISAN earthquake analysis soft ware. Instrumental data of the IRIS global seismo graphic network, regional seismological networks of the SB GS RAS, the Baikal Branch of the Geophysical Survey the Siberian Branch of the Russian Academy of Sciences, and the seismological network of the Far Eastern Branch of the Russian Academy of Sciences were used as the input information. A clear and stable solution was based on 64 polarities of the first motions recorded in the vertical component. Its stereogram is shown in the inset in Fig. 1. The solution describes the strike slip mechanism with subvertical nodal planes, one of which is oriented almost strictly to the west, and the other, to the south. The EROF point of continuous GPS observations closest to the earthquake epicenter (117 km) included into the geodynamic network of the Far Eastern Branch, Russian Academy of Sciences, did not record The Largest Recent Earthquake in the Upper Amur Region on October 14, 2011: First Results of Multidisciplinary Study

The “methane eruption” on Lake Baikal in 1912 as an effect of a strong earthquake

New factual materials on methane eruptions into the water of Lake Baikal in August 1912 are considered. The regional press is used as the information source. The spatio-temporal relationship between the gas eruptions and earthquakes, observed in South Baikal in 1912, is revealed, which permits us to consider this effect as a consequence of seismic activity intensification. Intensive methane discharge into water is a potential danger for the lake’s ecosystem as was proved by massive death of water organisms in South Baikal in 1912 during the described phenomena.

Seismicity of the Southern Siberian Platform: Spatiotemporal Characteristics and Genesis

Analysis of instrumental data on earthquakes of 1960–2005 in the Irkutsk amphitheater shows that the majority of the earthquakes form a wide (150–300 km) band of diffuse seismicity along the marginal suture of the Siberian platform. In accordance with established regular spatiotemporal patterns of the distribution of earthquakes, this band belongs to the Sayan-Baikal seismic belt, associated with the destruction process at the boundary of large lithospheric blocks. The band is located on the northern periphery of the belt and, the deformable substrate being highly monolithic, this sharply weakens the seismicity within the band. Because of the tectonic origin of earthquakes in such a vast platform territory, undoubted evidence for induced seismicity around the Angara cascade reservoirs, and the intense economic development of the region, the problem of seismic hazard in the southern Siberian platform should be regarded as one of the most significant objects of geodynamic research.

“Great East Siberian” earthquake of February 1, 1725 (M = 8.2): Revision of the main parameters in the light of additional data and modern requirements

The earthquake of February 1 (January 21 in the Julian Calendar), 1725 was for a long time considered not only the earliest precisely dated historical seismic event in eastern Siberia, but also the most powerful earthquake for the entire period of recorded seismic events in the region: M = 8.2 (Novyi katalog…, 1977). The epicenter location (Stanovoy Upland) and the magnitude of the event were assessed on the basis of very scanty historical data, as well as using paleoseismogeological information. The February 1, 1725 event received the name “The Great East Siberian” earthquake and served for decades as decisive evidence for the assessment of the seismic hazard and seismic zoning of the northeastern flank of the Baikal rift zone. However, the solution of the focal parameters in the (Novyi katalog…, 1977) has caused serious doubts. In this paper a newly elaborated version is proposed that is based on a detailed reevaluation of the initially known macroseismic information, as well as additional historical data that previously had not come to the attention of seismologists. As the result, a different solution of the focal parameters (51.8° N; 113.0° E, eastern Transbaikalia) and a significantly lower magnitude (M = 6.0) compared with the parameters given in (Novyi katalog…, 1977) were obtained. The presented solution makes us more attentive to the estimates of seismic hazard in east Transbaikalia based on historical data, as well as to the historical data themselves.

The earthquake of July 22, 2011 (M w = 4.5) in a low-seismicity area of the Argun region

The paper considers the Argun earthquake of July 22, 2011 (Mw = 4.5), which occurred in the Argun River valley in a low-seismicity territory in China. The focal parameters of the earthquake (depth of the hypocenter, moment magnitude, scalar seismic moment, and focal mechanism) were determined by calculating the seismic moment tensor from the amplitude spectra of surface waves and the data on the signs of the first arrivals of body waves at regional stations. The solution of the focal mechanism makes it possible to assume a relationship between the earthquake focus and a fault with a northeastern strike bordering the southeastern side of the Argun Basin (in Chinese territory). The Argun earthquake was felt in Russia with an intensity of II–III to V at the epicentral distances up to 255 km. The intensity of shaking did not exceed values suggested by new GSZ-2012 and GSZ-2014 seismic zoning maps of Russian territory. Nevertheless, the question on the possible occurrence of stronger earthquakes in the studied region remains open.