**4. OSC area in the East Sayany**

This area embraces seismic active interblock zones separating the Sayany Block from NEP and from the Hangay Block in the east part of the Tuva Republic, southwest of Buryat Republic and adjacent part of north Mongolia (**Figure 2**). Its length is 350 km, width – 100-216 km and the total seismic energy volume achieves 1.805.11·1016J. The most intensive seismic energy releasing up to 109−11J characterizes the left bank of the Ka-Hem River in the upper stream and south from it in both sides of the Russia and Mongolia Frontier. The majority of epicenters correspond to events with M3–6 and M of some of them in the Mongolia territory increase up to 7–9.

In the OSC area's north parts near latitudinal thrusts to the north predominate with the left-lateral slip component. The stead left-lateral slips take place south between Sayany and Hangay blocks. Transverse north – northeast subsidiary faults develop inside the Sayany Block. The majority of tectonic stress axis's have the strike changing from near longitudinal in the west up to northeast in the east. The transition from transpression to transtension conditions takes place in south Siberia from west to east within interblock zones separating NEP from blocks, which situate south. Thrusts predominate on Altai and west Sayany, where instrumentally measuring crust shortening reaches 2-4 mm/y [12]. They changes on slips with the thrust component in the east Sayany and further east on left-lateral slips with the extension component in the Tunka depression approaching to the Lake Baikal. More easts within the Baikal Rift System the extension completely predominates. The tectonic stress tensors' analysis confirms this changing as well as our field investigation together with V.A. San'kov near the Lake Baikal and in Transbaikalia [5, 13].

The steady NEP displacement east characterizes regarded territory with velocities 25-26 mm/y and the azimuth 89° NE on the Novosibirsk Station and 104.5° SE on the Badary Station north of the east Sayany. Permanent azimuth deflections east and southeast correspond to the plate small clockwise rotation. The maximal difference between measuring and a model vector of horizontal displacement reaches 68° in the NEP – Sayany interblock zone, where active slips predominate. The Novosibirsk Station records the uplift at 8.0 mm/y. The lesser uplift with velocities 2.0–2.5 mm/y characterizes the east Sayany. High magnetic anomalies up to +50 - +100*n* coincide with interblock zones between NEP, Altai and Sayany blocks as well as with faults within them (*http://projects.gtk.fi/WDMAM/project/perugia/index.html*).

The negative gravitational anomalies at the Bouguer Reduction up to −50 – -150mGal, characterize the great part of central Asia, including Hangay and Sayany

#### **Figure 2.**

*The OSC area in east Sayany takes place within seismic active zones separating the Sayany block from NEP and the Hangay block. See the summary legend for the Figure 1.*

**7**

**Figure 3.**

*with the possible mantle plume rising.*

*Tentative Intracontinental Seismic Activity in South Siberia and Russian Far East*

blocks. In the Junggar–Altai interblock zone they reach even −200mGal [14]. High negative values of these anomalies correspond to significant increasing of the crust thickness within the above-mentioned blocks. HF values increase in the interblock zone between Sayany – NEP up to 47-150 μW/m−2 and between Hangay – Sayany blocks within the Hubsugul Rift up to 80–140. The increasing HF values often coincide with areas, which situate above supposed mantle plumes [15]. One of those plumes can situate after seismic tomography data at the boundary between Hangay and Sayany blocks in east Tuva in depth 100-150 km (see **Figure 3**). There the lithosphere thickness decreases down to 70-50 km and the temperature at the depth

*Central Asian mantle anomalies. For block, transitional zone and plate boundaries see summary legend for the Figure 1. Gray lines of different types limit projections of S-waves' velocity slowing-down up to ≤4.2–4.25 km/sec−1 on 50, 100, 150, 200, 250, and 300 km depth levels after seismic tomography data. Authors of the paper drew these lines after data contained in the work [16]. Note that both parts of OSC areas with possible earthquakes' M to 7–9 (see Figures 1, 2) coincide with near isometric outline (counter), under which S-wave velocities slow down at the depth 150 km. That allows supposing the connection of the increase seismicity there* 

The increased level of seismicity can characterize in the future two parts of the OSC area with possible earthquakes M to 7–9. The first of them takes place on the left bank of the Ka-Hem River in its upper stream between the Lake Tere-Hol in the west and the Russia – Mongolia frontier in the east (see **Figure 2**). The second part situates south from it in both sides of the Russia and Mongolia Frontier. The hypocenters' concentration takes places at the depth 9-1 km, so we can wait mainly destructive events in this part of the area. Earthquakes with M4.99–5.99 are more

50 km can reach 1000–1200° C after relation of isotopes 3He/4He [17].

**5. The OSC area in the Baikal system and northwest Transbaikalia**

The OSC area includes territory between NEP and the Amurian Block (or the small plate in other interpretation [18]) within the Baikal Rift System and its northeast continuation (**Figure 4**). The area length is 950 km, its width – 11-230 km

probable in future within the rest territory of the OSC area.

*DOI: http://dx.doi.org/10.5772/intechopen.95073*

*Tentative Intracontinental Seismic Activity in South Siberia and Russian Far East DOI: http://dx.doi.org/10.5772/intechopen.95073*

#### **Figure 3.**

*Central Asian mantle anomalies. For block, transitional zone and plate boundaries see summary legend for the Figure 1. Gray lines of different types limit projections of S-waves' velocity slowing-down up to ≤4.2–4.25 km/sec−1 on 50, 100, 150, 200, 250, and 300 km depth levels after seismic tomography data. Authors of the paper drew these lines after data contained in the work [16]. Note that both parts of OSC areas with possible earthquakes' M to 7–9 (see Figures 1, 2) coincide with near isometric outline (counter), under which S-wave velocities slow down at the depth 150 km. That allows supposing the connection of the increase seismicity there with the possible mantle plume rising.*

blocks. In the Junggar–Altai interblock zone they reach even −200mGal [14]. High negative values of these anomalies correspond to significant increasing of the crust thickness within the above-mentioned blocks. HF values increase in the interblock zone between Sayany – NEP up to 47-150 μW/m−2 and between Hangay – Sayany blocks within the Hubsugul Rift up to 80–140. The increasing HF values often coincide with areas, which situate above supposed mantle plumes [15]. One of those plumes can situate after seismic tomography data at the boundary between Hangay and Sayany blocks in east Tuva in depth 100-150 km (see **Figure 3**). There the lithosphere thickness decreases down to 70-50 km and the temperature at the depth 50 km can reach 1000–1200° C after relation of isotopes 3He/4He [17].

The increased level of seismicity can characterize in the future two parts of the OSC area with possible earthquakes M to 7–9. The first of them takes place on the left bank of the Ka-Hem River in its upper stream between the Lake Tere-Hol in the west and the Russia – Mongolia frontier in the east (see **Figure 2**). The second part situates south from it in both sides of the Russia and Mongolia Frontier. The hypocenters' concentration takes places at the depth 9-1 km, so we can wait mainly destructive events in this part of the area. Earthquakes with M4.99–5.99 are more probable in future within the rest territory of the OSC area.
