A. V. Klyuchevskii

Stresses and seismicity at the present stage of evolution of the Baikal rift zone lithosphere

The paper presents results of studying stresses and seismicity of the lithosphere in the Baikal rift zone at the present (instrumental) stage of its evolution. These results are well consistent with the theory of self-organization of complex dynamic systems and can be used for the classification of certain features and properties of the Baikal rift seismogenic system studied. Application of the theory of nonlinear dynamic systems to the analysis of stresses and seismicity allowed us to develop a concept of a complex spatiotemporal structure of the stress state of the lithosphere and seismicity in the region. In terms of this concept, the distribution of strong earthquakes in time is explained in terms of bifurcations (catastrophes) of stresses in the rift zone. Extrapolation of the results indicates that a stress catastrophe in lithosphere of the rift zone can take place in the next few years years, which increases the probability of occurrence of strong (M ≈ 7) earthquakes in the Baikal region. A model with bifurcation of triple equilibrium most consistent with the phase image of regional stresses is proposed as a scenario of the stress state evolution in the lithosphere of the Baikal rift zone.

Stress-strain state of the lithosphere in the southern Baikal region and northern Mongolia from data on seismic moments of earthquakes

Investigation and understanding of the present-day geodynamic situation are of key importance for the elucidation of the laws and evolution of the seismic process in a seismically active region. In this work, seismic moments of nearly 26000 earthquakes with Kp ≥ 7 (MLH ≥ 2) that occurred in the southern Baikal region and northern Mongolia (SBNM) (48°–54°N, 96°–108°E) from 1968 through 1994 are determined from amplitudes and periods of maximum displacements in transverse body waves. The resulting set of seismic moments is used for spatial-temporal analysis of the stress-strain state of the SBNM lithosphere. The stress fields of the Baikal rift and the India-Asia collision zone are supposed to interact in the region studied. Since the seismic moment of a tectonic earthquake depends on the type of motion in the source, seismic moments and focal mechanisms of earthquakes belonging to four long-term aftershock and swarm clusters of shocks in the Baikal region were used to “calibrate” average seismic moments in accordance with the source faulting type. The study showed that the stress-strain state of the SBNM lithosphere is spatially inhomogeneous and nonstationary. A space-time discrepancy is observed in the formation of faulting types in sources of weak (Kp = 7 and 8) and stronger (Kp ≥ 9) earthquakes. This discrepancy is interpreted in terms of rock fracture at various hierarchical levels of ruptures on differently oriented general, regional, and local faults. A gradual increase and an abrupt, nearly pulsed, decrease in the vertical component of the stress field Sv is a characteristic feature of time variations. The zones where the stress Sv prevails are localized at “singular points” of the lithosphere. Shocks of various energy classes in these zones are dominated by the normal-fault slip mechanism. For earthquakes with Kp = 9, the source faulting changes with depth from the strike-slip type to the normal-strike-slip and normal types, suggesting an increase in Sv. On the whole, the results of this study are well consistent with the synergism of open unstable dissipative systems and are usable for interpreting the main observable variations in the stress-strain state of the lithosphere in terms of spatiotemporal variations in the vertical component of the stress field Sv. This suggests the influence of rifting on the present-day geodynamic processes in the SBNM lithosphere.

Attractor structures of riftogenesis in the lithosphere of Baikal Rift System

The study results of modern geodynamics and tectonophysics of the lithosphere of Baikal Rift System (BRS) are generalized. By the data on radii of dislocations, three areas of maximal strain-strength anisotropy of the medium are distinguished, while analysis of seismic moments of earthquakes has showed that in these parts of the lithosphere mostly dip-slip fault-causing quakes of various energy classes take place; i.e., riftogenesis processes dominate. Within the framework of the theory of nonlinear dissipative dynamical systems, these areas are classified as attractor structures of riftogenesis (ASR). ASRs are located in the central part and in the flanks of the BRS, and they form nonlinearity and instability of modern geodynamical and tectonophysical processes in the lithosphere, which are manifested in seismicity of the Baikal Region and Mongolia.

Attractor structures of riftogenesis as an attribute of the cenozoic stage in evolution of the lithosphere within the Baikal rift system

In accordance with the principle of actualism, theresults of studying the modern geodynamics and tectonophysics of the Baikal rift system (BRS) have beenapplied for development and broadening of ideasabout the Cenozoic Stage of the lithosphere in theregion. Identification of three attractor structures ofriftogenesis (ASR) has allowed the theory of moderngeodynamics and tectonophysics of the BRS to bedeveloped: the regional energy sources for geodynamical and seismic reconstructions have been includedinto the pattern, and their nature and effects have beendescribed in the first approximation. Within theframework of the theory of dissipative dynamical systems of selforganization and nonlinear media, theASRs have been considered as an attribute of Cenozoic geodynamics of the BRS lithosphere: they formednonlinearity and instability of geodynamical and tectonophysical processes in this period of time. Threestages of volcanic activation of the BRS lithosphere areexplained by sequential origination and developmentof three ASRs: firstly, in the South Baikal Depression(Late Cretaceous–Paleocene), then in the KhubsugulDepression (Eocene–Oligocene), and finally in theMuya Depression (postOligocene). The start of theCenozoic riftogenic stage of the Baikal Depressionabout 70 Ma BP is explained by appearance of theASR is the South Baikal Depression. Two stages of rifting, an increase in the rate of rift processes in the BRS,and propagation of riftogenesis southwestwards andnortheastwards from the South Baikal Depression areattributed to the appearance and gradual twolateralevolution of the ASRs in the Khubsugul (southwesternflank) and Muya (northeastern flank) depressions.In the hierarcy of natural systems, intracontinentalrift systems, which consist of deeplaid fault zones andzones functioning in a relatively narrow thermodynamical regime, are classified as mesosystems [1].N.A. Logachev et al. [2, 3] found the main features ofthe Cenozoic riftogenesis dynamics of the intracontinental BRS: evolution of the process southwestwardsand northeastwards from the South Baikal Depression, which is the historical core of the BRS, tookplace during two stages, namely, slow and rapid rifting.Such systems that change in space and time are identified as dynamic [4], and it follows from the theorythat dissipative dynamical systems in geological–geophysical media should have spatiotemporal attractors;in the lithosphere of a rift system, these attractors areto be classified as attractors of riftogenesis (spacedomains and time periods where normal faultrelatedearthquakes, which are typical for this mode, dominate). It has been found that temporal attractors of riftogenesis in the BRS lithosphere correspond to assemblage structure within the framework of the evolutional scenario with bifurcation of triple equilibrium[5]. The detailed study and generalization of theseparameters of seismic sources enabled the identification of three ASRs in the lithosphere of the region:South Baikal, Khubsugul, and Muya depressions [6](Fig. 1). ASRs are the main peculiarity of regionallithosphere since they form nonlinearity and instability of modern geodynamical and tectonophysical processes in the BRS lithosphere.In the present paper, the ASRs are considered as anattribute of Cenozoic nonlinear geodynamics of theBRS lithosphere. Their inclusion in the history ofregional Cenozoic geodynamics allows us to supplement the available description [2, 3, 7] with modernconcepts about the origin and evolution of riftogenesisin the BRS lithosphere within the framework of theselforganization theory for dissipative systems andnonlinear media. We lean upon the actualism principle here; it is not understood as a statement about similarity between present and past geological–geophysical processes, but as a statement that systems preserve

Correlation of Earthquake Occurrence Rates in the Baikal Region and Mongolia: Episodes of Synchronism

Variations in annual numbers of earthquakes (the earthquake occurrence rate) that hit the Baikal region and Mongolia during the period from 1964 through 2001 are studied in this work. Correlation analysis of the different-length series of annual numbers N of earthquakes of representative energy classes makes it possible to reveal the effects of synchronous changes in the earthquake occurrence rate in seven regions and eleven areas in the Mongolia-Baikal region, located far apart. The analysis of the shock occurrence rate revealed episodes of short-period synchronization of seismic processes in the Mongolia-Baikal region at the end of the 1960s, early in the 1980s, and in the middle of the 1990s. The episode of synchronization in the earthquake occurrence rate in the early 1980s is observed in all the territories under study, but the episode at the end of the 1960s is less distinctly seen in Mongolia and is revealed mainly in the data series with a length of three years. The synchronization in the seismicity in Mongolia and in the southern PreBaikal region in 1995 requires further investigations, involving the dynamic parameters of the earthquake sources. The observed synchronism in the annual number of earthquakes indicates that the seismic processes become active nearly simultaneously over the huge territory of the Mongolia-Baikal region and produce a short-term coherent change in the shock occurrence rate in the spatial-temporal distribution of the seismicity. The observed spatial and temporal correlation in the seismicity is a sign of the seismogenic link between the Baikal region and Mongolia.

Synchronization episodes in annual numbers of earthquakes in the Mongolian-Baikal Region

Correlation analysis of annual numbers of earthquakes for Baikal Region and Mongolia allowed us to discover episodes of synchronized change in velocity of the seismic current in the territory of seven territories and twelve sections of the Mongolian-Baikal Region (MBR), substantially distant from each other. The three episodes of short-term synchronization in seismic process in the MBR were detected, namely, in the late 1960s, in the early 1980s, and in the mid 1990s. The episode of the early 1980s was observed in all the territories, while the episode of the late 1960s was expressed more weakly in Mongolia and distinguished mostly with an implementation length of three years. The episode of the mid 1990s requires further study with the use of parameters for seismic sources. The observed synchronization for annual numbers of earthquakes is evidence for the fact that activation of seismic process takes place almost simultaneously throughout the huge territory of the MBR during stress reconstruction in the lithosphere of the Baikal Rift Zone (BRZ); this activation forms a short-term coherent change in the velocity of quakes current for the spatial-temporal distribution of seismicity, which shows the seismogenic relation between Baikal Region and Mongolia.

Episodes of high correlation between annual rates of earthquakes: The Baikal Rift Zone

Correlation analysis techniques were used to study variations in the annual rates N of completely reported earthquakes with energy class K ≥ 8 that occurred from 1964 to 2001 in the Baikal Rift Zone (BRZ), in three subregions within that zone, and in six areas. This correlation analysis of samples of annual rates of earthquakes N with different observation periods revealed two statistically significant episodes of short-lived synchronization between the seismic processes in the BRZ, in the late 1960s and in the late 1970s to the early 1980s. The 1970–1980 episode stands out because of its duration and the highest correlation level; this makes it the dominant phenomenon in the Baikal Rift seismicity synchronization. The observed synchronization episodes between annual rates of earthquakes show that the seismic process was activated at about the same time in different subregions of the BRZ, thus producing short-lived coherent increases in seismicity rates.

Diagnosis of stressed states in the lithosphere of the Baikal Rift System

The results of investigation of the structure and dynamics of stresses at the modern (instrumental) stage of the evolution of the lithosphere in the Baikal Rift System (BRS) are presented. The application of the theoretical methods of nonlinear dynamic systems for diagnosis of the stressed states allowed us to formulate the concept of the complex stressed state of the BRS lithosphere. In this concept, the time distribution of large earthquakes is explained by bifurcations (catastrophes) of stresses. The analysis of the results obtained points to possibilities of a catastrophe of stresses in the BRS lithosphere in the next few years and the consequent realization of large earthquakes with magnitude M ≈ 7 . A model of triple equilibrium bifurcation is suggested for the evolution of stresses in the BRS lithosphere. Rapid processes of stress rearrangement in the BRS lithosphere are consistent with the behavior of complex self-organizing unstable thermodynamic systems [1]. Since they are observed synchronously in three zones of the maxima of structural force inhomogeneities, we classify the BRS as a spatiotemporal open self-organized nonlinear dissipative system [2]. A complex system is considered self-organized if it maintains its instability at a level sufficient for effective resistance to the variations in the active medium, thus saving itself from collapse. We understand self-organization as an antientropic process of buildup, interaction, and maintenance of coherence between the elements of the system with increase of its complexity and formation of the attractor structure. Complex synergetic phenomena are exemplified in the literature as different processes of self-organization in physicochemical and biological systems. Their analysis allows us to establish the general features, which are characteristic of complex phenomena, and formulate the main principles irrespective of the specific nature of the system. Each of such processes has unique features. The role of nonlinearity, fluctuations, oscillations, bifurcations, and the attractor is manifested very clearly in these examples. Therefore, when modeling the behavior of geophysical systems, one should use the advantage of new perspectives discovered by science during the investigation of complex behavior of nonlinear dynamic systems. This is especially important in light of the modern tendencies of forecasting and controlling the seismic process, since forecast and control imply knowledge of the properties and peculiarities of the system under control. They begin with gathering of information about the state of the system, its links, and the logic of its functioning.

Focal Parameters of Strong Earthquakes of Baikal Area: The Main Regularities

971 The structure of sampling containing the focal mechanisms of strong earthquakes of CisBaikalia is analyzed, and the average distributions of parameters are obtained on the entire sampling and depending on the slip type in a seismic source. The main regularities are formed by the quantitatively predominant quakes with normal faulting mechanisms. After the sam� plings subdivision into normal faulting, strikeslip, and reverse faulting mechanisms, both significant dif� ferences and coincidences of focal mechanisms were found. The correspondence between average dipping angles , moduli of slipping angles SLIP, and strike azimuths STK indicates possible displacements on the zones of the same faults. The average dipping angles for slips of different types exceed 50°, indicating a steep sink of the fault zones. Focal parameters enable us to estimate the stressed state of a medium and the structure of ruptures in a fault zone in which an earthquake has occurred. This information is unique because there are no other methods of definition and parametric description of the geophysical state for rocks at great depths yet. Understanding of how important the solution of this problem is for the region of CisBaikalia has been reached long time ago (1), and the systematic defini� tion of focal mechanisms for strong earthquakes (energy class is KР ≥ 10) has been carried out in the region for more than half a century. Information about the focal mechanisms for strong earthquakes for the region of CisBaikalia has been presented in numerous publications, but unfortunately there is no united database yet. The sampling containing the focal mech� anisms of strong earthquakes of CisBaikalia, which is used in the present study, was compiled by the author on the basis of published sources in order to compare the focal mechanisms and the dynamical parameters of earthquake sources (2). The results of statistical aver DP processing of focal parameters for 335 strong (KР≥10) earthquake that occurred in the period since 1950 until 1998 in the region of CisBaikalia characterize the dis� tributions of the available solutions of focal mecha� nisms.

Estimating the energy of seismotectonic deformations in the lithosphere of the Baikal Rift Zone

Estimation and comparison of the energy of seismotectonic deformations in the lithosphere of the Baikal Rift Zone (BRZ) based on observations of large (M ≥ 6) earthquakes for the period of instrumental recording (1950–2002), for a historical period lasting 210 years (1740–1949), and inferred from palaeo-seismological materials for the past 2000 years, all indicate that the hypothesis of a stationary seismic process is appropriate for the region. The locations of maxima of the density of seismotectonic strain energy released during the time intervals under investigation show that most of the failures in the lithosphere occurred approximately in the same areas, which may be interpreted as stress concentrators. The isolines of increased density for the energy of seismotectonic deformations align themselves along the rift features from southwest to northeast in the Baikal region and this allows one to treat the BRZ lithosphere as an extended zone of enhanced, inhomogeneous, energy release of endogenous geotectonic processes. We assessed the power of the seismotectonic processes that reflect the release of endogenous energy through earthquakes. Identification of areas with deficits in the energy of seismotectonic deformations (“energy gaps”) is an important step toward long-term solution of seismic-safety problems for the Baikal region.